Why are we using i as a counter in loops

Question

why are we using

for (int i = 0 ; i < count ; i++){ }

why the i

why not

for (int a = 0; a < count; a++){ }

I do it, you do it, everyone does it but WHY?

*edit

I found out an old saying about FORTRAN which is more funny than correct which says "god is real, everything else above is an integer".

"god" would be a variable name stating with a g so it would be in the real domain, while everything else above (excluding h for the joke's purpose) would be an integer

It seems that the original saying was in fact : "God is real, unless declared integer". Apologies to everyone citing me in their phd thesis

Answer 1

i = iterator.

Answer 2

i = integer

Comes from Fortran where integer variables had to start with the letters I through N and real variables started with the other letters. Thus I was the first and shortest integer variable name. Fortran was one of the earliest programming languages in widespread use and the habits developed by programmers using it carried over to other languages.

EDIT: I have no problem with the answer that it derives from mathematics. Undoubtedly that is where the Fortran designers got their inspiration. The fact is, for me anyway, when I started to program in Fortran we used I, J, K, ... for loop counters because they were short and the first legally allowed variable names for integers. As a sophomore in H.S. I had probably heard of Descartes (and a very few others), but made very little connection to mathematics when programming. In fact, the first course I took was called "Fortran for Business" and was taught not by the math faculty, but the business/econ faculty.

For me, at least, the naming of variables had little to do with mathematics, but everything due to the habits I picked up writing Fortran code that I carried into other languages.

Answer 3

I think it's most likely derived from index (in the mathematical sense) - it's used commonly as an index in sums or other set-based operations, and most likely has been used that way since before there were programming languages.

Answer 4

It's funny how everyone has a different take on this. I always assumed that it stood for 'index'. The fact that everyone immediately realizes what it means is enough for me.

Answer 5

I use it for a number of reasons.

• Usually my loops are int based, so you make a complete triangle on the keyboard typing "int i" with the exception of the space I handle with my thumb. This is a very fast sequence to type.

• The "i" could stand for iterator, integer, increment, or index, each of which makes logical sense.

With my personal uses set aside, the theory of it being derived from FORTRAN is correct, where integer vars used letters I - N.

Answer 6

Possibly historical ?

FORTRAN, aurguably the first high level language, defined i,j,k,l,m as Integer datatypes by default, and loops could only be controlled by integer variable, the convention continues ?

eg:

do 100 i= j,100,5 .... 100 continue ....

Answer 7

Most probably we are using i because we learn to program from examples.

Early examples of for(;;){} loops in C seem to use i as the iteration variable. Checking around the Internet shows this to remain true (see http://en.wikipedia.org/wiki/For_loop).

Once we learn a pattern (i, j, k for nested loops) they become habits and moreover become accepted, even expected, norms for collaboration.

This doesn't make it right or optimal to continue, of course.

Answer 8

Mathematicians were using i,j,k to designate integers in algebra (subscripts, series, summations etc) long before (e.g 1836 or 1816) computers were around (this is the origin of the FORTRAN variable type defaults). The habit of using letters from the end of the alphabet (...,x,y,z) for unknown variables and from the beginning (a,b,c...) for constants is generally attributed to Rene Descartes, (see also here) so I assume i,j,k...n (in the middle of the alphabet) for integers is likely due to him too.

Answer 9

Because int starts with an i :)

Answer 10

i = iterator, i = index, i = integer

Which ever you figure "i" stands for it still "fits the bill".

Also, unless you have only a single line of code within that loop, you should probably be naming the iterator/index/integer variable to something more meaningful. Like: employeeIndex

BTW, I usually use "i" in my simple iterator loops; unless of course it contains multiple lines of code.

Answer 11

Unless you need to know the specific index in an array... Might aswell use a foreach construct;

foreach $item (@list) { do($item); }

lol perl.

Answer 12

We're not. At least not if we're using any sort of decent naming convention.

Answer 13

When I 'learned' BASIC on the BBC and ZX Spectrum the convention was to use 'n'. When I moved to more curly bracket languages the convention shifted to 'i'.

Answer 14

i = index I think, at least that's how I look at it

Answer 15

I'm quite sure that i comes from index (I'm a mathematician, after all).

Actually when I was high-schooler, my loop variables were usually called idx, jdx and kdx.

Answer 16

Actually, since I had to do a lot of coding in Matlab, where i is predefined as the unit imaginary number, I have switched to k as the default loop counter... =)