strtof- return a wrong number

Question

edit-

So I understand that there is no solution to this problem. Does anyone know a function in C that returns the 31-38 index from the array(str)?

Hey i have a problem with strtof function and i cant find a solution to the problem on the Internet, Can anyone help please?

here a simple function:

float getCord(int placeOnArray, char str[80]){
   char str2[7];
   for (int i=placeOnArray ;i<placeOnArray+7;i++){
      str2[i-placeOnArray]=str[i];
   }
   float f1;
   f1=strtof(str2,NULL);
   return f1;

    }

the input- str= ATOM 1 N ASN A 1 -4.890 -12.296 -15.280 1.00 30.36 N , place on array=31

The desired result: -4.890 ,the returned result: -4.88999987

Use `double` and `strtod`. It looks like floating point problem. — i486, Oct 22 '18 at 11:45
Are you aware of the fact that floating-point math is inherently inexact? — Angew is no longer proud of SO, Oct 22 '18 at 11:46
@papi read [this SO article](https://stackoverflow.com/a/588014/898348) — Jabberwocky, Oct 22 '18 at 11:47
Try `printf("%6.3f\n", -4.88999987); /* -4.890 */` See [code running on ideone.com](https://ideone.com/0t1LY8) — pmg, Oct 22 '18 at 11:48
@user3121023 yes but here it's not really important because `strtof` will stop at the `' '` anyways. But the code is admittedly sloppy. — Jabberwocky, Oct 22 '18 at 11:50
@user3121023 you're right, but anyway there is no need to copy anything, you can call `strtof` directly on the pointer into the original string — Jabberwocky, Oct 22 '18 at 11:55
@papi ... and `place on array=31`: shouldn't it be 19 instead of 31? — Jabberwocky, Oct 22 '18 at 11:57
@papi not the answer to your question, but your `getCord` function is wrong, see user3121023's comments. This is simpler and works. `float getCord(int placeOnArray, char str[]) { return strtof(str + placeOnArray, NULL); }` — Jabberwocky, Oct 22 '18 at 12:02
So I understand that there is no solution to this problem. Does anyone know a function in C that returns the 31-38 index from the array(str)? — papi, Oct 22 '18 at 12:03
@papi no, there is no solution, because there is no problem. See my answer. — Jabberwocky, Oct 22 '18 at 12:13
*"Does anyone know a function in C that returns the 31-38 index from the array(str)?"* Maybe `strncpy(dst, src + 31, 8); dst[8] = 0;` — pmg, Oct 22 '18 at 12:13
@papi your edit completly changed the meaning of the question.... — Jabberwocky, Oct 22 '18 at 12:17

Jabberwocky · Answer 1 · 2018-10-22T12:20:22.853

strtof works as expected:

Consisder this little program:

#include <stdio.h> 
#include <stdlib.h>

int main () 
{
  float f = strtof("4.890", NULL);
  printf("%10.10f\n", f);
}

The output is:

4.8899998665

There is no exact representation of 4.890 in floating point. You need to read this SO article:

Is floating point math broken?

Also your getCord function is wrong:

This is a simpler and correct alternative:

float getCord(int placeOnArray, char str[]) {
  return strtof(str + placeOnArray, NULL);
}

There is no need to extract the string representing the number, you can directly scan for the number in the original string. strtof will stop at the first character different from 0-9 and '.'. Change the first line in the program above to float f = strtof("4.890 ", NULL);, the output will be the same.

score 0 · Answer 2 · answered Oct 22 '18 at 11:52

desired result: -4.890 ,the returned result: -4.88999987

There will always be some error in a floating point operation. This is expected and should be taken care of by the programmer.

That is why you cannot use f1==f2 for floating point numbers. You need to check that the numbers are within the given error range. For floats this is FLT_EPSILON

score 0 · Answer 3 · answered Oct 22 '18 at 13:28

//
//  main.c
//
//
//  Created by myhaspl on 2018/10/22.
//   myhaspl@myhaspl.com.
//   https://dmyhaspl.github.io/
//

#include <stdlib.h>
#include <stdio.h>
char mytoken[10];
char* getCord(int placeOnArray, char mystr[80]){
    const char s[2] = " ";
    char str2[7];

    char* token=mytoken;
    for (int i=placeOnArray ;i<placeOnArray+7;i++){
        str2[i-placeOnArray]=mystr[i];
    }
    strcpy(mytoken,strtok(str2, s));
    printf("%s\n",mytoken);
    return mytoken;
}
int main(){
    char str[80] = "ATOM 1 N ASN A 1 -4.890 -12.296 -15.280 1.00 30.36 N";
    char* token=getCord(17,str);
    printf("%s\n",token);
    float number=atof(token);
    printf( "%f\n", number );
    return 0;
}

-4.890
-4.890
-4.890000

strtof- return a wrong number

So I understand that there is no solution to this problem. Does anyone know a function in C that returns the 31-38 index from the array(str)?

3 Answers3