for loops and range()

One value at a time

Predict what this prints.

for letter in "Selam":
    print(letter)

S
e
l
a
m

for letter in "Selam": means "for each character in the text, call it letter and run the block". The name after for is the loop variableloop variableThe name a for loop gives to each value in turn. Each round, it refers to the next value.: each round it refers to the next value, and you don't update it yourself. Ge'ez works the same way, one character per round:

for letter in "ሰላም":
    print(letter)

Change the text to your own name.

Counting with range()

range() gives a series of whole numbers to loop over. Predict this one carefully.

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

0
1
2
3
4

range(5) starts at 0 and stops before 5. Give it two numbers, a start and a stop, and it starts where you say: range(1, 6) gives 1 to 5. A third number is the step, and a negative step counts down:

for n in range(10, 0, -2):
    print(n)

The stop is never included, even counting down: 0 isn't printed. Try range(1, 6), then range(0, 20, 5).

range() only takes whole numbers, so a number that came from input() needs converting first:

Back to the stop. "Stops before" causes the most common loop bug of all, the off-by-oneoff-by-oneA bug where a loop runs one round too many or too few, usually because of where it starts or stops. error:

A running total

To add up 1 to 100, keep a running total: start it at 0 before the loop, and add each number to it inside the loop.

total = 0
for n in range(1, 101):
    total = total + n
print(total)

n is the loop's counter, and total is the running total. Where total = 0 goes matters. Here it's inside the loop by mistake; predict what this prints.

for n in range(1, 4):
    total = 0
    total = total + n
print(total)

3

Step through it and watch total drop back to 0 each round:

3

The last visit to line 1 finds no numbers left, so the loop ends. The fix is to move total = 0 above the loop, so it runs once: the code then prints 6, the same shape as the 1-to-100 program. A running total always starts before the loop.

The loop variable is not in charge

In a while loop, you change the counter yourself. In a for loop, you don't, and changing it has no effect on the next round. Predict:

for i in range(3):
    print(i)
    i = 10

0
1
2

So which loop should you use? If you know what you're going through, or how many rounds (every letter, 1 to 100, 10 rows), use for. If you're repeating until something happens (a valid answer, a savings goal), use while.