control-flow.md

Control Flow and Conditional Statements

if

If-statements are used to execute a block of code on when a particular condition has been met.

my_bool = True
if my_bool:
    print("The condition is True.")

Output:

The condition is True.

We may put mathematical expressions in place of booleans, as these will evaluate to True or False.

num = 16
if num > 10:
    print(str(num) + " is greater than 10.")

Output:

16 is greater than 10.

Try running the code below:

if False:
    print("What will happen now?")

Output:

This time nothing happens, because our boolean is False.

Like in the case of loops, we have to indent a block of code to let Python know that this code should only be run the condition in our if-statement is True.

if False:
    print("This won't be printed.")
print("This will be printed because it's outside our if-statement.")

Output:

This will be printed because it's outside our if-statement.

Further information on using mathematical operators in Python can be found here.

if...else

An if-else statement allows us to run different code depending on whether or not a condition is True or False. This means that at least one of the blocks of code in or if-else statement will be run no matter what.

if False:
    print("This won't be run.")
else:
    print("This will be run, because our if-condition is False.")

Output

This will be run, because our if-condition is False.

On the flip side, when our first condition is True, the code in the else-block is skipped.

if True:
    print("This will be run, because our if-condition is True.")
else:
    print("This won't be run, because our if-condition is True.")

Output

This will be run, because our if-condition is True.

if-else statements allow us to account for both the True and False conditions for a boolean.

elif

The elif keyword allows us to check if multiple conditions are True. If our first condition is False, we can move to check if another set of conditions are True by using elif. Consider the example below:

age = 25

if age < 13:
    print("You are a child")
elif age < 20:
    print("You are a teenager")
elif age < 30:
    print("You are in your twenties (Vicenarian)")
elif age < 40:
    print("You are in your thirties (Tricenarian)")
elif age < 50:
    print("You are in your forties")
elif age < 60:
    print("You are in your fifties")
else:
    print("You are a senior citizen")

Output:

You are in your twenties (Vicenarian)

break and continue

break and continue are two words that allow us to alter the flow of loops.

break

break terminates a loop immediately. Take the example below:

animals = ["dog", "cat", "bird", "pig", "elephant"]

for animal in animals:
    if animal == "pig":
        break
    print(animal)

Output:

dog
cat
bird

Because the loop was "broken" upon reaching the element "pig", we only see the first three items printed.

continue

The continue keyword allows us to "skip" a specific iteration of a loop and move on to the next iteration without executing the remaining code within that code block.

animals = ["dog", "cat", "bird", "pig", "elephant"]

for animal in animals:
    if animal == "pig":
        continue
    print(animal)

Output:

dog
cat
bird
elephant

Summary

• if Statements
  • Used to execute a block of code when a specific condition is met.
  • Can handle mathematical expressions that evaluate to True or False.
  • Code blocks within if statements must be indented.
• if...else Statements
  • Executes different code based on whether a condition is True or False.
  • At least one block of code will be executed.
• elif Statements
  • Checks multiple conditions sequentially.
  • If the first condition is False, subsequent conditions are evaluated using elif.
• break Statement
  • Terminates the loop immediately upon encountering it.
• continue Statement
  • Skips the current iteration of the loop and moves to the next iteration.

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