CS1111: Problem Solving�using Computers

Rupesh Nasre.

​

Mentor TAs: Returaj, Akshay, Narayana, Sankaranarayanan,

Ruheena, Pranshu, Dipanshu, Deepankar, Amit

IIT Madras

November 2022

Placement in Computer Science

• CS1111: Problem Solving and Coding
• CS1200: Proofs, Counting
• CS2200: Computation Theory
• CS2300: Overview of Digital World
• CS2600: Hardware
• CS2700: Efficient Implementation
• CS2800: Algorithms
• CS3100: Ways of Programming
• CS3300: Translation (Programmer and Machine)
• CS3500: Resource Management (User and Machine)

CS1111 is a foundational

subject feeding into all the

other CS courses.

Learning Outcomes

• Model a given problem computationally.
• Identify a solution to the problem.
• Decompose the solution into a sequence of logical steps.
• Implement the steps in a computer program.
• Solve the problem with the program.
• Iterate through the solution as required.

Problem

A

p

p

r

o

a

c

A

h

e

s

I

m

p

l

e

m

e

A

n

t

a

t

i

A

o

n

s

...

...

Our first problem: Make tea.

1. Take tea-powder.
2. Take sugar.
3. Take milk.
4. Boil together.
5. Tea is ready!

// how much?

// where is it?

// what if I don't have milk?

// for how long?

​

Even for intellectuals such as humans, we need more information.

For dumb machines such as computers, we need to be precise.

Programming is about precise understanding;

so precise that even a machine should be able to follow.

foodviva.com

Data

Algorithm

Your laptop is unlikely to be able to make tea. But then ...

Is your tea vending machine a computer?

What if it can give you tea, coffee, milk, hot water, ...?

How about a calculator?

​

A computer is programmable.

A vending machine or a calculator are not. They can perform only pre-programmed computation.

Then how about your iPads or smart phones?

amazon.com

computerhope.com

Logistics

• Course credits: 3-0-0-3-6-12
• if (date == Nov 9 or date == Nov 16) {
• theoryAt(RJN 102, Wed 14, Thu 15, Fri 8);
• labAt(CSE DCF, Thu 9);
• } else {
• theoryAt(RJN 102, Wed 11, Thu 15, Fri 8);
• labAt(CSE DCF, Thu 9);
• }
• We will use replit platform for the labs.

Logistics

• Evaluation
• 30% Lab + 15% Q1 + 15% Q2 + remaining% EndSem
• Every lab (except the first) is evaluated.
• Attendance: Standard institute rules apply.
• course webpage (slides, codes, information).
• Moodle / Google Group
• Will be used as a communication mechanism.
• Join moodle here.

To get the MOST out of this course

• Keep hands away from WhatsApp.
• Solve questions during classwork.
• Keep a copy with you. Take notes.
• Ask questions (others also haven’t understood).
• Do not let a few dominate the discussion.

First Program

• Unfortunately, this is Tamil for a Bengali person.
• And our mother-tongue is C. So we will have to follow the C syntax.
• Where do you write this program?
• On Linux: text editor, vi, VS code, nano, sublime, ...
• On replit

print “Hello World!“

A line with # indicates a preprocessor directive (such as #if, #pragma, #define).

Hello World!

#include <stdio.h>

​

int main() {

printf("Hello World!\n");

}

​

Please allow me to use printf.

Entry function

Print this to the screen when the program is executed.

This is a header file.

(check /usr/include/stdio.h)

Returns an exit code (integer) to the shell.

Formatted printing

Newline

This semicolon is required.

End of statement (like a full-stop).

() indicate arguments to the function (e.g., sin(x)).

Block or body of the function in {…}.

#include <stdio.h>

int main( )

{printf ( "Hello World!\n")

;

​

}

Whitespace can be added freely (almost).

Whitespace means space, tab, newline.

main(){}

Smallest C program

#include <stdio.h>

​

int main() {

printf("Hello World!\n");

}

​

A Small Problem

• This English-like program can be understood by humans, but not by machines – such as your laptop.
• Computers understand only 0 and 1.
• How about writing our code in binary?
• Possible, but not very motivating.
• Is it possible to write in C and the machine reads binary?

• We will use a translator!
• On replit and your Linux laptops, the compiler is gcc.

#include <stdio.h>

​

int main() {

printf("Hello World!\n");

}

​

Compiler

11000110

01010011

01010100

11101000

11111110

Tamil

Bengali

C

Machine code

$ gcc hello.c

$ a.out

Hello World!

$

Command to compile. Translates .c file to a.out.

Run your program (or execute it).

Output of your program.

Command prompt to type the next command.

gcc is also a big program written by many people, such as you.

So are firefox, chrome, minesweeper, powerpoint, Windows OS, Android OS, ...

#include <stdio.h>

​

int main() {

printf("Hello World!\n");

}

​

#include <stdio.h>

​

int main() {

printf("Bye World!\n");

}

​

#include <stdio.h>

​

int main() {

printf("printf(printf)\n");

}

​

#include <stdio.h>

​

int main() {

printf("Hello “);

printf(“World!\n");

}

​

#include <stdio.h>

​

int main() {

printf("Hello "

“World!\n“);

}

​

#include <stdio.h>

​

int main() {

printf("Hello \

World\n");

}

​

#include <stdio.h>

​

int main() {

printf("3*5 = 15\n");

}

​

#include <stdio.h>

​

int main() {

printf(3*5);

}

​

#include <stdio.h>

​

int main() {

printf(TN‘s CM);

}

​

3*5 = 15

Compiler issues a warning,

but compiles to a.out.

a.out prints

Segmentation fault (core dumped)

Compiler issues a warning

and an error.

What does a.out print?

We need to understand printf a little better.

printf

Placeholders, to be replaced by people.

