Difference between revisions of "Range-based for loop"
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(→Usage: add output) |
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=== Usage === | === Usage === | ||
+ | {| class="wikitable" | ||
+ | |- | ||
+ | ! Code | ||
+ | ! Output | ||
+ | |- style="vertical-align:top;" | ||
+ | | | ||
<source lang="C"> | <source lang="C"> | ||
for RANGE(i, -1, 5) { | for RANGE(i, -1, 5) { | ||
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} | } | ||
</source> | </source> | ||
+ | || | ||
+ | <pre> | ||
+ | -1 | ||
+ | 0 | ||
+ | 1 | ||
+ | 2 | ||
+ | 3 | ||
+ | 4 | ||
+ | </pre> | ||
+ | |- style="vertical-align:top;" | ||
+ | | | ||
+ | <source lang="C"> | ||
+ | for REVERSE_RANGE(i, 5, -1) { | ||
+ | printf("%d\n", i); | ||
+ | } | ||
+ | </source> | ||
+ | || | ||
+ | <pre> | ||
+ | 4 | ||
+ | 3 | ||
+ | 2 | ||
+ | 1 | ||
+ | 0 | ||
+ | -1 | ||
+ | </pre> | ||
+ | |} | ||
[[Category:C]] | [[Category:C]] | ||
[[Category:Programming]] | [[Category:Programming]] | ||
[[Category:C preprocessor snippets]] | [[Category:C preprocessor snippets]] |
Latest revision as of 08:43, 28 April 2020
One of my pet peeves is the regular counting for loop in C where you have to repeat the variable name 3 times. This is error-prone in the case of nested loops, since the compiler has no way to know that you meant to increment j and not i. To prevent this kind of error, you can use a few helper macros that iterate over a range (forwards or backwards).
Definitions
// low .. (high - 1)
#define RANGE(var, low, high) \
(typeof(0 ? (low) : (high)) var = (low), _end = (high); var < _end; ++var)
// https://stackoverflow.com/a/5458283
// (high - 1) .. low
#define REVERSE_RANGE(var, high, low) \
(typeof(0 ? (low) : (high)) var = (high), _end = (low); var-- > _end; )
This automatically deduces the type of the index variable, so that e.g. RANGE(i, -1, 1) will use int, while RANGE(i, 0UL, 1UL) will use unsigned long. Note that typeof is a GNU/gcc extension, so this will not work on e.g. MSVC.
Usage
Code | Output |
---|---|
for RANGE(i, -1, 5) {
printf("%d\n", i);
}
|
-1 0 1 2 3 4 |
for REVERSE_RANGE(i, 5, -1) {
printf("%d\n", i);
}
|
4 3 2 1 0 -1 |