The for loop

What you will be able to do

After this lesson you can

• write a for loop that repeats lines a number of times,
• say what the loop variable holds in each round,
• see which lines are inside the loop and which are after it, and
• go through the letters of a text.

Repeating without retyping

You want to print the message Good morning five times. You could write five print() lines. But what about a thousand times? Nobody wants to type a thousand lines. This is where a loop helps. A loop is a way to say: do this, again and again.

Python's most common loop is the for loop. It repeats some lines once for every value in a row of values.

How the loop works

Look at the first loop, for i in range(3):.

• for starts the loop. The line ends with a colon, just like if.
• i is the loop variable. It is a name that you choose. In each round it holds the next value of the row. The row here is 0, then 1, then 2.
• range(3) makes the row of values. We study it in the next lesson.
• The lines under it, indented by four spaces, are the body. They run once in each round.
• When the values are used up, the loop is over. The program goes on with the first line that is not indented. In our program, that is print("---"), which runs only once.

So the first loop prints Round 0, Round 1 and Round 2. It is three rounds, and the loop variable holds 0, 1 and 2.

The body can have many lines

The body is every line that is indented under the for.

The indent is not optional

What if you forget the indent?

The loop variable starts at 0, so in round 1 it is 0 and in the last round it is one less than n.

Going through a text

A for loop can take a text instead of a number row. In each round, the loop variable holds the next letter. Look at the second loop of the first program: for letter in "python": shows p, then y, then t, and so on.

The words after in can be anything that has a sequence of items: a text, a row of numbers, or (in Chapter 11) a list. The loop does the same thing for all of them: one round for every item, in order.

What is the loop variable?

The loop variable is an ordinary variable. Python gives it the next value at the start of each round. You can use it in the body, as in number * 2. After the loop, it still holds the last value. You can choose a good name: i for a counter, letter for letters, number for numbers. A good name makes the loop easy to read.

Common mistakes

• Forgetting the colon at the end of the for line.
• Forgetting the indent of the body.
• Putting a line inside the loop when it belongs after, or the other way round. Check the indent of each line.
• Changing the loop variable inside the body and expecting it to change the loop. The next round starts with the next value of the row anyway.
• Using i outside a loop before it has any value (a NameError).

Practice

Start with two quick questions. Each New numbers gives you a fresh one, so you can practise as long as you like.

Then the labs and the problems:

1. Warm-up. Open the lab Say it five times. Write a loop that shows a word a number of times.
2. Build. Open the problem Count from 1 to 5 and the problem A times table.
3. Stretch. Open the problem Trace the loop and fill in the value of the loop variable in each round, before you run it.

Remember these

• A for loop repeats its indented lines, once for every value of the row.
• The loop variable holds the current value. You choose its name.
• The first line that is not indented is after the loop, and runs once.
• A text works too: one round for each letter.

Check yourself

Try each one on your own first. If you are stuck, open a hint. Then check with Show the answer.

1. How many times does for i in range(4): print("Hi") show Hi?

2. What does for letter in "cat": print(letter) show?

3. Why does the program give an error if the line after for is not indented?