#include <stdio.h>

​

int main() {

printf("3*5 = %d\n", 3*5);

}

​

3*5 = 15

printf here took two arguments

“3*5 = %d\n“ and 3*5

​

The first argument in double-quotes

is called a format-string.

​

This format-string is our bus without people,

but with their placeholders.

Placeholder for decimal integer.

printf

Placeholders, to be replaced by people.

#include <stdio.h>

​

int main() {

printf("3*5 = %d, a-z are %d letters, 5 is 50%% of %d\n", 3*5, 26, 10);

}

​

3*5 = 15, a-z are 26 letters, 5 is 50% of 10

printf

#include <stdio.h>

​

int main() {

printf("3*5 = %d, a-z are %d letters, 5 is 50%% of %d\n", 3*5, 26, 10);

}

​

3*5 = 15, a-z are 26 letters, 5 is 50% of 10

• What if there is a mismatch in the number of placeholders and the number of arguments?
• For a correct program, the two should match.
• You can play around with these numbers to know the behavior of the compiler / runtime, but it would not fetch you much w.r.t. the application semantics.
• Why does C have such a cryptic way for simple printing?

There are more format specifiers,

which we will study soon.

Future Connect:

printf permits variable

number of arguments.

Classwork: Find outputs.

#include <stdio.h>

​

int main() {

printf("3*5\n = 15\n");

}

​

#include <stdio.h>

​

int main() {

printf("3*5 = %d\n", 16);

}

​

#include <stdio.h>

​

int main() {

printf("15 is 15% of 100\n");

}

​

3*5

= 15

3*5 = 16

15 is 1531263471240f 100

printf

#include <stdio.h>

​

int main() {

printf("3*5 = %d, a-z are %d letters, 5 is 50%% of %d\n", 3*5, 26, 10);

}

​

There are more format specifiers,

which we will study soon.

Data types

Why do we need data types?

• Numbers are of different types (number of students vs. height).
• Text vs. numbers vs. roll number
• Academic record vs. bank account transactions
• ...

Some of these are provided by C.

Others can be created by us.

printf Format Specifiers

Hello, which course is this?

1111

Hi! Welcome to CS1111.

printf(“%s of %s\n“,

“Summer“, “69“);

printf(“%s has %d students\n“,

“CS1111“, 87);

printf(“The value of PI“);

printf(“ is %f“, 3.1428);

printf(“%d“, 3);

printf(“ idiots“);

printf(“name = %s\n“, “Khan“);

printf(“age = %d\n“, 20);

printf(“height = %lf ft\n“, 5.8);

printf(“weight is 50kg\n“);

printf(“CS%d is %s course\n“,

1111, “foundational“);

printf(“Hexadecimal has \

%X symbols.\

\n“, 16);

printf(“32 1s is %x in hex \

and %o in octal\n“,

0xFFFFFFFF, 0xFFFFFFFF);

printf(“%cS%x\n“, ‘C‘, 4369);

We need to understand printf a little better.

This was output. Does C have a capability to take input?

We want this to be typed by the user.

Hello, which course is this?

1111

Hi! Welcome to CS1111.

We want this to be typed by the user.

printf(“Hello, which course is this?\n“);

​

What user enters here should appear below.

​

printf(“Hi! Welcome to CS%d\n“, ???);

We know how to print this.

How about this?

How does our brain remember?

It stores the information in memory cells.

Can we also do the same?

But which cell to access?

Hmm... Let's name the cells.

Hello, which course is this?

1111

Hi! Welcome to CS1111.

We want this to be typed by the user.

printf(“Hello, which course is this?\n“);

​

What user enters is stored in cell cell1.

​

printf(“Hi! Welcome to CS%d\n“, cell1);

We know how to print this.

How about this?

How does our brain remember?

It stores the information in memory cells.

Can we also do the same?

But which cell to access?

Hmm... Let's name the cells.

Where is this cell1 stored?

Inside your computer.

But where?

Well, in memory.

We call it random access memory.

Hello, which course is this?

1111

Hi! Welcome to CS1111.

We want this to be typed by the user.

printf(“Hello, which course is this?\n“);

​

What user enters is stored in cell cell1.

​

printf(“Hi! Welcome to CS%d\n“, cell1);

We know how to print this.

How about this?

Where is this cell1 stored?

Inside your computer.

But where?

Well, in memory.

We call it random access memory.

printf(“Hello, which course is this?\n“);

​

scanf(“%d“, cell1);

​

printf(“Hi! Welcome to CS%d\n“, cell1);

This code doesn't compile.

gcc says it doesn't know cell1.

Let's compile and run it.

Data

Algorithm

int cell1;

printf(“Hello, which course is this?\n“);

scanf(“%d“, cell1);

printf(“Hi! Welcome to CS%d\n“, cell1);

Hello, which course is this?

1111

Hi! Welcome to CS1111.

We want this to be typed by the user.

printf(“Hello, which course is this?\n“);

​

What user enters is stored in cell cell1.

​

printf(“Hi! Welcome to CS%d\n“, cell1);

We know how to print this.

How about this?

Where is this cell1 stored?

Inside your computer.

But where?

Well, in memory.

We call it random access memory.

printf(“Hello, which course is this?\n“);

​

scanf(“%d“, cell1);

​

printf(“Hi! Welcome to CS%d\n“, cell1);

​

​

Let's compile and run it.

int cell1;

printf(“Hello, which course is this?\n“);

scanf(“%d“, cell1);

printf(“Hi! Welcome to CS%d\n“, cell1);

Hello, which course is this?

1111

Segmentation fault (core dumped)

A New Problem

• Say you want to get your room/house painted.

​

• Which of the following whatsapp messages would help a painter reach and paint your house?
• Hi, my house color is white.
• Hi, my house is around 1500 sq ft.
• "Hi, I have %d members in my house.", 4
• Hi, my house address is 670, New Nandanvan.

