Repeating with for

Some work has to happen more than once: say every number from 1 through 10, add up a list of prices, print a row for each guest. A for loop repeats the lines under it, once for each number in a range. When those things are already gathered together, Lists and records shows how for walks a list. When you want one field, a total, how many there are, an average, the records in order, or the records inside other records, Asking for data does that in one line. for stays for the step each record needs on its own.

Counting from one number to another

for i in 1..3
  say i

1
2
3

Read it as: “for each number i in 1 through 3, say i.”

• i is the number for this pass. The first pass it is 1, then 2, then 3.
• 1..3 includes both ends. The loop runs for 1, 2, and 3, and then stops.
• The lines to repeat go below for, indented by exactly 2 spaces.
• Each pass, i moves to the next number by itself. You don’t write set for it.

A range that starts after it ends does nothing. The lines under it are skipped, and the program continues:

for i in 3..1
  say i
say "done"

done

for i in 5..5 runs once, with i as 5.

Keeping a total

The loop’s own name can’t be changed with set, and a line inside the loop can’t create a new value, because that value would be created again on every pass. Create a var before the loop, then change it inside:

var total = 0
for n in 1..4
  set total = total + n
say total

10

That adds 1, then 2, then 3, then 4. 10 is their sum.

The end of the range is decided once, before the first pass. Changing a value inside the loop doesn’t move the finish line.

The name lives inside the loop

i exists on the lines under the for, and nowhere else:

for i in 1..3
  say i
say i

If a value with that name already exists, the loop borrows the name while it runs. Afterwards the earlier value is still there:

i = 10
for i in 1..2
  say i
say i

1
2
10

The range itself sees the earlier value. for i in i..12, when i is 10, counts from 10 through 12.

Common mistakes

The repeated lines have to be indented by exactly 2 spaces. On the same line as for, or back at the left edge, they aren’t part of the loop.

A range is two dots. Three dots is a mistake:

for i in 1...3
  say i

Both ends of the range have to be numbers:

for i in 1.."3"
  say i

And the loop can’t create a value:

for i in 1..3
  label = i

Create label before the loop with var, then use set inside.

Try it

1. Say the numbers from 1 through 5.
2. Add those numbers into a var total, and say the total when the loop is finished. It should be 15.
3. Try for i in 5..5. How many lines does it print?