for loop macro coding style

Question

One of my tutors at university suggests using macros to reduce repetition in c99 code, like this.

#define foreach(a, b, c) for (int a = b; a < c; a++)
#define for_i foreach(i, 0, n)
#define for_j foreach(j, 0, n)
#define for_ij for_i for_j

Which can be used like this:

for_ij { /*do stuff*/; }
for_i  { /*do stuff*/; }

Another tutor with industrial background discourages its use, claiming it was seen as an anti-pattern at his former employer (but he does not know the reason behind this) . In fact by grepping through source code of large projects one rarely finds those constructs outside of short academic examples.

My question is: why is this construct so rarely used in practice? Is it dangerous to some extent?

You are in effect creating your own dialect using the preprocessor. Doig so is always fraught with peril and impedes understanding of anyone else (or yourself later). Anyway, this is a question more for Software Engineering, not SO. — Deduplicator
– Deduplicator, Commented Dec 11, 2016 at 20:20
As an analogy, imagine a book where the preface stated that "SR" would replace "said Robert" and "TND" would replace "The next day" . You could read it but it would be bizarre to say the least — M.M
– M.M, Commented Dec 11, 2016 at 21:57
When I was learning C (a looong time ago), I was smarter than your first tutor and did #define forloop for (int i = 0; i < n; i++) {. By adding the { to the macro I saved even more repetitions! Modern text editors aren't happy about the imbalanced braces, but that was not a problem in the good old days... — rodrigo
– rodrigo, Commented Dec 12, 2016 at 1:07
Listen to the second tutor — the one with the industrial background. His advice is more sensible, IMNSHO. That sort of macro is seldom used because the saving in typing is not large enough to warrant the increase in options for confusion. — Jonathan Leffler
– Jonathan Leffler, Commented Dec 12, 2016 at 6:20

Community · Accepted Answer · 2017-04-13 12:38:59Z

This is such a perfect illustration of the gap between academia and the real world, it is hard to believe. But it looks too weird to be made up.

To answer your question: NO this kind of construction is not used in practice and it is risky as it hides information, using implicit variables and conditions.

Here are some questions the casual reader will ponder about:

what is the upper bound? It is not so obvious it should be n.
what is the actual range for i?
it is 0 based?
does it include n or stop before?

C programmers are very good at reading idiomatic constructions such as

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

Hiding the details of this loop logic saves nothing, is counter-productive, error prone and should be outlawed by local coding conventions. The only reason it isn't in my teams is nobody came up with such a weird idea.

You may need to use caution with the university tutor who wants these abbreviations used, possibly an APL nostalgic, and avoid conflict. You might want to play some code golfing with him, there is a stack exchange dedicated to that and he will love how people waste countless hours shaving bytes from their source codes...

But you should follow the other tutor's advice and write clear and explicit code, carefully indented and spaced out, with meaningful names where it matters and short ones where their use is obvious. Favor idiomatic constructions where possible as it makes the code more readable, less error prone and better fit for optimizing compilers.

Chris Dodd · Accepted Answer · 2016-12-11 21:23:33Z

The problem is that the reduction in repetition (and thus the improvement in readability) is fairly trivial. You're reducing

for (int i = 0; i < n; i++)
    for (int j = 0; j < n; j++)

to

for_ij

which is not much of an improvement, as the variable names i, j, and n are all short so repeating them is not an issue.

At the same time, you're hiding the parameter n -- the latter loop is implicitly dependent on n, even though n appear nowhere in for_ij or (likely) in the body of the for loop either. Hiding important information makes reading code harder -- in order to read this, you need to know the critical importance of n, which you can only see by searching through two levels of macro indirection.

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