Gives some information but the painter can‘t change the color.

Allows the painter to go and change the color.

In addition, the painter can get the other information also!

Coming back to the old problem

​

​

​

​

​

​

​

• Hi, my house address is 670, New Nandanvan.

int cell1;

printf(“Hello, which course is this?\n“);

scanf(“%d“, cell1);

printf(“Hi! Welcome to CS%d\n“, cell1);

Informs scanf of the value in cell1, but does not allow scanf to change it.

We need to send address of cell1.

???

cell1

670, New Nandanvan

int cell1;

printf(“Hello, which course is this?\n“);

scanf(“%d“, 670, New Nandanvan);

printf(“Hi! Welcome to CS%d\n“, cell1);

Allows the painter to go and change the color.

In addition, the painter can get the other information also!

Coming back to the old problem

int cell1;

printf(“Hello, which course is this?\n“);

scanf(“%d“, cell1);

printf(“Hi! Welcome to CS%d\n“, cell1);

Informs scanf of the value in cell1, but does not allow scanf to change it.

We need to send address of cell1.

???

cell1

670, New Nandanvan

int cell1;

printf(“Hello, which course is this?\n“);

scanf(“%d“, 670, New Nandanvan);

printf(“Hi! Welcome to CS%d\n“, cell1);

???

cell1

670, RAM

int cell1;

printf(“Hello, which course is this?\n“);

scanf(“%d“, &cell1);

printf(“Hi! Welcome to CS%d\n“, cell1);

Note that printf is interested in the value, and not in changing it.

What if I

pass &cell1

to printf?

What if I

I pass cell1

to scanf?

Coming back to the old problem

???

cell1

670, RAM

int cell1;

printf(“Hello, which course is this?\n“);

scanf(“%d“, &cell1);

printf(“Hi! Welcome to CS%d\n“, cell1);

Hello, which course is this?

1111

Hi! Welcome to CS1111

Hello, which course is this?

6023

Hi! Welcome to CS6023

1111

cell1

670

6023

cell1

424

passed to scanf

passed to printf

syntactic sugar

address

value

Hello, which course is this?

CS1111

Hi! Welcome to CS0

0

cell1

35213944

int year;

scanf(“%d“, &year);

printf(“%s of %d\n“,

“Summer“, year);

int nstuds;

scanf(“%d“, &nstuds);

printf(“%s has %d students\n“,

“CS1111“, nstuds);

float pi;

printf(“Value of pi?“);

scanf(“%f“, &pi);

printf(“The value of PI“);

printf(“ is %f“, pi);

short nidiots;

scanf(“%d“, &nidiots);

printf(“%d“, nidiots);

printf(“ idiots“);

int age; double height;

scanf(“%d“, &age);

printf(“age = %d\n“, age);

scanf(“%lf“, &height);

printf(“height = %lf ft\n“, height);

printf(“weight is 50kg\n“);

long int course;

scanf(“%ld“, &course);

printf(“CS%d is %s course\n“,

course, “foundational“);

int base16;

scanf(“%x“, &base16);

printf(“Hexadecimal has \

%X symbols.\

\n“, base16);

int max32;

scanf(“%x“, &max32);

printf(“32 1s is %x in hex \

and %o in octal\n“,

max32, max32);

char c;

int num;

scanf(“%c“, &c);

scanf(“%d“, &num);

printf(“%cS%x\n“, c, num);

18

age

4315348

5.88

431534C

height

Problem: Find age from birth year.

// create cell birthyear

​

// take input from user in cell birthyear

​

// create cell age

​

// calculate (2022 – birthyear) and store in age.

​

// output age

​

Algorithm and Implementation

Comment

Assignment

int A, b, C;

A = 0;

b = 1 - A;

C = A - b;

float celcius, farenheit;

scanf("%f", &celcius);

farenheit = 9*celcius/5 + 32;

scanf("%d", &num);

mod5 = num % 5;

printf("%d mod 5 is %d\n",

num, mod5);

char c;

int num;

scanf(“%c“, &c);

scanf(“%d“, &num);

printf(“%cS%x\n“, c, num);

Modulus or remainder

// create cell birthyear

int birthyear;

// take input from user in cell birthyear

scanf("%d", &birthyear);

// create cell age

int age;

// calculate (2022 – birthyear) and store in age.

age = (2022 – birthyear);

// output age

printf("Your age is %d\n", age);

0

A

1

b

-1

C

Problem: Find your team number.

// create cell for last two digits of roll number

​

// take input from user in cell rollnumber

​

// create cell for team number

​

// find rollnumber / 10 + 1 and store in teamnumber

​

// output team number

​

Expect last two digits alone.

char c;

int num;

scanf(“%c“, &c);

scanf(“%d“, &num);

printf(“%cS%x\n“, c, num);

// create cell for last two digits of roll number

int rollnumber;

// take input from user in cell rollnumber

scanf("%d", &rollnumber);

// create cell for team number

int teamnumber;

// find rollnumber / 10 + 1 and store in teamnumber

teamnumber = (rollnumber / 10 + 1);

// output team number

printf("Your team is Team %d\n", teamnumber);

• Here is your replit team mapping.
• Team 1: Roll numbers CS22B001 – 10
• Team 2: Roll numbers CS22B011 – 20
• Team 9: Roll numbers CS22B081 -- 90

15

Your team is Team 2

78

Your team is Team 8

20

Your team is Team 3

Testing helps find bugs.

teamnumber = ((rollnumber - 1) / 10 + 1);

15

Your team is Team 2

78

Your team is Team 8

20

Your team is Team 2

Given a team number, find the first and the last roll numbers in that input team.

(assume 90 students)

Extent the program for full roll number.

