Python Bootcamp�Day 2: Basic Syntax and Control Flow

Overview

• How do we represent data?�
• How do we make decisions?�
• How do we repeat commands?

Basic Syntax

Building Blocks

• Input�Getting data from outside world���
• Output�Displaying result of a program���
• Sequential Execution�Perform states one after another

• Conditional Execution�Check for certain conditions and execute or skip a set of instructions�
• Repeated Execution�Perform a set of instructions repeatedly until a condition is met�
• Reuse�Give a set of instructions a name and then reuse those instructions

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Values

• Every object in has a value.�
• Every value belongs to a certain type which determines the behavior and properties of that object.

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Expressions

An expression is a combination of values, variables, and operators.�

An operator is a symbol that represents a kind of computation.

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Every expression has a value when evaluated.�

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Variables

We can store the results of an expression into a variable by using the the assignment.

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Variable

A name or label that refers to stored data�(has a name, type, and value)�

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Statements

A statement is unit of code that Python can execute (ex. print, assignment).

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A program is a series of statements.

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A statement does not necessarily have a value.

Activity: Tracing

Trace the execution of the following code:

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# Program 1

x = 1

y = 2

print x, y

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x = 1

y = 1

print x, y

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# Program 2

x = 1

y = 2

z = x + y

print x, y, z

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x = y

y = x

x * y

print x, y, z

Containers

• Lists�A collection of values.�
• Tuples�A fixed set of values.�
• Dictionaries�A table of name, value pairs.�
• Sets�A collection that lets us know if a value is contained.

Activity: Computations

Write code that computes the following:

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• Area of a circle with radius of 5�
• Sum of the elements of the list [5, 7, 4]

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Bugs

• Syntax Errors�Violated the grammar or syntax rules of the programming language�
• Logic Errors�Program has correct syntax, but there is a mistake in the order or construction of statements�
• Semantic Errors�Program has correct syntax and is logically correct, but doesn’t solve the problem you intend to solve

Conditional Execution

Logical Expressions

A logical expression evaluates to a truth value, in essence True or False.

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Example

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>>> length = 4

>>> width = 2

>>> length < width

False

Logical Operators

== equal to�!= not equal to�>= greater than or equal to�> greater than�< less than�<= less than or equal to��and both must be true�or one or both must be true�not reverses the truth value

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Compare Two Objects

Glue Logical Expressions

Conditional Statements

We use conditional statements to check boolean expressions (which we call conditions), and change the behavior of the program accordingly.

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if condition: if n == 0:

statement(s) print ‘n is zero’

Alternative Execution

Alternative execution is when we have two possibilities and the condition determines which one gets executed.�

if condition:� statement(s) # Condition is True�else:

statement(s) # Condition is False

Activity: Range Check

Write code that checks if a number is between 5 and 10 (inclusive).

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• To read a number use the raw_input function�� number = int(raw_input('Number? '))��
• Print 'Yes!' if the number is in range, otherwise, print 'No!'

Chained Conditionals

When we want to check more than one condition, we can use chained conditionals.�Each condition is checked in the order that they appear; if one is triggered, then the rest are skipped.

if condition 1:� statements(s)�elif condition 2:� statements(s)�elif condition 3:� statements(s)�else:� statements(s)

Nested Conditionals

Conditional statements can be nested or placed inside another conditional statement.

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Example

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if a > b:

if a < c:

print ‘a is between b and c’

Activity: Odd and Divisible

Write code that checks if the number entered in by the user is both odd and divisible by three.

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• You can get the remainder of a division operation by using the modulus operator %�� 4 % 2 == 2�
• Print 'Yes!' if the number is meets both conditions, otherwise, print 'No!'

Iteration

While: Overview

We can use a while statement to repeatedly execute a series of tasks (i.e. a loop).

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Syntax Example

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while condition: n = 0

statement(s) while n < 10:

print n

n = n + 1

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While: Control Flow

• Evaluate condition (which is a boolean expression).�
• If condition is False, then exit loop and continue program.�
• If condition is True, then execute statements in body and then go to step 1.

While: Notes

• Each time through the body of a loop is called an iteration.�
• The condition normally contains a variable that changes within the body of the loop so that we can eventually exit the loop.�
• If a loop never exits, then this is called an infinite loop.

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Activity: Debugging

The following code is suppose to sum all the numbers between 0 and 9 (inclusive). Unfortunately, it has bugs. Correct these errors.

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n = 0

total = 0

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while n < 9:

total + n

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print total

Activity: Loop Exercises

• Find the smallest item in a list of numbers.�
• Simulate rolling a die until we reach a 5.

For: Overview

Iterating through a definite set of objects is a common looping pattern, so Python provides the for statement.

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Syntax Example

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for item in dataset: for i in [0, 1, 2, 3]:

statement(s) print i

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For: Control Flow

• Extract one element from the dataset.�
• Execute the body of for loop with item bound to the element.�
• Go to step 1.

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For: Range

We can generate a list of numbers using the range function:

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>>> range(0, 3)

[0, 1, 2]

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>>> for i in range(0, 3):� print i

0

1

2

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For: Enumerate

We can generate a list of pairs consisting of the index and the item using enumerate:

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>>> list(enumerate([‘a’, ‘b’, ‘c’]))

[(0, 'a'), (1, 'b'), (2, 'c')]

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>>> for index, letter in enumerate(‘abc’):

print index, letter

0 a

1 b

2 c

Activity: Loop Exercises

• Find the smallest item in a list of numbers.�
• Simulate rolling a die until we reach a 5.

Break

We can exit a loop immediately by using the break statement

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import random

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while True:

r = random.randint(0, 10)

if r == 0:

break

print r

print r

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Continue

We can skip the rest of the loop body and move on to the next iteration by using continue

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while True:

r = random.randint(0, 10)

if r == 0:

break

if r % 2:

continue

print '{0} is even'.format(r)

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Activity: Guessing Game

Write code that generates a random number and asks the user to guess what it is.

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Guess the number: 1� Higher!

Guess the number: 3� Lower!

Guess the number: 2� Hooray, you got it!�

• If the guess is too low, tell the user to guess 'Higher!'�
• If the guess is too high, tell the user to guess 'Lower!'�
• End the program when the user has found the number