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10382886
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2012-04-30T11:24:52.617
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2012-04-30T11:24:52.617
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If floats are 32-bit IEEE-754 and ints are 32-bit too and if there are no +infinity, -infinity and <code>NaN</code> values, we can compare floats as ints with a little trick: <pre><code>#include <stdio.h> #include <limits.h> #include <assert.h> #define C_ASSERT(expr) extern char CAssertExtern[(expr)?1:-1] C_ASSERT(sizeof(int) == sizeof(float)); C_ASSERT(sizeof(int) * CHAR_BIT == 32); int isGreater(float* f1, float* f2) { int i1, i2, t1, t2; i1 = *(int*)f1; i2 = *(int*)f2; t1 = i1 >> 31; i1 = (i1 ^ t1) + (t1 & 0x80000001); t2 = i2 >> 31; i2 = (i2 ^ t2) + (t2 & 0x80000001); return i1 > i2; } int main(void) { float arr[9] = { -3, -2, -1.5, -1, 0, 1, 1.5, 2, 3 }; float thr; int i; // Make sure floats are 32-bit IEE754 and // reinterpreted as integers as we want/expect { static const float testf = 8873283.0f; unsigned testi = *(unsigned*)&testf; assert(testi == 0x4B076543); } thr = -1.5; for (i = 0; i < 9; i++) { printf("%f %s %f\n", arr[i], "<=\0> " + 3*isGreater(&arr[i], &thr), thr); } thr = 1.5; for (i = 0; i < 9; i++) { printf("%f %s %f\n", arr[i], "<=\0> " + 3*isGreater(&arr[i], &thr), thr); } return 0; } </code></pre> Output: <pre><code>-3.000000 <= -1.500000 -2.000000 <= -1.500000 -1.500000 <= -1.500000 -1.000000 > -1.500000 0.000000 > -1.500000 1.000000 > -1.500000 1.500000 > -1.500000 2.000000 > -1.500000 3.000000 > -1.500000 -3.000000 <= 1.500000 -2.000000 <= 1.500000 -1.500000 <= 1.500000 -1.000000 <= 1.500000 0.000000 <= 1.500000 1.000000 <= 1.500000 1.500000 <= 1.500000 2.000000 > 1.500000 3.000000 > 1.500000 </code></pre> Of course, it makes sense to precalculate that final integer value in <code>isGreater()</code> that's used in the comparison operator if your threshold doesn't change. If you are afraid of undefined behavior in C/C++ in the above code, you can rewrite the code in assembly.
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1. POEfficient floating point comparison (Cortex-A8)
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1. USAlexey Frunze
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