Problem: Find team members.

char c;

int num;

scanf(“%c“, &c);

scanf(“%d“, &num);

printf(“%cS%x\n“, c, num);

// create data

char teamid;

int startroll, endroll;

// take input

scanf("%c", &teamid);

// find the roll numbers

startroll = (...);

endroll = startroll + 9;

// output

printf("%d to %d\n", startroll, endroll);

• Here is your replit team mapping.
• Team A: Roll numbers CS22B001 – 10
• Team B: Roll numbers CS22B011 – 20
• Team I : Roll numbers CS22B081 -- 90

A

1 to 10

I

81 to 90

C

21 to 30

Given a team id, find the first and the last roll numbers in that team.

(assume 90 students)

Characters are stored as integers.

‘A‘ is 65, ‘B‘ is 66, …

(teamid + 5) is allowed in C.

startroll = (teamid – 65) * 10 + 1;

As a good programming practice

• Avoid implicit type conversion.
• Avoid constants such as 65.

startroll = ((int)teamid – ‘A‘) * 10 + 1;

Problem: Find endsem percentage.

char c;

int num;

scanf(“%c“, &c);

scanf(“%d“, &num);

printf(“%cS%x\n“, c, num);

// create data

int labs, quizzes, endsem;

// take input

scanf("%d%d", &labs, &quizzes);

// do computation

endsem = (100 – (labs + quizzes));

// output

printf("Endsem %% is %d\n", endsem);

• 30% Lab + 15% Q1 + 15% Q2 + remaining% EndSem

​

endsem = (100 – labs – quizzes);

​

int nonendsem;

nonendsem = labs + quizzes;

endsem = 100 – nonendsem;

int remaining;

remaining = 100;

scanf(“%d“, &labs);

remaining = remaining – labs;

scanf(“%d“, &quizzes);

remaining = remaining – quizzes;

printf(“Endsem %% is %d\n“, remaining);

???

labs

???

quizzes

???

remaining

100

30

70

30

40

30

30

Endsem % is 40

Problem: Find sum.

int n;

// read n

scanf("%d", &n);

// compute sum

int sum = n * (n + 1) / 2;

// print sum

printf("Sum of first %d numbers is %d\n", n, sum);

char c;

int num;

scanf(“%c“, &c);

scanf(“%d“, &num);

printf(“%cS%x\n“, c, num);

Σn = 1 + 2 + 3 + ... + n

= n * (n + 1) / 2

Σ2ⁱ = 1 + 2 + 4 + 8 + ... n terms

= 2ⁿ – 1

Needs pow() function or a loop.

pow(x, y) returns x^y.

Does your code compile?

Problem: Find probability.

int nCards = 52;

int nKings = 4;

int nSpades = 13;

int nKingAndSpade = 1;

int nNonkingNonspade = nCards -

(nKings + nSpades - nKingAndSpade);

float prob = nNonkingNonspade / nCards;

printf("Probability of nonking, nonspade is %f\n",

prob);

char c;

int num;

scanf(“%c“, &c);

scanf(“%d“, &num);

printf(“%cS%x\n“, c, num);

A card is drawn at random from a deck of well-shuffled cards. Find the probability of it being neither a king nor a spade.

Find the issue

with this code.

Problem: Find the line.

float x1, y1, x2, y2;

scanf("%f%f%f%f", &x1, &y1, &x2, &y2);

float m = (y2 – y1) / (x2 – x1);

float c = (y1 – m*x1);

printf("Equation of the line is y = %.2fx + %.2f\n", m, c);

printf("Equation of the line is y = %gx + %g\n", m, c);

printf("Equation of the line is y = %.2ex + %.2e\n", m, c);

char c;

int num;

scanf(“%c“, &c);

scanf(“%d“, &num);

printf(“%cS%x\n“, c, num);

Given two points on a line, find its equation in y = mx + c format.

3.2 3 9.6 5

Equation of the line is y = 0.31x + 2.00

Equation of the line is y = 0.3125x + 2

Equation of the line is y = 3.12e-01x + 2.00e+00

Problem: Print tabular.

char name1[20], name2[20], name3[20];

int m11, m12, m13, m21, m22, m23, m31, m32, m33;

​

scanf("%s%d%d%d", &name1, &m11, &m12, &m13);

scanf("%s%d%d%d", &name2, &m21, &m22, &m23);

scanf("%s%d%d%d", &name3, &m31, &m32, &m33);

​

int t1, t2, t3;

t1 = m11 + m12 + m13;

t2 = m21 + m22 + m23;

t3 = m31 + m32 + m33;

​

printf("%-12s%5d%5d%5d = %5d\n", name1, m11, m12, m13, t1);

printf("%-12s%5d%5d%5d = %5d\n", name2, m21, m22, m23, t2);

printf("%-12s%5d%5d%5d = %5d\n", name3, m31, m32, m33, t3);

char c;

int num;

scanf(“%c“, &c);

scanf(“%d“, &num);

printf(“%cS%x\n“, c, num);

Read names and marks of three students and print the names and total in a table.

Rajesh 1 43 43 = 87

SomeshSingh 23 55 6 = 144

JK 21 21 21 = 63

char array or string

Future Connect:

Replacing &name1 with

name1 also works.

Homework Problem: Number game.

char c;

int num;

scanf(“%c“, &c);

scanf(“%d“, &num);

printf(“%cS%x\n“, c, num);

Choose a number from 0..9.

Multiply by 5.

Add 3 to it.

Double it up.

Choose another number from 0..9 and add to it.

​

Tell me the number. I will tell you both the chosen numbers.

Computer System

limbd.org

Your files reside here.

e.g., hello.c, a.out

printf

scanf

18

age

4315348

executes a.out

Hello World!

All C Keywords

www.programiz.com

Summary

• Hello World!
• Formatted input, output
• Problem Solving with assignments

CS1111: Problem Solving�using Computers

Rupesh Nasre.

