While

while expr
  line of code
  line of code
end while

while allows you to repeat a block of lines as long as a condition is true.
As the test is carried out from the start, a while block might never be run.
Two other statements can be used in all loops:
- break; to exit from a loop (example 2)
- continue; to interrupt the current iteration and start over from the begining of the loop. (example 3)
Loops of any kind can be nested, but not intertwined.

Correct:

for i=1 to 5
repeat
..
..
until a = "" $
end for

Incorrect:

for i=1 to 5
repeat
..
..
end for
until a = "" $

Example 1:
(A classic mistake is to forget to increment the control variable, something that is done automatically by the for loop)

// Table of 4
i = 1
while i < 5
print(int(i), "* 4 = ", i * 4,"\n")
i = i + 1
end while

1 * 4 = 4.0000
2 * 4 = 8.0000
3 * 4 = 12.0000
4 * 4 = 16.0000

Example 2:

k = 1
while k < 10
print(k, "\n")
if k = 3
break
end if
k = k + 1
end while

1
2
3

Example 3:

l = 0
while l < 5
l = l + 1
if l = 2
continue
end if
print(l, "\n")

end while

1
3
4
5