Coding
How to Understand Loops in Programming
By Daniyal Ahmed · · 4 min read
Quick answer
A loop repeats a block of code, saving you from writing the same instructions many times. A for loop repeats a set number of times or over a collection; a while loop repeats as long as a condition holds. The main danger is an infinite loop, where the stopping condition is never met.
Why loops exist
Loops let a program repeat an action without you writing it out each time. Instead of copying the same instructions a hundred times, you write them once inside a loop that runs a hundred times.
This is one of the fundamental ideas in programming. Almost every useful program repeats work, and loops are how that repetition is expressed.
For loops
A for loop typically repeats a set number of times, or once for each item in a collection. Use it when you know how many times to repeat, or when you want to process every item in a list.
For loops are ideal when the number of repetitions is known or when you are working through a collection item by item.
While loops
A while loop repeats as long as a condition remains true, checking the condition before each repetition. Use it when you do not know in advance how many times to repeat — you repeat until something changes.
While loops suit situations where the stopping point depends on what happens during the loop, rather than a fixed count.
Choosing between them
The choice comes down to whether you know the number of repetitions. Known count or a collection to process suggests a for loop; repeating until a condition changes suggests a while loop.
Both can often be made to do the same job, but choosing the natural one for the situation makes code clearer and less error-prone.
Avoiding infinite loops
The classic loop bug is an infinite loop, where the stopping condition is never met and the program runs forever. This usually happens when something inside the loop fails to move it towards its end.
For while loops especially, make sure something inside the loop changes so the condition will eventually become false. Checking this prevents the commonest loop error.
Loops and logic
Loops combine with conditions and variables to do real work — counting, summing, searching, building results. Tracing a loop by hand, noting how variables change each time round, is the way to understand it.
This tracing, following the loop step by step for a small case, is the single most useful technique for understanding what a loop actually does.
Frequently asked questions
What is a loop in programming?+
A structure that repeats a block of code, so you write instructions once and run them many times instead of copying them. Almost every useful program repeats work, and loops express that repetition.
What's the difference between a for loop and a while loop?+
A for loop repeats a set number of times or over a collection; a while loop repeats as long as a condition holds. Use for when you know the count, and while when you repeat until something changes.
How do I choose which loop to use?+
Ask whether you know the number of repetitions. A known count or a collection to process suggests a for loop; repeating until a condition changes suggests a while loop. Choosing the natural one makes code clearer.
What is an infinite loop?+
A loop whose stopping condition is never met, so it runs forever. It usually happens when nothing inside the loop moves it towards its end, so ensure something changes to make the condition eventually false.
How do I understand what a loop does?+
Trace it by hand for a small case, noting how the variables change each time round. This step-by-step tracing is the single most useful technique for understanding a loop's behaviour.
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