​

Mentor TAs: Ahmed, Vivek, Vimala, Akash,

Kankan, Rahul, Swati, Akshay, Ashok, Keshav

IIT Madras

May 2022

Placement in Computer Science

• CS1111: Problem Solving and Coding
• CS1200: Proofs, Counting
• CS2200: Computation Theory
• CS2300: Overview of Digital World
• CS2600: Hardware
• CS2700: Efficient Implementation
• CS2800: Algorithms
• CS3100: Ways of Programming
• CS3300: Translation (Programmer and Machine)
• CS3500: Resource Management (User and Machine)

CS1111 is a foundational

subject feeding into all the

other CS courses.

Learning Outcomes

• Model a given problem computationally.
• Identify a solution to the problem.
• Decompose the solution into a sequence of logical steps.
• Implement the steps in a computer program.
• Solve the problem with the program.
• Iterate through the solution as required.

Problem

A

p

p

r

o

a

c

A

h

e

s

I

m

p

l

e

m

e

A

n

t

a

t

i

A

o

n

s

...

...

Our first problem: Make tea.

• Take tea-powder.
• Take sugar.
• Take milk.
• Boil together.
• Tea is ready!

// how much?

// where is it?

// what if I don't have milk?

// for how long?

​

Even for intellectuals such as humans, we need more information.

For dumb machines such as computers, we need to be precise.

Programming is about precise understanding;

so precise that even a machine should be able to follow.

foodviva.com

Data

Algorithm

Your laptop is unlikely to be able to make tea. But then ...

Is your tea vending machine a computer?

What if it can give you tea, coffee, milk, hot water, ...?

How about a calculator?

​

A computer is programmable.

A vending machine or a calculator are not. They can perform only pre-programmed computation.

Then how about your iPads or smart phones?

amazon.com

computerhope.com

Logistics

• Course credits: 3-0-0-3-6-12
• if (date <= May 12) {
• theoryAt(Mon 13 + 17, Wed 17, Thu 15 + 17, Fri 14);
• labAt(Thu 9, Fri 9);
• } else {
• theoryAt(Mon 13 + 14, Wed 14, Thu 10 + 14, Fri 9);
• labAt(Thu 15, Fri 15);
• }
• We will use replit platform for the labs.

Logistics

• Evaluation
• 56% labs + 15% midsem + remaining% endsem
• Every lab is evaluated.
• Attendance: Standard institute rules apply.
• Midsem and endsem dates will be populated on the course webpage (along with slides and codes).
• Moodle
• Will be used as a communication mechanism.
• Your responsibility to subscribe to it.
• Join here.

To get the MOST out of this course

• Keep hands away from WhatsApp.
• Solve questions during classwork.
• Keep a copy with you. Take notes.
• Ask questions (others also haven’t understood).
• Do not let a few dominate the discussion.

Takeaways

• The same problem can be solved using multiple algorithms. An algorithm may have more than one implementations.
• Programming demands precise computation.
• Computers are programmable.
• Stay away from WhatsApp.

First Program

• Unfortunately, this is Tamil for a Bengali person.
• And our mother-tongue is C. So we will have to follow the C syntax.
• Where do you write this program?
• On Linux: text editor, vi, VS code, nano, sublime, ...
• On replit

print “Hello World!“

A line with # indicates a preprocessor directive (such as #if, #pragma, #define).

Hello World!

#include <stdio.h>

​

int main() {

printf("Hello World!\n");

}

​

Please allow me to use printf.

Entry function

Print this to the screen when the program is executed.

This is a header file.

(check /usr/include/stdio.h)

Returns an exit code (integer) to the shell.

Formatted printing

Newline

This semicolon is required.

End of statement (like a full-stop).

() indicate arguments to the function (e.g., sin(x)).

Block or body of the function in {…}.

#include <stdio.h>

int main( )

{printf ( "Hello World!\n")

;

​

}

Whitespace can be added freely (almost).

Whitespace means space, tab, newline.

main(){}

Smallest C program

#include <stdio.h>

​

int main() {

printf("Hello World!\n");

}

​

A Small Problem

• This English-like program can be understood by humans, but not by machines – such as your laptop.
• Computers understand only 0 and 1.
• How about writing our code in binary?
• Possible, but not very motivating.
• Is it possible to write in C and the machine reads binary?

• We will use a translator!
• On replit and your Linux laptops, the compiler is gcc.

#include <stdio.h>

​

int main() {

printf("Hello World!\n");

}

​

Compiler

11000110

01010011

01010100

11101000

11111110

Tamil

Bengali

C

Machine code

$ gcc hello.c

$ a.out

Hello World!

$

Command to compile. Translates .c file to a.out.

Run your program (or execute it).

Output of your program.

Command prompt to type the next command.

gcc is also a big program written by many people, such as you.

So are firefox, chrome, minesweeper, powerpoint, Windows OS, Android OS, ...

#include <stdio.h>

​

int main() {

printf("Hello World!\n");

}

​

#include <stdio.h>

​

int main() {

printf("Bye World!\n");

}

​

#include <stdio.h>

​

int main() {

printf("printf(printf)\n");

}

​

#include <stdio.h>

​

int main() {

printf("Hello “);

printf(“World!\n");

}

​

#include <stdio.h>

​

int main() {

printf("Hello "

“World!\n“);

}

​

#include <stdio.h>

​

int main() {

printf("Hello \

World\n");

}

​

#include <stdio.h>

​

int main() {

printf("3*5 = 15\n");

}

​

#include <stdio.h>

​

int main() {

printf(3*5);

}

​

#include <stdio.h>

​

int main() {

printf(TN‘s CM);

}

​

3*5 = 15

Compiler issues a warning,

but compiles to a.out.

a.out prints

Segmentation fault (core dumped)

Compiler issues a warning

and an error.

