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Recap: Arrays

A collection of elements all of which have the same data type

Each array element is accessed using the array index (integer-valued)

For the above example, marks[0], marks[2], marks[499], marks[int_expr] where int_expr is integer-valued expression such that 0 <= int_expr <= 499

float marks[500];

marks[0]

marks[1]

marks[2]

marks[499]

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Recap: Array: Declaration and Initialization

Can be initialized at time of declaration itself

Can be partly initialized as well

Over initialization may crash

Better way to initialize is the following

Warning: uninitialized arrays contain garbage, not zeros

int a[6] = {3,7,6,2,1,0};

int a[6] = {3,7,6};

int a[6] = {1,2,3,4,5,6,7,8,9};

int a[] = {1,2,3,4,5,6,7,8,9};

I will figure out how much space needed

No need to specify the array size during declaration

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Array: Declaration and Initialization

Can declare the array first and initialize its elements later
The later initialization can be done using user-provided values (e.g., using scanf), or some expression, or using some fixed values

int i,tmp,a[5];

for(i=0;i<5;i++){

scanf(“%d”,&tmp);

a[i] = tmp;

}

Read a use-provided value

Assign the read value to the i^th element of the array

Can I run the loop as for(i=1;i<=5;i++)?

Yes, if you use a[i-1] = tmp; in loop’s body

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Array: Declaration and Initialization

Can declare the array first and initialize its elements later
The later initialization can be done using user-provided values (e.g., using scanf), or some expression, or using some fixed value

int i,a[5];

for(i=0;i<5;i++){

scanf(“%d”,&a[i]);

}

Directly read a user provided value into the i^th element of the array (the tmp variable is not needed)

Note: &a[i] is evaluated as &(a[i]) since [] has higher precedence than &

A shortcut for reading user provided values

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Operator Name	Symbol/Sign	Associativity
Brackets, array subscript, Post increment/decrement	(), [] ++, --	Left
Unary negation, Pre increment/decrement, NOT	-, ++, --, !	Right
Multiplication/division/ remainder	*, /, %	Left
Addition/subtraction	+, -	Left
Relational	<, <=, >, >=	Left
Relational	==, !=	Left
AND	&&	Left
OR	\|\|	Left
Conditional	? :	Right
Assignment, Compound assignment	=, +=, -=, *=, /=, %=	Right

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Array: Declaration and Initialization

Can declare the array first and initialize its elements later
The later initialization can be done using user-provided values (e.g., using scanf), or some expression, or using some fixed value

int i,a[5];

for(i=0;i<5;i++){

a[i] = i+1;

}

Assign a value of expression i+1 to the i^th element of the array

a[0]

a[1]

a[2]

a[3]

a[4]

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Array: Declaration and Initialization

Can declare the array first and initialize its elements later
The later initialization can be done using user-provided values (e.g., using scanf), or some expression, or using some fixed value

Assign a fixed (constant) value 10 to the i^th element of the array

int i,a[5];

for(i=0;i<5;i++){

a[i] = 10;

}

a[0]

a[1]

a[2]

a[3]

a[4]

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Tracing the execution of an array based program

include <stdio.h>

int main () {

int a[5];

int i;

for (i=0; i < 5; i= i+1) {

a[i] = i+1;

}

return 0;

}

a[0] a[1] a[2] a[3] a[4]

Let us trace the

execution of the program.

Statement becomes a[0] =0+1;

Statement becomes a[1] =1+1;

Statement becomes a[2] =2+1;

Statement becomes a[3] = 3+1;

Statement becomes a[4] = 4+1;

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Arrays: Some Example Programs

Create an integer array of size 100
Initialize elements with even index as 0
Initialize elements with odd index as 1

int i,a[100];

for(i=0; i<100; i=i+1){

if(i%2==0) a[i] = 0;

else a[i] = 1;

}

Method 1

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Arrays: Some Example Programs

Create an integer array of size 100
Initialize elements with even index as 0
Initialize elements with odd index as 1

int i,a[100];

for(i=0; i<100; i=i+2){

a[i] = 0;

a[i+1] = 1;

}

Method 2, without if-else

Incrementing the loop counter by 2

This for loop will run 50 times. Each iteration will assign values to 2 elements, one at odd index, one at even index

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Arrays: Some Example Programs

Check whether a sequence of numbers is a palindrome sequence

Greek origin word:

palin = again,

dromos = direction

Palindrome: Forward and

Reverse gives the same

sequence

Some palindromes:

1 2 3 4 5 4 3 2 1

1 2 3 3 2 1

Some non-palindromes:

1 2 3 4 5

1 2 3 3 4 1

9 0 4 0 8

int main(){

int a[100], temp, len = 0, i, flag = 1;

while(1){

scanf("%d", &temp);

if(temp == -1)

break;

a[len++] = temp;

}

for(i = 0; i < len; i++)

if(a[i] != a[len-i-1])

flag = 0;

if(flag) printf("YES");

else printf("NO");

return 0;

}

The while(1) loop keeps reading numbers until user enters -1, store each number as an element of the array named a

After the while(1) loop exits, len is the size of the array (indices are 0 to len-1)

This line does a[len] = temp; and then increments len

a[0]

a[1]

a[2]

a[len-1]

a[len-2]

Compares a[0] with a[len-1], then a[1] with a[len-2], and so on. If any pair does not match, set flag variable to 0

flag = 1 assumes that sequence is palindrome (set 0 if later found otherwise)

Let’s specify a maximum sequence size

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Arrays: Some Example Programs

#include <stdio.h>

int main() {

char s[100];

int count = 0;

int ch;

int i;

return 0;

}

ch = getchar();

while ( ch != EOF && count < 100) {

s[count] = ch;

count = count + 1;

ch = getchar();

}

i = count-1;

while (i >=0) {

putchar(s[i]);

i=i-1;

}

/*print_in_reverse */

/*read_into_array */

/* the array of 100 char */

/* counts number of input chars read */

/* current character read */

/* index for printing array backwards */

Read until user has

entered 100 chars or

the end-of-file (EOF)

special character

has been read.

Now print the characters

in reverse order

getchar() returns a single character entered by the user

putchar() prints a single character