Conditionals

Conditionals#

  • if

  • elif

  • else

Conditionals: if#

Conditionals are statements that check for a condition, using the if statement, and then only execute a set of code if the condition evaluates as True. 
condition = True

if condition:
    print('This code executes if the condition evaluates as True.')

# equivalent to above
if condition == True:
    print('This code executes if the condition evaluates as True.')

Class Question #1#

Replace --- below with something that will print ‘True’

  • A) I did it!

  • B) I think I did it!

  • C) I tried but am stuck.

  • D) I’m unsure where to start.

math = ---

if math:
    print('True')

Conditional: else#

After an if, you can use an else that will run if the conditional(s) above have not run. 
condition = False

if condition:
    print('This code executes if the condition evaluates as True.')
else: 
    print('This code executes if the condition evaluates as False')

Class Question #2#

Replace --- below with something that will print ‘False’.

  • A) I did it!

  • B) I think I did it!

  • C) I tried but am stuck.

  • D) I’m unsure where to start.

my_value = ---

if my_value:
    print('True')
else: 
    print('False')

Conditional: elif#

After an if statement, you can have any number of elif`s (meaning 'else if') to check other conditions. 
condition_1 = False
condition_2 = False

if condition_1:
    print('This code executes if condition_1 evaluates as True.')
elif condition_2:
    print('This code executes if condition_1 did not evaluate as True, but condition_2 does.')
else: 
    print('This code executes if both condition_1 and condition_2 evaluate as False')

elif without an else#

An else statement is not required, but if both the if and the elif condtions are not met (both evaluate as False), then nothing is returned.

condition_1 = False
condition_2 = False

if condition_1:
    print('This code executes if condition_1 evaluates as True.')
elif condition_2:
    print('This code executes if condition_1 did not evaluate as True, but condition_2 does.')

elif after an else does not make sense#

The order will always be if-elif-else…with only the if being required. If the elif is at the end…it will never be tested, as the else will have already returned a value once reached (and thus Python will throw an error).

## THIS CODE WILL PRODUCE AN ERROR
condition_1 = False
condition_2 = False

if condition_1:
    print('This code executes if condition_1 evaluates as True.')
else: 
    print('This code executes if both condition_1 and condition_2 evaluate as False')
elif condition_2:
    print('This code executes if condition_1 did not evaluate as True, but condition_2 does.')

Conditionals With Value Comparisons#

Any expression that can be evaluated as a boolean, such as value comparisons, can be used with conditionals. 
speed_limit = 65
speed = 85

if speed > speed_limit:
    ticket = True
else:
    ticket = False
    
print(ticket)

# string value comparisons also possible
a1 = 'not started'

if a1 == 'completed':
    action = 'grade'
elif a1 == 'in progress':
    action = 'wait'
elif a1 == 'not started':
    action = 'incomplete'
else:
    action = 'uncertain'
    
print(action)

Class Question #3#

What will the following code snippet print out:

my_condition = False

if my_condition:
    print("John")
elif not my_condition:
    print("Paul")
elif not my_condition:
    print("George")
else:
    print("Ringo")

  • A) John

  • B) Paul, George, Ringo

  • C) Paul

  • D) Paul, George

  • E) Ringo

Class Question #4#

What will the following code snippet print out:

if 1 + 1 == 2:
    print("I did Math")
elif 1/0:
    print("I broke Math")
else:
    print("I didn't do math")

  • A) I did Math

  • B) I broke Math

  • C) I didn’t do math

  • D) This code won’t execute

Class Question #5#

What will the following code snippet print out:

conditional = True
python = "fantastic"

if conditional:
    if python == "great":
        print("Yay Python!")
    else:
        print("Oh no.")
else:
    print("I'm here.")

  • A) Yay Python!

  • B) Oh no.

  • C) I’m here.

  • D) This code won’t execute

Properties of conditionals#

  • All conditionals start with an if, can have an optional and variable number of elif’s and an optional else statement

  • Conditionals can take any expression that can be evaluated as True or False.

  • At most one component (if / elif / else) of a conditional will run

  • The order of conditional blocks is always if then elif(s) then else

  • Code is only ever executed if the condition is met

Tying things together:#

Let’s create a function that calculates grades…

Code Style: Conditionals#

  • avoid single line statements

  • a single conditional (if + elif + else) has no additional line spaces between statements

Conditionals: Good Code Style#

value = 16

if value % 2 == 0:
    out = "even"
else:
    out = "odd"

Conditionals: Code Style to Avoid#

if value%2==0:out="even" # avoid statement on same line as conditional
                         # avoid blank line between if and else
else:out="odd"           # don't forget about spacing around operators!

Functions + Conditionals#

Let’s define a more interesting function than what we did in the functions lecture. Here we are using conditionals within our function.

# Determine if a value is even or odd
def even_odd(value): 

    if value % 2 == 0: 
        out = "even"
    else: 
        out = "odd"
        
    return out 

# Execute our function to check that
# it is working according to our expectations
even_odd(2)
Contents