for loops

A for loop goes through the items of a collection one at a time. It is the loop you will use most in Python.

fruits = ["apple", "banana", "cherry"]
for fruit in fruits:
    print("I like", fruit)

Each time round, the loop variable (fruit) refers to the next item. When the items run out, the loop ends; there is no counter to manage.

Looping over a string

A string is a sequence of characters, so you can loop over it too:

for letter in "Python":
    print(letter)

Keeping a running total

prices = [120, 45, 300, 15]
total = 0
for price in prices:
    total += price
print("Total:", total)

Counting matches

words = ["apple", "avocado", "banana", "apricot", "cherry"]
count = 0
for word in words:
    if word.startswith("a"):
        count += 1
print(count, "words start with 'a'")

enumerate(): the position and the item

When you need the index as well as the value, use enumerate():

players = ["Asha", "Leo", "Mei"]
for position, name in enumerate(players, start=1):
    print(f"{position}. {name}")

This is much cleaner than keeping your own counter.

zip(): two lists side by side

zip() pairs up items from several collections:

names = ["Asha", "Leo", "Mei"]
scores = [92, 78, 85]
for name, score in zip(names, scores):
    print(f"{name}: {score}")

zip stops when the shortest collection runs out.

for vs while

• Use for when you are going through a collection or a known number of steps.
• Use while when you repeat until something happens and don't know how many times that will take.