CSC108H1 Chap.9 While Loops and Index-Based Search
While Loops and Index-Based Search
Repeating while a condition holds
A while loop checks its condition at the start of every pass and repeats its body while the condition is True. Something in the body must move the loop toward stopping, or it runs forever.
While loops suit problems where you do not know in advance how many passes are needed.
Searching by index
Unlike a for loop, a while loop can walk a string by index and stop at the first match.
The guard that checks i against len(s) must come before any test that reads s[i], so that short-circuit evaluation stops the expression before an index runs past the end.
How while loops are assessed
Worksheet 15 asks you to fill in a missing loop condition and update line from a docstring, to count the steps of a rule applied repeatedly until a value reaches 1, and to recover a header, type contract and examples from a finished loop body.
Each of these is a tracing question in disguise, so practise them with a table rather than by running code.
What this chapter covers
- 01
The while condition and when it is checked
- 02
Progress lines and infinite loops
- 03
Stop conditions and their negation
- 04
Guarded searches over string indices
- 05
Trace tables for loop variables
- 06
Debugging loops in Wing 101
Worked example · free
Count the passes of a halving loop
- 2n takes the values 20, 10, 5, 2 and 1 on successive passes.
- 1The condition is True five times and False when n is 1.
- 1steps ends at 5.
Key terms
- Loop Condition
- The expression a while loop checks before each pass, repeating while it is True.
- Infinite Loop
- A loop whose condition never becomes False because nothing in the body moves it toward stopping.
- Guard
- A test placed first in an and expression so that a later, riskier test never runs when it would fail.
While Loops and Index-Based Search FAQ
When should I use a while loop instead of a for loop?
When you do not know in advance how many passes you need, or when you want to stop as soon as a condition is met, such as finding the first space in a string. A for loop always visits every element.
Why must the length guard come first in a search condition?
Because and stops at the first False. If i has reached len(s), the guard is False and s[i] is never read. Written the other way round, Python reads s[i] first and raises an IndexError when no match exists.
How do I write the condition for a while loop?
State when the loop should stop, then negate it. If you stop when you find a match or run out of characters, the loop continues while there is no match and there are characters left, two conditions joined by and.
What causes an infinite while loop?
A body that never changes what the condition depends on. If the condition checks n and nothing in the body updates n, the condition stays True and the loop never ends. Name the progress line before writing the rest of the body to avoid it.
How do I trace a while loop on paper?
Make a table with one column per variable and a column for the condition. Add a row each time the condition is checked, write its value, and stop at the first row where it is False. That row holds the values the code after the loop will use.
Exam move
For each Week 4 while-loop exercise, write the stop condition in words, negate it into the loop condition, and name the progress line before writing the rest. Trace every loop with a table holding one row per check of the condition. Then break one loop deliberately by removing its progress line or reversing its guard, and predict the failure. Add one loop that should run zero times and check that your table shows no passes.
Keep every table; a wrong row is the quickest way to find which line of a loop you misread.
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