q)1+where mod[;2] sum 0=(1_til 500) mod/: til 500
    1 4 9 16 25 36 49 64 81 100 121 144 169 196 225 256 289 324 361 400 441 484

I can also lift the core of the C++ code into a function and reuse it, which doesn't appear possible in the other language.

Can you give an example of some improvements you see that are directly obvious from the C code?

I'm especially interested in a process that reveals the sqrt optimisation.

> whereas the two lines above are impenetrable.

It takes practice to learn a new language. For now, you will have to take my word that the former of the lines is almost a literal translation of the problem, and to an experienced q developer, directly suggests the second line. I hoped the illustration would suggest this, but communicating alien things is hard.

Languages that have that property are extremely valuable to me: There are a lot of languages that have a nice notation leading from problem to code (lisp, haskell, etc; at least some of the time) but it is much more rare that we have a notation that directly suggest further (better) notations, and myself having programmed in C, C++, Perl, Python, Java, Forth, Fortran, Cobol, Lisp, and so on for decades, I'm telling you that this seems to happen more with array languages than with any other language or language-family that I've had experience with.

If that intrigues you, I can try and give you a crash course:

- til x is the vector of integers from 0 to x. In C it's a little like: r=calloc(sizeof(int),x);for(i=0;i<x;++i)r[i]=x;

- x mod y is modulo. Similar to x%y in C, but it generalises to vectors.

- x mod/: y shows each-right (/:) which composes to the operator on the left. This builds a 2d array of each x argument (columns) mod each y argument (rows).

- til[x] mod/: til[x] is thus the 2d array of integers from 0 to x mod each right the same list.

- x=y is equals. Similar to x==y in C, but it generalises to vectors.

- 0=til[x] mod/: til[x] is thus a bitmap of locations where (columns:0..x) mod (rows:0..x) = 0

- sum adds up the values in a vector. applied to a 2d array (including a bitmap) returns the sum of each column.

- functions are written with curly braces. In a function, x is the first argument, y is the second.

- {x*x} is thus a function that squares its result.

- mod[;2] is a projection. It is actually the function {x mod 2}

- where returns the locations of 1's in a list.

- where mod[;2] ... is thus the locations of odd values.

- sqrt x returns the square-root of x, expressed as a float. this too generalises to vectors, but we do not take advantage of this.

- ceiling x is the same as ceil(x) except it too generalises to vectors (we aren't using this).

With that in hand, you should be able to carefully decode each line: They're read from left to right, and q composes easily.

> I can also lift the core of the C++ code into a function and reuse it, which doesn't appear possible in the other language.

That's generally true for Python or Java, but q programmers have an excellent C binding. It's as simple as:

    k(0, "{where mod[;2] sum 0=til[x] mod/: til x}", kj(n), 0)

- kj(n) returns a variant (type tagged) from a long long.

Comments and good variable names, to begin with, which should be mandated by any style guide.

If you can't give your variables readable, understandable names you don't understand your code and nobody else is going to, either.

Anyway, good luck with things.

It's also the bare minimum required to have readable source code.

Most people don't like reading stuff that looks like it came out of a particularly aggressive obfuscator.