Range-based for loop: Difference between revisions
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One of my pet peeves is the regular counting <tt>for</tt> 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 <tt>j</tt> and not <tt>i</tt>. To prevent this kind of error, you can use a few helper macros that iterate over a range (forwards or backwards). |
One of my pet peeves is the regular counting <tt>for</tt> 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 <tt>j</tt> and not <tt>i</tt>. To prevent this kind of error, you can use a few helper macros that iterate over a range (forwards or backwards). |
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Definitions |
=== Definitions === |
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<source lang="C"> |
<source lang="C"> |
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// low .. (high - 1) |
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#define RANGE(var, low, high) \ |
#define RANGE(var, low, high) \ |
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(typeof(0 ? (low) : (high)) var = (low), _end = (high); var < _end; ++var) |
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// https://stackoverflow.com/a/5458283 |
// https://stackoverflow.com/a/5458283 |
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// (high - 1) .. low |
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#define REVERSE_RANGE(var, high, low) \ |
#define REVERSE_RANGE(var, high, low) \ |
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(typeof(0 ? (low) : (high)) var = (high), _end = (low); var-- > _end; ) |
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</source> |
</source> |
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Note that <tt>typeof</tt> is a GNU/gcc extension, so this will not work on e.g. MSVC. |
This automatically deduces the type of the index variable, so that e.g. <tt>RANGE(i, -1, 1)</tt> will use <tt>int</tt>, while <tt>RANGE(i, 0UL, 1UL)</tt> will use <tt>unsigned long</tt>. Note that <tt>typeof</tt> is a GNU/gcc extension, so this will not work on e.g. MSVC. |
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Usage |
=== Usage === |
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{| class="wikitable" |
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|- |
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! Code |
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! Output |
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|- style="vertical-align:top;" |
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| |
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<source lang="C"> |
<source lang="C"> |
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for RANGE(i, -1, 5) { |
for RANGE(i, -1, 5) { |
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printf("%d\n", i); |
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} |
} |
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</source> |
</source> |
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<pre> |
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-1 |
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0 |
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1 |
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2 |
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3 |
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4 |
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</pre> |
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|- style="vertical-align:top;" |
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| |
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<source lang="C"> |
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for REVERSE_RANGE(i, 5, -1) { |
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printf("%d\n", i); |
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} |
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</source> |
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<pre> |
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4 |
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3 |
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2 |
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1 |
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0 |
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-1 |
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</pre> |
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|} |
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[[Category:C]] |
[[Category:C]] |
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[[Category:Programming]] |
[[Category:Programming]] |
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[[Category:C preprocessor snippets]] |
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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 |