Loops: for and while

Why loops

Suppose you want to print a grade for each of 40 students. Writing 40 print lines is slow, error-prone and has to change every time the class changes. A loop runs the same code once for each item, however many there are.

Python has two kinds:

for loops

Registered: ECO 201
Registered: STA 211
Registered: GST 201
All done

How to read it:

1. for course in courses: takes the first item and calls it course.
2. The indented block runs with that value.
3. Python takes the next item, and runs the block again, until the list runs out.
4. Then it carries on with the first unindented line.

course is a variable name you choose. Pick one that describes a single item: for score in scores, for name in names.

Looping over text

A string is a sequence of characters, so a for loop goes through it one character at a time:

L
a
g
o
s

range: counting

range() produces a series of numbers, perfect for repeating something a set number of times:

range always stops before the stop number. The third number is the step.

Week 1
Week 2
Week 3
Week 4

Building up a result

Most useful loops accumulate something: a total, a count or a new list. The pattern is always the same: start with an empty value before the loop, update it inside, use it after.

A running total

Total spent: ₦10,500

(Python's sum(spending) does this in one step, but the pattern works for anything, not just adding.)

A counter

How many scores are 70 or above? Start at 0 and add 1 each time the condition is true:

2 scores are 70 or above

count += 1 is short for count = count + 1.

Building a new list

Start with an empty list and append to it:

[67, 81, 54, 72]

Loops with decisions

Put an if inside the loop to treat items differently. The if block is indented one more level:

67: B
81: A
54: C
72: A
45: F

enumerate: the position as well as the item

Sometimes you need each item's position too. enumerate() gives both:

1. ECO 201
2. STA 211
3. GST 201

zip: two lists side by side

zip() pairs up items from two lists, first with first, second with second:

ECO 201: 67 (pass)
STA 211: 81 (pass)
GST 201: 54 (pass)

while loops

A while loop keeps going as long as its condition is true. Use it when you don't know how many repeats you'll need.

You save ₦4,500 a week towards a ₦40,000 phone. How many weeks?

9 weeks: you'll have ₦40,500

Each time round, Python checks saved < 40000. While it's true, it runs the block. After week 9, saved is 40,500, the condition is false, and the loop stops.

break and continue

break stops a loop immediately. continue skips the rest of this round and moves on to the next item.

The first score below 50:

First fail: 44

The loop stops at 44 and never looks at 72 or 45.

Skip blank entries:

Hello Adaeze
Hello Musa
Hello Tobi

Loops inside loops

A loop can contain another loop. The inner one runs completely for each round of the outer one:

Mon 1
Mon 2
Mon 3
Tue 1
Tue 2
Tue 3

List comprehensions: a loop in one line

Building a new list from an old one is so common that Python has a short form, a list comprehension:

[67, 81, 54, 72]
[72, 86, 59, 77, 50]

Read [score + 5 for score in scores] as "score + 5, for each score in scores". It does exactly what the append loop does. Use it for simple cases; write the full loop when the logic gets longer.

When it goes wrong

for course in courses
    print(course)

SyntaxError: expected ':'

Missing colon after the for line.

for course in courses:
print(course)

IndentationError: expected an indented block after 'for' statement on line 1

Missing indentation inside the loop.

total = 0
for amount in [100, 200, 300]:
    total = 0
    total += amount
print(total)

300

Resetting inside the loop. total = 0 belongs before the loop. Inside, it wipes the total each time, so only the last amount survives.

Summary

Try it

Use these scores in Colab: