Lecture Notes 07: Conditionals and Operators

The IF statement

An If block is a piece of code that will be executed only when a condition (a boolean expression) is evaluated as True.

This is what it looks like:

if <condition> :
    <Block To Execute>

• The Keyword if
• a condition: expression that evaluates to True or False
• A colon (  :  )
• The block of code that is executed if the condition is true

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# before the if
x = 2

# if statement
if x > 0:
  print ('positive') # if body

# after the if
print ('done')

Comparison Operators

• \(<\): less than   example: x < 2
• \(<=\): less or equal than   example: x <= 2
• \(>\): greater than   example: x > 2
• \(>=\): greater or equal than   example: x >= 2

Equality

# before the if
x = 2

# if statement
if x == 2:
  print ('it is a two') # if body

# after the if
print ('done')

Difference

# before the if
x = 2

# if statement
if x != 0:
    print ('x is not zero')

# after the if
print ('done')

Complex expressions

# before the if
x = 6

# complex condition
if x%2 == 0:
    print ('x is divisible by 2')

# after the if
print ('done')

The IF-ELSE statement

In the example above, we have three options that can be executed.

The nature of the problem makes it so that one of them always is (a number has to be <0, 0, or >= than 0).

However, there are problems where a sequence of if statements might not be ideal, because there is always a default option: none of them get executed!

For example:

# before the if
x = 2
y = 2

# if statement
if x < y:
  print ('x is smallest')

# another (independent) if statement
if x > y:
  print ('y is smallest')

# after the if
print ('done')

if <condition> :
    <Block To Execute>
else:
    <Alternative Block To Execute>

# before the if
x = 2
y = 2

# if-else statement
if x < y:
  print ('x is smallest')
else:
  print ('y is smaller or equal than x')

# after the if
print ('done')

The IF-ELIF-ELSE statement

What happens when we want alternative paths, each with its own condition?

It turns out that you can combine IF and IF-ELSE statements to do anything, but to make things easier,
Python provides the if-elif-else block:

if <condition> :
    <Block To Execute>
elif <condition>:
    <Another Block To Execute>
else:
    <Default Block To Execute>

# before the if
x = 2
y = 2

# if-elif-else statement
if x < y:
    print ('x is smallest')
elif x > y:
    print ('y is smallest')
else:
    print ('x and y are equal')

# after the if
print ('done')

Scope and IF-blocks

A variable's scope is that of the innermost function, class or module in which they're assigned.

The IF/ELIF/ELSE block does not establish its own scope (This is different than in Java or C!).

Example:

x = -2

if x<0:
  y = x+1
  print('y is the next number:',y)
    
print(x)
print(y)

Cascading If statements

Sometimes, we want sub-conditions to be true before we perform an action.

Think of the following situation:

A professor issues two assignments X and Y.
And the rules for grading are:

1. If students flunk X (a passing grade is 60), they need a grade of 80 or above in Y to pass the class (C+)
.
2. If students pass X, and if they also get 80 or above in Y, students get a final grade of A (otherwise C+).

# before the if
x = 6

# a "nested" conditional
if x%2 == 0:
    print ('x is divisible by 2')
    if x%3 == 0:
        print ('x is divisible by 6')

# after the if
print ('done')

Forgotten Expressions

I forgot to show you some "shorthand" for some arithmetic operations:

• += is an increment operator.
  Example:
  Try this example in the Python Tutor
• -= is a decrement operator.
  Example:
  Try this example in the Python Tutor

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