#include <stdio.h>
#include <stdlib.h>
float sum (float *A, int len) // sum of table of floats
{
float ss = 0.0;
int i;
for(i=0; i < len; ++i)
ss +=A[i];
return ss;
}
void print(float A[][3], int row) //// procedure that prints sum of elements for each row in 2D table
{
int i, j;
float k;
for(i = 0; i < row; i++)
{
k=0.0;
for(j = 0; j < 3; j++)
{
k=A[i][j]+k;
}
printf (" %2.2f \n", k);
}
return;
}
int compare(const void *p,const void *q) // sort 2D table in ascending order by sum in rows
{
float *a = (float*)p;
float *b = (float*)q;
float l=sum(a,3);
float r=sum(b,3); // CORRECTLY WRITTEN COMPARE FUNCTION
if (l<r) return -1;
if (l==r) return 0;
else return 1;
}
/* int compare(const void *p,const void *q)
{
float *a = (float*)p;
float *b = (float*)q;
float l=sum(a,3);
float r=sum(b,3); // WRONGLY WRITTEN COMPARE FUNCTION
return l-r;
} */
int main()
{
float TAB_1[ ][3]= {{1.3,2.4,1.1},{4.9,5.9,0.},{5.1,5.1, 1.1},{6.1,7.0,0.3},{1.3,1.3, 3.1},
{1.3,1.3, 0.1},{4.4,4.3, 4.1},{1.3,1.2, 3.1},{1.3,1.3, 8.1}};
print(TAB_1,sizeof(TAB_1)/sizeof(TAB_1[0]) );
qsort(TAB_1,sizeof(TAB_1)/sizeof(TAB_1[0]),sizeof(TAB_1[0]),compare);
puts("");
print(TAB_1, sizeof(TAB_1)/sizeof(TAB_1[0]));
return 0;
}
Why commented function that I marked as WRONGLY WRITTEN COMPARE FUNCTION gives bad output? Is it a problem with float representation in C or with subtraction 2 floats? Could somebody explain?
The compare function with lr
is wrong if the difference between these two numbers is less than 1
, more accurately if abs(lr) < 1
. The reason is that the result of the floating point operation lr
is converted to int
, and anything between 0..0.99999..
will yield 0
, just as if these two numbers were equal (even if they aren't).
The following short program illustrates this:
int main() {
float f1 = 3.4;
float f2 = 3.7;
int compareResultWrong = f2 - f1;
int compareResultOK = (f1 < f2);
printf("result should be 1; Wrong: %d, OK: %d\n", compareResultWrong, compareResultOK);
}
Is it a problem with float representation in C or with subtraction 2 floats?
Neither first nor second - it is definitely wrong implementation.
Just look what occurs when -0.01
or 0.01
result is casted to int
result of function?
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