Loops

1. Why bother with loops?

Imagine you want to print “Hello” five times. You could write it out five times:

print("Hello")
print("Hello")
print("Hello")
print("Hello")
print("Hello")

But what about 100 times? Or a million? That is exactly what loops are for: say it once, repeat it as many times as you like. There are two kinds, and the trick is knowing which to reach for.

• for loop: use it when you know how many times (or you have a list of things to go through).
• while loop: use it when you don’t know how many times, and you just want to keep going until something becomes true.

2. The for loop

A for loop repeats its block once for each item it is given. The indented lines underneath are the body, the part that repeats.

for i in range(5):
    print("Hello")

That prints “Hello” five times. Two things to notice, exactly like with if:

• The first line ends with a colon :
• The repeating part is indented (4 spaces).

3. Counting with range

range makes a sequence of numbers for the loop to walk through. The variable (often i) holds the current number each time around:

for i in range(1, 4):
    print(i)

# prints 1, then 2, then 3  (4 is the stopping point, not included)

You can also count backwards with a step of -1, handy for a countdown:

for i in range(5, 0, -1):
    print(i)

# prints 5, 4, 3, 2, 1

4. Looping through things

A for loop can also walk through the letters of a word (or items in a list), no numbers needed:

for letter in "cat":
    print(letter)

# prints c, then a, then t

It reads almost like English: “for each letter in cat, print the letter.”

5. The while loop

A while loop keeps repeating as long as a condition is True. Use it when you don’t know how many goes it will take, like asking for a password until the user gets it right:

password = ""

while password != "swordfish":
    password = input("Password? ")

print("Welcome in!")

It checks the condition, runs the body, then checks again, over and over, until the condition is finally False. Here it keeps asking while the password is wrong, and only moves on once it’s right.

6. Watch out: infinite loops

A while loop only stops when its condition becomes False. If nothing inside the loop ever changes that condition, it runs forever, an infinite loop:

count = 0

while count < 5:
    print(count)
    # oops! count never changes, so this never stops

The fix is to make sure something in the loop moves you towards the exit; here, adding count = count + 1 so count eventually reaches 5:

count = 0

while count < 5:
    print(count)
    count = count + 1   # now it counts up and stops at 5

Golden rule: every while loop needs a way to end. Before you run it, ask yourself: “what will eventually make this condition False?”

7. Quick check Part 1 of 3

Five quick questions. This is half of completing the module. The two coding tasks below make up the other half.