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This noncompliant code example takes the mean of 10 identical numbers and checks to see if the mean matches this number. It should match because the 10 numbers are all 10.1. Yet, because of the imprecision of floating-point arithmetic, the computed mean does not match this number.
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#include <stdio.h>
/* Returns the mean value of the array */
float mean(float array[], int size) {
float total = 0.0;
size_t i;
for (i = 0; i < size; i++) {
total += array[i];
printf("array[%d%zu] = %f and total is %f\n", i, array[i], total);
}
if (size != 0)
return total / size;
else
return 0.0;
}
enum { array_size = 10 };
float array_value = 10.1;
int main(void) {
float array[array_size];
float avg;
size_t i;
for (i = 0; i < array_size; i++) {
array[i] = array_value;
}
avg = mean( array, array_size);
printf("mean is %f\n", avg);
if (avg == array[0]) {
printf("array[0] is the mean\n");
} else {
printf("array[0] is not the mean\n");
}
return 0;
}
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array[0] = 10.100000 and total is 10.100000 array[1] = 10.100000 and total is 20.200001 array[2] = 10.100000 and total is 30.300001 array[3] = 10.100000 and total is 40.400002 array[4] = 10.100000 and total is 50.500000 array[5] = 10.100000 and total is 60.599998 array[6] = 10.100000 and total is 70.699997 array[7] = 10.100000 and total is 80.799995 array[8] = 10.100000 and total is 90.899994 array[9] = 10.100000 and total is 100.999992 mean is 10.099999 array[0] is not the mean |
Compliant Solution
This The noncompliant code may can be fixed by replacing the floating-point numbers with integers for the internal additions. Floats are used only when printing results and when doing the division to compute the mean.
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#include <stdio.h>
/* Returns the mean value of the array */
float mean(int array[], int size) {
int total = 0;
size_t i;
for (i = 0; i < size; i++) {
total += array[i];
printf("array[%d%zu] = %f and total is %f\n", i, array[i] / 100.0, total / 100.0);
}
if (size != 0)
return ((float) total) / size;
else
return 0.0;
}
enum {array_size = 10};
int array_value = 1010;
int main(void) {
int array[array_size];
float avg;
size_t i;
for (i = 0; i < array_size; i++) {
array[i] = array_value;
}
avg = mean(array, array_size);
printf("mean is %f\n", avg / 100.0);
if (avg == array[0]) {
printf("array[0] is the mean\n");
} else {
printf("array[0] is not the mean\n");
}
return 0;
}
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Tool | Version | Checker | Description |
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| Can detect violations of this recommendation. In particular, it checks to see if the arguments to an equality operator are of a floating-point type |
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