What does a.out print?

We need to understand printf a little better.

printf

Placeholders, to be replaced by people.

#include <stdio.h>

​

int main() {

printf("3*5 = %d\n", 3*5);

}

​

3*5 = 15

printf here took two arguments

“3*5 = %d\n“ and 3*5

​

The first argument in double-quotes

is called a format-string.

​

This format-string is our bus without people,

but with their placeholders.

Placeholder for decimal integer.

printf

Placeholders, to be replaced by people.

#include <stdio.h>

​

int main() {

printf("3*5 = %d, a-z are %d letters, 5 is 50%% of %d\n", 3*5, 26, 10);

}

​

3*5 = 15, a-z are 26 letters, 5 is 50% of 10

printf

#include <stdio.h>

​

int main() {

printf("3*5 = %d, a-z are %d letters, 5 is 50%% of %d\n", 3*5, 26, 10);

}

​

3*5 = 15, a-z are 26 letters, 5 is 50% of 10

• What if there is a mismatch in the number of placeholders and the number of arguments?
• For a correct program, the two should match.
• You can play around with these numbers to know the behavior of the compiler / runtime, but it would not fetch you much w.r.t. the application semantics.
• Why does C have such a cryptic way for simple printing?

There are more format specifiers,

which we will study soon.

Future Connect:

printf permits variable

number of arguments.

Classwork: Find outputs.

#include <stdio.h>

​

int main() {

printf("3*5\n = 15\n");

}

​

#include <stdio.h>

​

int main() {

printf("3*5 = %d\n", 16);

}

​

#include <stdio.h>

​

int main() {

printf("15 is 15% of 100\n");

}

​

3*5

= 15

3*5 = 16

15 is 1531263471240f 100

printf

#include <stdio.h>

​

int main() {

printf("3*5 = %d, a-z are %d letters, 5 is 50%% of %d\n", 3*5, 26, 10);

}

​

There are more format specifiers,

which we will study soon.

Data types

Why do we need data types?

• Numbers are of different types (number of students vs. height).
• Text vs. numbers vs. roll number
• Academic record vs. bank account transactions
• ...

Some of these are provided by C.

Others can be created by us.

printf Format Specifiers

Hello, which course is this?

1111

Hi! Welcome to CS1111.

printf(“%s of %s\n“,

“Summer“, “69“);

printf(“%s has %d students\n“,

“CS1111“, 87);

printf(“The value of PI“);

printf(“ is %f“, 3.1428);

printf(“%d“, 3);

printf(“ idiots“);

printf(“name = %s\n“, “Khan“);

printf(“age = %d\n“, 20);

printf(“height = %lf ft\n“, 5.8);

printf(“weight is 50kg\n“);

printf(“CS%d is %s course\n“,

1111, “foundational“);

printf(“Hexadecimal has \

%X symbols.\

\n“, 16);

printf(“32 1s is %x in hex \

and %o in octal\n“,

0xFFFFFFFF, 0xFFFFFFFF);

printf(“%cS%x\n“, ‘C‘, 4369);

We need to understand printf a little better.

This was output. Does C have a capability to take input?

We want this to be typed by the user.

Hello, which course is this?

1111

Hi! Welcome to CS1111.

We want this to be typed by the user.

printf(“Hello, which course is this?\n“);

​

What user enters here should appear below.

​

printf(“Hi! Welcome to CS%d\n“, ???);

We know how to print this.

How about this?

How does our brain remember?

It stores the information in memory cells.

Can we also do the same?

But which cell to access?

Hmm... Let's name the cells.

Hello, which course is this?

1111

Hi! Welcome to CS1111.

We want this to be typed by the user.

printf(“Hello, which course is this?\n“);

​

What user enters is stored in cell cell1.

​

printf(“Hi! Welcome to CS%d\n“, cell1);

We know how to print this.

How about this?

How does our brain remember?

It stores the information in memory cells.

Can we also do the same?

But which cell to access?

Hmm... Let's name the cells.

Where is this cell1 stored?

Inside your computer.

But where?

Well, in memory.

We call it random access memory.

Hello, which course is this?

1111

Hi! Welcome to CS1111.

We want this to be typed by the user.

printf(“Hello, which course is this?\n“);

​

What user enters is stored in cell cell1.

​

printf(“Hi! Welcome to CS%d\n“, cell1);

We know how to print this.

How about this?

Where is this cell1 stored?

Inside your computer.

But where?

Well, in memory.

We call it random access memory.

printf(“Hello, which course is this?\n“);

​

scanf(“%d“, cell1);

​

printf(“Hi! Welcome to CS%d\n“, cell1);

This code doesn't compile.

gcc says it doesn't know cell1.

Let's compile and run it.

Data

Algorithm

int cell1;

printf(“Hello, which course is this?\n“);

scanf(“%d“, cell1);

printf(“Hi! Welcome to CS%d\n“, cell1);

Hello, which course is this?

1111

Hi! Welcome to CS1111.

We want this to be typed by the user.

printf(“Hello, which course is this?\n“);

​

What user enters is stored in cell cell1.

​

printf(“Hi! Welcome to CS%d\n“, cell1);

We know how to print this.

How about this?

Where is this cell1 stored?

Inside your computer.

But where?

Well, in memory.

We call it random access memory.

printf(“Hello, which course is this?\n“);

​

scanf(“%d“, cell1);

​

printf(“Hi! Welcome to CS%d\n“, cell1);

​

​

Let's compile and run it.

int cell1;

printf(“Hello, which course is this?\n“);

scanf(“%d“, cell1);

printf(“Hi! Welcome to CS%d\n“, cell1);

Hello, which course is this?

1111

Segmentation fault (core dumped)

A New Problem

• Say you want to get your room/house painted.

