for or while? Choosing the Right Loop

You have two loops in Python, and each one is the better tool for a different job.

The choice in one question

Do I know how many times to repeat, or what to go through?

• Yes: use for. Go through every item of a list, count from 1 to 10, or repeat a fixed number of times.
• No: use while. Repeat until something happens: the user types the right password, a number gets small enough, or a game ends.

Side by side

What can go wrong

A while loop needs something inside it that changes the condition. If nothing changes, the loop never ends. This is called an infinite loop. In the loop above, the line savings = savings * 2 is what finally makes savings < 100 false.

Three habits keep you safe:

1. Before you run a while loop, say which line changes the condition.
2. Check that the condition can become false.
3. Run it first with small numbers.

Mistakes to avoid

• Forgetting to change the variable in a while loop.
• Using while when for is simpler. Going through a list is a job for for.
• Changing the list while a for loop goes through it. Make a copy or build a new list.

Think it through

Write your answers in full sentences. Try each one before you read the pointers.

1. For each job, say which loop you would use and why. (a) Print the 7 times table. (b) Keep asking for a password until it is correct. (c) Add all the marks in a list. (d) Halve a number again and again until it is smaller than 1.
2. A program has count = 1 and then while count < 5: with only a print(count) inside. Explain why it never stops, and give the one line that fixes it.
3. The doubling program above uses a while loop. Can you write it with a for loop? What would you need to add, and why is while a more natural fit?
4. Trace this by hand and write down every number it prints: set n = 3, then while n > 0: print n and subtract 1 from n, and after the loop print "go". Then change "subtract 1" to "subtract 2" and trace it again.
5. A student says, "while is better than for because it is more powerful." Give your opinion, and give one case in which for is clearly the better choice.

Pointers for your answers

1. (a) for, because you know it is 10 times. (b) while, because you do not know how many tries. (c) for, because you go through a list. (d) while, because you do not know how many halvings.
2. Nothing inside changes count, so count < 5 stays true. Add the line count = count + 1.
3. You would need a number of repeats that is large enough and a break when the savings reach 100. This is awkward, because you do not know the number of days in advance.
4. With "subtract 1" the output is 3, 2, 1, then go. With "subtract 2" the values are 3 and 1, and then n becomes -1 and the loop stops, so it prints 3, 1, go.
5. while can do any for job, but a for loop is shorter and safer for a list, because it cannot run forever.