When I’m doing coding interviews I always like to start off and say I’m a big fan of very long variable names. “As descriptive as you can be” I say. Then I get to my first for loop. Instead of i I use “iterator” and then when I start a nested loop I use “jiterator” and it always gets a laugh.

i is for index. j is simply the next letter and we're too lazy to think up something meaningful

It depends. x and y are either elements or coordinates, a and b usually elements though in e.g. Haskell reserved (by convention) for type variables.

The i j k l series is reserved for indices. n m etc. are the counts of something, as such you'll see i counting up to n. Both are due to mathematical sum notation and general mathematical convention. Random google result:

Let x1, x2, x3, …xn denote a set of n numbers. x1 is the first number in the set. xi represents the ith number in the set.

...if you're using a language in which you use i often chances are you should stop coding in C and get yourself a language with iterators. Manual loops are a bug magnet.

A useful tip I picked up was to use ii instead of j for an inner loop. It's far more distinct than j.

If for some terrible reason you have even more inner loops you can easily continue the trend i, ii, iii, iiii, iiiii - or iv, v if you're feeling roman

x is used for map, filter, etc. a and b are used for sorts, comparisons and merges. y might be used if I'm doing multiple lambda expressions (but that means I'm in a bad place already). I have no idea why, but these are firm rules in my brain.

People who name iterators with one letter have no soul.

It's my understanding that i,j are conventionally used in mathematics which carried over into programming, but specifically it comes from Fortran in which all integer variables start with "I" through "N" based on said mathematical convention

In old FORTAN variable starting with I...N are integers. This is how the practice began.

I generally use a for each type loop or a map because I am usually applying some function across a collection, and in both cases I use the singular name from the collections plural.

’Cities.map(city -> …)’

For (val city in cities)

If I actually need the index for some reason I still prefer loop structures that give me the index and the item together

*note syntax pulled out of my head and not necessarily belonging to any specific language.

For ( city, index in cities)

cities.map((city, index) -> … )

If I need to double loop a matrix array I would use rowIndex and ColIndex for the indexes.

I always thought i for index when iterating through an array. Then you can't use i again in a nested loop so j follows.

Tho sometimes x, y if the array represents coordinates.

Only a maniac would use a, b.

I find it hard to read when these are together:

• i, j, l
• n, m, u, v, w

From all the possible character combinations, somehow the lookalike combinations are among the most popular. Yes, probably comes from math. I hated it even more when my math prof's i and j on the board were indistinguishable.

I prefer index variable names that are two words. The second word is always 'index' and the first word describes the enumerable objects. carIndex, productIndex, thingIndex

I'm not paid by the character count. Longer and more descriptive is better. Long lines that go past your 1080p monitor are probably not long because of variable names but because you insist on doing many things in one line (quit doin' that). For small functions this isn't necessary, but too often I'm shunted to the middle of a big function with two or three indecies doing acrobatics over one another and while working on it I have to constantly remind myself that this i and j mean particular things.

If you use I,j,k.. what do you do if you need another? I hate seeing lowercase l as a var.

I like range-based for loops. You can just name the iterator after the object that it actually is. Have to be a little careful though, if the container is named a plural noun, and the natural name is the same word minus the easy-to-miss 's'.

for <thing> in <amount> do...

I think it's a convention taken from math notation conventions.

I used starcraft references in mine till the project lead demanded I knock it off.

The protoss quotes were perfect.

Int index = 0 But you shorten the name to Int I = 0

OMG the PRNDL

I don't like i and j since they are commonly used for imaginary numbers. I like to start on n. Probably because I do DSP.