​

• Which of the following whatsapp messages would help a painter reach and paint your house?
• Hi, my house color is white.
• Hi, my house is around 1500 sq ft.
• "Hi, I have %d members in my house.", 4
• Hi, my house address is 670, New Nandanvan.

Give some information but the painter can‘t change the color.

Allows the painter to go and change the color.

In addition, the painter can get the other information also!

Coming back to the old problem

​

​

​

​

​

​

​

• Hi, my house address is 670, New Nandanvan.

int cell1;

printf(“Hello, which course is this?\n“);

scanf(“%d“, cell1);

printf(“Hi! Welcome to CS%d\n“, cell1);

Informs scanf of the value in cell1, but does not allow scanf to change it.

We need to send address of cell1.

???

cell1

670, New Nandanvan

int cell1;

printf(“Hello, which course is this?\n“);

scanf(“%d“, 670, New Nandanvan);

printf(“Hi! Welcome to CS%d\n“, cell1);

Allows the painter to go and change the color.

In addition, the painter can get the other information also!

Coming back to the old problem

int cell1;

printf(“Hello, which course is this?\n“);

scanf(“%d“, cell1);

printf(“Hi! Welcome to CS%d\n“, cell1);

Informs scanf of the value in cell1, but does not allow scanf to change it.

We need to send address of cell1.

???

cell1

670, New Nandanvan

int cell1;

printf(“Hello, which course is this?\n“);

scanf(“%d“, 670, New Nandanvan);

printf(“Hi! Welcome to CS%d\n“, cell1);

???

cell1

670, RAM

int cell1;

printf(“Hello, which course is this?\n“);

scanf(“%d“, &cell1);

printf(“Hi! Welcome to CS%d\n“, cell1);

Note that printf is interested in the value, and not in changing it.

What if I

pass &cell1

to printf?

What if I

I pass cell1

to scanf?

Coming back to the old problem

???

cell1

670, RAM

int cell1;

printf(“Hello, which course is this?\n“);

scanf(“%d“, &cell1);

printf(“Hi! Welcome to CS%d\n“, cell1);

Hello, which course is this?

1111

Hi! Welcome to CS1111

Hello, which course is this?

6023

Hi! Welcome to CS6023

1111

cell1

670

6023

cell1

424

passed to scanf

passed to printf

syntactic sugar

address

value

Hello, which course is this?

CS1111

Hi! Welcome to CS0

0

cell1

35213944

int year;

scanf(“%d“, &year);

printf(“%s of %d\n“,

“Summer“, year);

int nstuds;

scanf(“%d“, &nstuds);

printf(“%s has %d students\n“,

“CS1111“, nstuds);

float pi;

printf(“Value of pi?“);

scanf(“%f“, &pi);

printf(“The value of PI“);

printf(“ is %f“, pi);

short nidiots;

scanf(“%d“, &nidiots);

printf(“%d“, nidiots);

printf(“ idiots“);

int age; double height;

scanf(“%d“, &age);

printf(“age = %d\n“, age);

scanf(“%lf“, &height);

printf(“height = %lf ft\n“, height);

printf(“weight is 50kg\n“);

long int course;

scanf(“%ld“, &course);

printf(“CS%d is %s course\n“,

course, “foundational“);

int base16;

scanf(“%x“, &base16);

printf(“Hexadecimal has \

%X symbols.\

\n“, base16);

int max32;

scanf(“%x“, &max32);

printf(“32 1s is %x in hex \

and %o in octal\n“,

max32, max32);

char c;

int num;

scanf(“%c“, &c);

scanf(“%d“, &num);

printf(“%cS%x\n“, c, num);

18

age

4315348

5.88

431534C

height

Problem: Find age from birth year.

// create cell birthyear

​

// take input from user in cell birthyear

​

// create cell age

​

// calculate (2022 – birthyear) and store in age.

​

// output age

​

Algorithm and Implementation

Comment

Assignment

int A, b, C;

A = 0;

b = 1 - A;

C = A - b;

float celcius, farenheit;

scanf("%f", &celcius);

farenheit = 9*celcius/5 + 32;

scanf("%d", &num);

mod5 = num % 5;

printf("%d mod 5 is %d\n",

num, mod5);

char c;

int num;

scanf(“%c“, &c);

scanf(“%d“, &num);

printf(“%cS%x\n“, c, num);

Modulus or remainder

// create cell birthyear

int birthyear;

// take input from user in cell birthyear

scanf("%d", &birthyear);

// create cell age

int age;

// calculate (2022 – birthyear) and store in age.

age = (2022 – birthyear);

// output age

printf("Your age is %d\n", age);

0

A

1

b

-1

C

Problem: Find your team number.

// create cell for last two digits of roll number

​

// take input from user in cell rollnumber

​

// create cell for team number

​

// find rollnumber / 10 + 1 and store in teamnumber

​

// output team number

​

Expect last two digits alone.

char c;

int num;

scanf(“%c“, &c);

scanf(“%d“, &num);

printf(“%cS%x\n“, c, num);

// create cell for last two digits of roll number

int rollnumber;

// take input from user in cell rollnumber

scanf("%d", &rollnumber);

// create cell for team number

int teamnumber;

// find rollnumber / 10 + 1 and store in teamnumber

teamnumber = (rollnumber / 10 + 1);

// output team number

printf("Your team is Team %d\n", teamnumber);

• Here is your replit team mapping.
• Team 1: Roll numbers CS21B001 – 10
• Team 2: Roll numbers CS21B011 – 20
• Team 9: Roll numbers CS21B081 -- 86

15

Your team is Team 2

78

Your team is Team 8

20

Your team is Team 3

Testing helps find bugs.

teamnumber = ((rollnumber - 1) / 10 + 1);

15

Your team is Team 2

78

Your team is Team 8

20

Your team is Team 2

Given a team number, find the first and the last roll numbers in that input team.

