Control Statements #

I’ve got my own mind. Wanna make my own decisions. When it has to do with my life. I wanna be the one in control. —Janet Jackson, Control

Controlling Puppet

Conditional Statements #

It is very common for programs to execute statements based on some conditions. In this section we will learn about Python’s if, else, and elif statements.

Relational operators #

Relational operators are used to compare values. It either returns True or False according to the condition.

Operator Meaning Example

> Greater than - True if left operand is greater than the right x > y

< Less than - True if left operand is less than the right x < y

== Equal to - True if both operands are equal x == y

!= Not equal to - True if operands are not equal x != y

>= Greater than or equal to - True if left operand is greater than or equal to the right x >= y

<= Less than or equal to - True if left operand is less than or equal to the right x <= y

=======

Operator	Meaning	Example
>	Greater than - True if left operand is greater than the right	x > y
<	Less than - True if left operand is less than the right	x < y
==	Equal to - True if both operands are equal	x == y
!=	Not equal to - True if operands are not equal	x != y
>=	Greater than or equal to - True if left operand is greater than or equal to the right	x >= y
<=	Less than or equal to - True if left operand is less than or equal to the right	x <= y
=======

The if statement #

The if statement evaluates a boolean condition. If the results of the evaluation is True, the code block following the if statement is executed. If the evaluation is False, the code block is not executed.

# control.py
today = "Tuesday"

if today == "Tuesday":
  print("We get to write code!")

Create a file using the code above and run the command python control.py in your terminal.

$ python control.py
We get to write code!

Adding else #

The else statement only works when following an if statement and the block code following the else gets executed only when the if statement evaluates to False.

# control.py (modified)
today = "Wednesday"

if today == "Tuesday":
  print("We get to write code!")
else:
  print("Time to learn Scrum!")

Running the modified file, should provide this output:

$ python control.py
Time to learn Scrum!

The elif statement #

Similar to the else statement, an elif statement only works when following an if statement but the elif allows you to evaluate another boolean condition. If the elif statement evaluates to True the code block following will be executed.

Membership operators #

Keywords in and not in are membership operators in Python. They are used to test whether a value or variable is found in a container (string, list, or dictionary). In a dictionary we can only test for presence of key, not the value.

Operator Meaning Example

in True if value/variable is found in the sequence 5 in x

not in True if value/variable is not found in the sequence 5 not in x

=======

Operator	Meaning	Example
in	True if value/variable is found in the sequence	5 in x
not in	True if value/variable is not found in the sequence	5 not in x
=======

# control.py (modified)
today = "Saturday"
code_days = ["Tuesday", "Thursday"]
scrum_days = ["Monday", "Wednesday", "Friday"]

if today in code_days:
  print("We get to write code!")
elif today in scrum_days:
  print("Time to learn Scrum!")
else:
  print("Time to relax!")

Make the above changes to the control.py file and run it in the terminal.

$ python control.py
Time to relax!

Let’s use the if-elif-else statement combination to help with assigning grades.

# letter_grade.py
num = int(input("Enter your numeric grade: "))
if num >= 90:
  letter_grade = 'A'
elif num >= 80:
  letter_grade = 'B'
elif num >= 70:
  letter_grade = 'C'
elif num >= 60:
  letter_grade = 'D'
else:
  letter_grade = 'F'

print("Your letter grade is " + letter_grade )

For Loops #

Thus far, we have limited ourselves to short sequences of instructions that are executed one after the other. Next we will introduce repetition statements which repeat an action. Each repetition of the action is known as a pass or an iteration.

The Python for loop is a control statement that supports a definite number of iterations.

# for_loop.py
weekdays = ["Monday", "Tuesday", "Wednesday", "Thursday", "Friday"]

for day in weekdays:
  print(day)

for loops are also great for iterating over numbers and making calculations.

# avg_loop.py
l = [5, 6, 10, 20]

sum = 0
for num in l:
    sum += num

avg = sum / len(l)
print(avg)

While Loops #

The for loop executes for a definite number of iterations. In some situations, the number of iterations needed is not predictable. The logic needed requires a condition to be met before completing its work.

# blackjack.py
import random

card_values = [2, 3, 4, 5, 6, 7, 8, 9, 10, 10, 10, 10, 11]
sum = 0
turn = "hit"
while turn == "hit":
  sum += random.choice(card_values)
  print("You currently have: " + str(sum))
  turn = input("What do you want to do? ")

if sum >= 21:
  print("You went over 21!")
else:
  print("You stopped at: " + str(sum))

As a final note on while loops, the command break can also be used to exit a loop.

Nested Loops #

Loops can be nested in that you can put any type of loop inside of any other type of loop. For example a for loop can be inside a while loop or vice versa. Let’s expand the the simple Blackjack code such that it allows us to play multiple games.

# blackjack.py (modified)
import random

card_values = [2, 3, 4, 5, 6, 7, 8, 9, 10, 10, 10, 10, 11]
play_again = 'yes'
while play_again == 'yes':
  sum = 0
  turn = "hit"
  while turn == "hit":
    sum += random.choice(card_values)
    print("You currently have: " + str(sum))
    turn = input("What do you want to do? ")

  if sum >= 21:
    print("You went over 21!")
  else:
    print("You stopped at: " + str(sum))

  play_again = input("Want to play again? ")