Why do we use `i` as the variable to control a Javascript loop?

Question

I always wondered why we use i in Javascript loops couldn't it be any letter? Just wondering why i seems to be the default one.

for (i = 0; i < 5; i++) {
  text += "The number is " + i + "<br>";
}

i doesn't matter , i use (x) most of the time. (i) for index may be — Aleks, May 23 '19 at 02:59
https://softwareengineering.stackexchange.com/questions/86904/why-do-most-of-us-use-i-as-a-loop-counter-variable — spender, May 23 '19 at 02:59
https://stackoverflow.com/questions/4137785/why-are-variables-i-and-j-used-for-counters — Trevor Keller, May 23 '19 at 03:00
x and n are also pretty common because of mathematical formulas, but you should do yourself a favor and use something more descriptive whenever possible. — JasonB, May 23 '19 at 03:02
I did a fairly long drawn search thanks for showing me the original question/answers — bradpotts, May 23 '19 at 03:15

Intervalia · Answer 1 · 2022-03-28T18:04:07.720

4

There are several variable names that are commonly used in loops:

i is probably the most common since it is only one letter and stands for index.

Your code example should define i like this:

for (let i = 0; i < 5; i++) {
  text += `The number is ${i}<br>`;
}

Otherwise it is created on the global scope and that just shouldn't happen.

edited Mar 28 '22 at 18:04

answered May 23 '19 at 02:59

Intervalia

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`var` would also be an improvement over nothing at all. `let`, despite the obnoxious name, is teh new hotness. https://caniuse.com/#search=let – Jan Kyu Peblik May 23 '19 at 03:00

score 2 · Answer 2 · answered May 23 '19 at 03:02

2

It's an historical artifact from Fortran, which originally made all variables beginning with i, j, ... n implicitly integer (a convention common to mathematical notation as well, at least for i, j and k), and all other names (a-h, o-z) implicitly real. A lot of code ended up borrowing that convention for throwaway integer variable names, using i for the outer loop, j for the next loop in, and so on.

There's no real rhyme or reason to it (aside from i often being used to mean "index" or "integer"), just a common convention. The convention makes it okay to use i (because we all know what you mean); if you didn't use it, you'd probably want a spelled out name, not some other random single letter name that fails to describe what the variable represents.

answered May 23 '19 at 03:02

ShadowRanger

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Thanks for the Origin Story! I always assumed short variable names due to small memory sizes. – Intervalia May 23 '19 at 14:41
@Intervalia: I mean, that's part of why Fortran did this. They limited names to six characters initially, and made them implicitly typed based on the first letter. Remember that Fortran was originally written on punchcards, the ultimate in fixed width fields. Even later on, memory on early compile hosts is measured in kilobytes (except the *byte* wasn't standard!), so you have to make some compromises to reduce source code size, reduce symbol table overhead, etc. IBM 1401 FORTRAN's compiler [ran in core memory of 8000 six bit characters](https://en.wikipedia.org/wiki/Fortran#IBM_1401_FORTRAN). – ShadowRanger May 23 '19 at 14:55
Back when I worked at Intel we were often told: "The code is perfect, now go reduce the size by 1K and don't lose any functionality." I did a lot of Assembly development back then, but never got to develop on a mainframe. – Intervalia May 23 '19 at 16:51

score 1 · Accepted Answer · answered May 23 '19 at 02:59

It's no reason whatsoever - it's just a naming convention that signifies an index (letter i) or loop counter (iteration). You can use whichever variable name you like:

let text = "";
for (anExtremelyLongCounterName = 0; anExtremelyLongCounterName < 5; anExtremelyLongCounterName++) {
  text += "The number is " + anExtremelyLongCounterName + "<br>";
}
document.write(text);

Also note that currently your code is implicitly declaring a global variable. Use let or var to make it better.

Philip Wrage · Answer 4 · 2019-05-23T03:08:35.387

You could use any letter or any valid variable name for that matter. When I first learned to program my teacher instructed us to use lcv for loop control variable.

However to answer your original question, i is very often used to control loop iteration in most languages that have a similar for loop construct, and I believe it derives from the first letter of the word increment. Then by convention if you have nested loops the next loop control variable will be letters following i in the alphabet: j, then k, etc.

As I mentioned before, though, it can be any variable name you choose. But who wants to do that much typing for a throwaway variable?

Why do we use `i` as the variable to control a Javascript loop?

4 Answers4