(assume 90 students)

Extent the program for full roll number.

Problem: Find team members.

char c;

int num;

scanf(“%c“, &c);

scanf(“%d“, &num);

printf(“%cS%x\n“, c, num);

// create data

char teamid;

int startroll, endroll;

// take input

scanf("%c", &teamid);

// find the roll numbers

startroll = (...);

endroll = startroll + 9;

// output

printf("%d to %d\n", startroll, endroll);

• Here is your replit team mapping.
• Team A: Roll numbers CS21B001 – 10
• Team B: Roll numbers CS21B011 – 20
• Team I : Roll numbers CS21B081 -- 90

A

1 to 10

I

81 to 90

C

21 to 30

Given a team id, find the first and the last roll numbers in that team.

(assume 90 students)

Characters are stored as integers.

‘A‘ is 65, ‘B‘ is 66, …

(teamid + 5) is allowed in C.

startroll = (teamid – 65) * 10 + 1;

As a good programming practice

• Avoid implicit type conversion.
• Avoid constants such as 65.

startroll = ((int)teamid – ‘A‘) * 10 + 1;

Problem: Find endsem percentage.

char c;

int num;

scanf(“%c“, &c);

scanf(“%d“, &num);

printf(“%cS%x\n“, c, num);

// create data

int labs, midsem, endsem;

// take input

scanf("%d%d", &labs, &midsem);

// do computation

endsem = (100 – (labs + midsem));

// output

printf("Endsem %% is %d\n", endsem);

• 56% labs + 15% midsem + remaining% endsem

​

endsem = (100 – labs – midsem);

​

int nonendsem;

nonendsem = labs + midsem;

endsem = 100 – nonendsem;

int remaining;

remaining = 100;

scanf(“%d“, &labs);

remaining = remaining – labs;

scanf(“%d“, &midsem);

remaining = remaining – midsem;

printf(“Endsem %% is %d\n“, remaining);

???

labs

???

midsem

???

remaining

100

56

44

15

29

56

15

29

Problem: Find sum.

int n;

// read n

scanf("%d", &n);

// compute sum

int sum = n * (n + 1) / 2;

// print sum

printf("Sum of first %d numbers is %d\n", n, sum);

char c;

int num;

scanf(“%c“, &c);

scanf(“%d“, &num);

printf(“%cS%x\n“, c, num);

Σn = 1 + 2 + 3 + ... + n

= n * (n + 1) / 2

Σ2ⁱ = 1 + 2 + 4 + 8 + ... n terms

= 2ⁿ – 1

Needs pow() function or a loop.

pow(x, y) returns x^y.

Does your code compile?

Problem: Find probability.

int nCards = 52;

int nKings = 4;

int nSpades = 13;

int nKingAndSpade = 1;

int nNonkingNonspade = nCards -

(nKings + nSpades - nKingAndSpade);

float prob = nNonkingNonspade / nCards;

printf("Probability of nonking, nonspade is %f\n",

prob);

char c;

int num;

scanf(“%c“, &c);

scanf(“%d“, &num);

printf(“%cS%x\n“, c, num);

A card is drawn at random from a deck of well-shuffled cards. Find the probability of it being neither a king nor a spade.

Find the issue

with this code.

Problem: Find the line.

float x1, y1, x2, y2;

scanf("%f%f%f%f", &x1, &y1, &x2, &y2);

float m = (y2 – y1) / (x2 – x1);

float c = (y1 – m*x1);

printf("Equation of the line is y = %.2fx + %.2f\n", m, c);

printf("Equation of the line is y = %gx + %g\n", m, c);

printf("Equation of the line is y = %.2ex + %.2e\n", m, c);

char c;

int num;

scanf(“%c“, &c);

scanf(“%d“, &num);

printf(“%cS%x\n“, c, num);

Given two points on a line, find its equation in y = mx + c format.

3.2 3 9.6 5

Equation of the line is y = 0.31x + 2.00

Equation of the line is y = 0.3125x + 2

Equation of the line is y = 3.12e-01x + 2.00e+00

Problem: Print tabular.

char name1[20], name2[20], name3[20];

int m11, m12, m13, m21, m22, m23, m31, m32, m33;

​

scanf("%s%d%d%d", &name1, &m11, &m12, &m13);

scanf("%s%d%d%d", &name2, &m21, &m22, &m23);

scanf("%s%d%d%d", &name3, &m31, &m32, &m33);

​

int t1, t2, t3;

t1 = m11 + m12 + m13;

t2 = m21 + m22 + m23;

t3 = m31 + m32 + m33;

​

printf("%-12s%5d%5d%5d = %5d\n", name1, m11, m12, m13, t1);

printf("%-12s%5d%5d%5d = %5d\n", name2, m21, m22, m23, t2);

printf("%-12s%5d%5d%5d = %5d\n", name3, m31, m32, m33, t3);

char c;

int num;

scanf(“%c“, &c);

scanf(“%d“, &num);

printf(“%cS%x\n“, c, num);

Read names and marks of three students and print the names and total in a table.

Rajesh 1 43 43 = 87

SomeshSingh 23 55 6 = 144

JK 21 21 21 = 63

char array or string

Future Connect:

Replacing &name1 with

name1 also works.

Homework Problem: Number game.

char c;

int num;

scanf(“%c“, &c);

scanf(“%d“, &num);

printf(“%cS%x\n“, c, num);

Choose a number from 0..9.

Multiply by 5.

Add 3 to it.

Double it up.

Choose another number from 0..9 and add to it.

​

Tell me the number. I will tell you both the chosen numbers.

Computer System

limbd.org

Your files reside here.

e.g., hello.c, a.out

printf

scanf

18

age

4315348

executes a.out

Hello World!

All C Keywords

www.programiz.com

Summary

• Hello World!
• Formatted input, output
• Problem Solving with assignments