Flow Control

Intro

Up until now, all the code we've written was executed in top-down order. Sometimes it's necessary to change the way a program flows, for example, making a program that can decide weather or not to run a piece of code.

In Python, we have 3 flow control structures:

1. If statement
2. For loop
3. While loop

If

• If statements are Python's decision making structure.
• The decisions are made by checking the truth value of a condition.
• The blocks of code that are inside an If structure are only run if the condition is True.

If Cheat Table

Operator	Meaning
==	equality test
!=	inequality test
>	greater than
<	less than
>=	greater than or equal to
<=	less than or equal to
and	logical and
or	logical or
not	logical not
True	logical true
False	logical false

Usually If structures are followed by an optional Else clause. The code inside the Else clause is only run if the condition in its corresponding If structure was False.

Example

day = int(input("Weekday? (1-7) "))
if day == 1 or day == 7:  # You can test multiple condition at the same time
    print("Weekend")

else:
    print("Work day")

• If structures can be nested indefinitely (If structures inside If structures).
• We can also make use of Elif structures. These follow either another If or Elif structure and are just like an Else clause but with their own condition to test added.

Example

#!/usr/bin/env python3

day = int(input("Weekday? (1-7) "))

if (day == 1):  # Parentheses are optional
    print("Sunday")

elif day == 2:
    print("Monday")

elif day == 3:
    print("Tuesday")

elif day == 4:
    print("Wednesday")

elif day == 5:
    print("Thursday")

elif day == 6:
    print("Friday")

elif day == 7:
    print("Saturday")

else:
    print("Number out of the expected range (1-7)")

Example

number = 123

guess = int(input("Your guess (tries left = 3)? "))
if number == guess:
    print("You got it right with 1 try!!")

else:
    guess = int(input("Your guess (tries left = 2)? "))
    if number == guess:
        print("You got it right with 2 tries!!")

    else:
        guess = int(input("Your guess (tries left = 1)? "))
        if number == guess:
            print("You got it right with 3 tries!!")

        else:
            print("You didn't manage to guess the number")

For

The For loop is one of the two loops available in Python. We use this loop when we want to repeat a code block a known, finite, number of times.
The For loop makes heavy use of the range object.

• A range is defined as follows: range(start, [end, [step]]).
• The interval is closed on the left side and open on the right side [start, end).
• The step parameter is optional and by default is 1.
• We can also create range objects with only 1 parameter: range(3). These are the same as: range(0, 3).

Another structure of a For loop will be presented in Iteration.

Example

number = 123

for i in range(3):
    print(3 - i, "guess(es) left")
    guess = int(input("Your guess? "))

    if number == guess:
        print("You got it right!!")
        # We use the break keyword to end a loop early
        break

    else:
        print("Better luck next time.")
        # We use the continue keyword to go straight to the next loop iteration
        # (We just wanted to show it in this case)
        continue

if number != guess:
    print("You didn't manage to guess the number.")

The For loop can be followed by an Else clause. The block of code inside the Else clause is executed once after the For loop is over, unless we reach a break keyword inside the For loop.

Example

number = 123

for i in range(3):
    print(3 - i, "guess(es) left")
    guess = int(input("Your guess? "))

    if number == guess:
        print("You got it right!!")
        # We use the break keyword to end a loop early
        break

    else:
        print("Better luck next time.")

else:
    print("You didn't manage to guess the number.")

While

While is the second and last available loop in Python. We use this loop when we want to repeat a code for an unknown amount of times (while a condition is True).

Example

number = 123
choice = "yes"

guess = int(input("Your guess? "))

# Parantheses are optional
# The tested condition works the same as in the If structure
while (number != guess) and (choice == "yes"):
    choice = input("You didn't get it right, would you like to try again? (yes/no) ")
    guess = int(input("Your guess? "))

if choice == "yes":
    print("You got it right!!")

else:
    print("Better luck next time.")

Sections

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Next: Data Structures