Lesson 2 reading

The for loop

Remember the "smart way" from Lesson 1?

print(phrase)
print(phrase)
print(phrase)
print(phrase)
print(phrase)
print(phrase)
print(phrase)
print(phrase)

Eight copies of the same line is still not very smart. Computers exist to repeat things — we just need a way to say "do this 8 times":

phrase = "I will not talk in class"

for i in range(8):
    print(phrase)

range(8) produces the numbers 0 through 7, and the loop runs the indented code once for each of them. The indentation rule is the same one you learned with if: the indented lines belong to the loop.

The loop variable

That i is a variable, and it changes on every trip through the loop. Print it and watch:

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

0
1
2
3
4

Two things to notice: it starts at 0, not 1, and it stops before 5. Programmers count from zero — you still get exactly 5 numbers, just shifted.

Since i is a real number, you can compute with it:

for i in range(5):
    print(i ** 2)

That prints the first five square numbers: 0, 1, 4, 9, 16.

Loops + if

The loop body can contain anything — including an if statement. This checks every number from 0 to 9, using the even/odd expression from the lesson:

for i in range(10):
    if i % 2 == 0:
        print(i, "even")
    else:
        print(i, "odd")

Note the double indentation: the if belongs to the loop, and each print belongs to its branch of the if.

Try it

Before running each one, predict the exact output. Then run it and check yourself:

for i in range(3):
    print("ha")

for i in range(4):
    print(i * 10)

for i in range(6):
    if i % 2 == 0:
        print(i)