If, For & While¶

So far, programs run from top to bottom, one line at a time. Control flow changes that: if makes decisions, and for and while repeat things.

if, elif, else¶

An if statement runs its indented code only when a condition is True:

fruit = "apple"

if fruit == "apple":
    print("Apples are red")

Add elif ("else if") to check another condition when the first one was false, and else to catch everything left over. Python checks them from top to bottom and runs only the first one that matches:

fruit = "grape"

if fruit == "apple":
    print("Apples are red")
elif fruit == "grape":
    print("Grapes are purple")      # this one runs
else:
    print("Does not compute")

Comparisons¶

Operator	Means
`==`	equal to
`!=`	not equal to
`<` `>`	less than, greater than
`<=` `>=`	less than or equal, greater than or equal

Combine conditions with and, or and not:

if temperature > 30 and not fan_on:
    print("Turning the fan on")

for loops¶

A for loop runs its code once for every item in a list, or for every number in a range.

fruits = ["apple", "grape", "banana"]

for fruit in fruits:
    print(fruit)

fruit is a new variable that holds the current item each time round the loop. You can call it anything.

range() gives you a sequence of numbers:

for x in range(5):             # 0, 1, 2, 3, 4
    print(x)

for x in range(100, 1000, 50): # 100, 150, 200, ... 950
    print(x)

range() stops before the end number

range(5) counts 0 to 4, not 0 to 5. And range(100, 1000, 50) starts at 100, goes up by 50, and stops before reaching 1000.

Loops and if statements work together:

for fruit in ["apple", "grape", "banana", "pineapple"]:
    if len(fruit) > 5:
        print(fruit + " has more than 5 letters")
    else:
        print(fruit + " has 5 or fewer letters")

while loops¶

A while loop keeps running as long as its condition stays True:

x = 10
while x > 0:
    print(x)
    x -= 1          # count down; without this the loop would never end
print("Liftoff!")

while True: the main loop¶

Almost every microcontroller program has a while True: loop. True is always true, so it runs forever: check the buttons, update the screen, check the buttons again, and so on, until the power goes off.

while True:
    if button.was_pressed():
        print("Pressed!")

break¶

break jumps straight out of a loop, even a while True: one:

count = 0
while True:
    count += 1
    if count == 5:
        break
print("Stopped at", count)

Stuck in a loop?

If a program runs forever when it shouldn't, press Stop in Thonny (Ctrl+F2), or press Ctrl+C in the Shell.

Next: Functions