Loops: Quick Revision

A brief note on Loops. Read it before a test. Try each question at the end before you open its answer.

The for loop

• A for loop repeats its indented lines once for every value it is given: for name in ["Ali", "Bina", "Chen"]:.
• The loop variable holds the current value. You choose its name.
• The body is the indented lines. The first line that is not indented comes after the loop, and runs once.
• A text works too: for letter in "cat": has one round for each letter.

Counting with range()

• The stop value is never included. To reach the number n, the stop must be n + 1.
• range(a, b) gives b - a numbers. A negative step counts down.

Loops that build answers

• Make the starting variable before the loop. Change it inside. Show it after.
• A total starts at 0, a count starts at 0, and a product starts at 1.
• A trace table, with one row for each round, shows what the loop really does.

A loop inside a loop

• The inner loop runs completely in every round of the outer loop.
• Multiply the two numbers of rounds to find how often the inner body runs: 3 rounds of 4 is 12.
• print(..., end="") keeps the output on the same line. A plain print() after the inner loop starts a new line.
• Give the two loop variables different names, such as row and col.

The while loop

• while condition: repeats its body as long as the condition is True. The condition is asked before every round, even the first.
• Every while loop needs three things: a start, a test and a change. If nothing changes, the loop never ends.
• break leaves the loop at once. continue skips the rest of this round and goes to the next.
• while True: runs until a break, so the break must be able to happen.
• A sentinel, such as 0, can mark the end of a list of numbers. It is not part of the data.

A worked example

A water tank holds 50 litres. Every hour, half of the water is used, in whole litres. How many hours pass before it holds 10 litres or less?

Follow it with a trace table. The test comes before every round.

1. The start. litres is 50 and hours is 0, before the loop.
2. The change. Each round halves litres with //, so the test can become false.
3. The end. After round 3, 6 > 10 is false. The program shows 6 litres after 3 hours.

Practice with new numbers

A program to try

Mira reads 25 pages of a 180-page book every day. The program counts the days and draws a bar of # for every 10 pages read.

Mistakes to avoid

• A missing indent. The body must be indented. Without it, Python stops with IndentationError: expected an indented block after 'for' statement on line 1.
• Off by one. range(1, 10) stops at 9. To include 10, write range(1, 11).
• The start inside the loop. total = 0 inside the loop sets the total back to 0 in every round. Put it before the loop.
• A product that starts at 0. Anything times 0 is 0, so a product starts at 1.
• A loop that never ends. If the body never changes what the test asks about, the loop runs forever. Check the start, the test and the change.

Check yourself

1. A hall has 6 rows of 9 seats. A program prints one ticket for every seat, with a loop over the seats inside a loop over the rows. How many tickets does it print?

2. A rocket countdown must show 10, 8, 6, 4 and 2. Which range() gives exactly these numbers?

3. Rohan wants the product of 1, 2, 3, 4 and 5. He writes product = 0 before the loop and product = product * k inside it. What does he get, and how does he fix it?

4. A loop goes through the scores 12, 0, 7, -1 and 9. It must skip a 0 and stop for good at -1. Which line does each job, and which total does it reach?

5. For each job, choose for or while: (a) show the names of the 12 months; (b) keep asking for a password until it is right.