Looping on a Condition

A for loop walks a collection. You know how many passes there will be before it starts.

A while loop repeats as long as a condition stays true. You don't know the count in advance: keep asking until they enter a valid answer, keep withdrawing until the balance runs out, keep playing until someone wins.

While loops

PYTHON

count = 0
while count < 3:
    print(count)
    count += 1

JAVASCRIPT

let count = 0;
while (count < 3) {
  console.log(count);
  count += 1;
}

Compare → Nearly identical. Python needs the colon and indentation; JavaScript needs parentheses around the condition and braces around the body. The three-part structure is the same: initialize before, test each pass, change inside.

EXAMPLE

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YOUR TURN

Your turn → Each pass checks the condition again: 0<3, 1<3, 2<3, then 3<3 is false and the loop exits. Change it so it prints 1 through 5, then change it again so it prints only the even numbers up to 10.

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Syntax has to be exact. Every quote, bracket and parenthesis needs its partner, and in Python the spaces at the start of a line count. One slip and the program won't run; the message will say where.

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