The for Loop

1How it is written

for variable in collection:
    statement
    statement
next statement

Three parts to notice. for and in are both keywords, and both are compulsory. The name between them is the loop variable — you choose it, and Python creates it for you. After the colon comes the usual indented block.

# printing hello 5 times
for i in range(1, 6):
    print('hello')

hello
hello
hello
hello
hello

range(1, 6) produces 1, 2, 3, 4, 5 — five values — so the block runs five times. The loop variable i is not used inside the block here; it is still there, quietly taking each value in turn.

2What Python actually does

Six steps, and every one of them matters:

1. Work out the collection: range(1, 6) becomes the values 1, 2, 3, 4, 5.
2. Create the loop variable — here, i.
3. Take the next value from the collection and put it in i.
4. Jump into the body and run it.
5. At the end of the body, go back up to the header.
6. Repeat 3 to 5 until there are no values left. Then the loop ends and the program carries on below it.

Step 5 is the one that makes it a loop, and it is invisible in the code — nothing on the page points back upwards. Step through it below and watch the pointer make that jump:

nothing yet

3Using the loop variable

The moment the body mentions i, every round does something different:

# the loop variable is a real variable — you can use its value
for i in range(1, 6):
    print('Round', i)

Round 1
Round 2
Round 3
Round 4
Round 5

Which is all a multiplication table is:

# the table of a number entered by the user
n = int(input('Enter a number: '))

for i in range(1, 11):
    print(n, 'x', i, '=', n * i)

Enter a number: 7
7 x 1 = 7
7 x 2 = 14
7 x 3 = 21
7 x 4 = 28
7 x 5 = 35
7 x 6 = 42
7 x 7 = 49
7 x 8 = 56
7 x 9 = 63
7 x 10 = 70

range(1, 11), not range(1, 10) — because stop is excluded, and the table needs the 10.

4Building up an answer across the rounds

A variable made before the loop survives every round, so the loop can add to it a little at a time. This is how a program totals things:

# the sum of all the numbers from 1 to n
n = int(input('Enter a number: '))
s = 0

for i in range(1, n + 1):
    s = s + i

print('Sum =', s)

Enter a number: 5
Sum = 15

Two placements decide whether this works. s = 0 is above the loop, so it happens once — inside, it would reset to 0 every round and the total would never grow past the last number. And print() is at the margin, so it happens after the loop is done — indented, it would print a running total on every round.

5for without range()

range() is the commonest thing to loop over, not the only one. A for loop will walk through any sequence — a string, a list, a tuple — handing you one item at a time:

# a for loop can walk through a string, one character at a time
name = 'Asha'

for ch in name:
    print(ch)

A
s
h
a

# ... or through a list, one item at a time
marks = [72, 91, 65]

for m in marks:
    print('Mark:', m)

Mark: 72
Mark: 91
Mark: 65

Notice there is no counting and no indexing here. The loop variable holds the item itself — the character, the mark — not its position. This is the form to reach for whenever the question is “do something to each one”.

6What goes wrong

The colon and the indentation fail exactly as they did with if, and the messages name the statement:

for i in range(1, 6):
print('hello')

  File "for_no_indent.py", line 2
    print('hello')
    ^
IndentationError: expected an indented block after 'for' statement on line 1

The one that is new belongs to for alone. The thing after in has to be something with values in it. A plain number is not:

for i in 5:
    print('hello')

Traceback (most recent call last):
  File "not_iterable.py", line 1, in <module>
    for i in 5:
TypeError: 'int' object is not iterable

Iterable means “can be gone through one item at a time”. A string, a list, a tuple and a range all can. The number 5 cannot — to repeat five times, ask for range(5), which turns the 5 into five values.

7Try it

Run the table, then change range(1, 11) to range(1, 6), then to range(10, 0, -1). Then replace the range with a list of your own.

# the table of a number entered by the user
n = int(input('Enter a number: '))

for i in range(1, 11):
    print(n, 'x', i, '=', n * i)

8Recap