CODEWITHSUNDEEP
cfloating-pointprintf
Jennifer
Jennifer

Reputation: 119

Return printf val of float of type char *str

Possible Duplicate

Function should return printf value of float of type char str*

Inputs:

char *string = "%f";
float k = 1.0005325678910

Function:

My function takes input string %f and float value 1.0005325678910 as inputs storing the value of printf in char *answer.Here I am unable to allocate memory for char *answer

int length(char* string){
     int i = 0;
     while (string[i] != '\0')
        i++;
    return i;
 }

 char *res(char *string, float k){
      int len = length(string); //len is a function which return length of the string
      char *answer = (char*)malloc(sizeof(char)*len);
      sprintf(answer,format, n);
      //printf("%05d", 50);
      //printf("%s", sqlAnswers);
     return answer;
 }

Expected Output:

char *output =  1.000532;

Bug: allocation of memory to char *answer Output: 1.000533

My code should not round the last digit.

Edit: Ex: If my float k = 6.143211 then expected Output should be 6.14 but the function returns 6.15 which rounds the last digit.

Upvotes: 0

Views: 90

Answers (2)

Eric Postpischil
Eric Postpischil

Reputation: 223553

It appears you are trying to do this:

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


char *res(char *format, float k)
{
    //  Ask snprintf to report how many bytes are needed.
    int t = snprintf(NULL, 0, format, k);
    if (t < 0)
    {
        fprintf(stderr, "Encoding error.\n");
        exit(EXIT_FAILURE);
    }

    //  Allocate space for those bytes and a null terminator.
    char *result = malloc(t+1);
    if (!result)
    {
        fprintf(stderr, "Allocation error.\n");
        exit(EXIT_FAILURE);
    }

    //  Format the number.
    snprintf(result, t+1, format, k);

    return result;
}


int main(void)
{
    char *b = res("%f", 1.0005325678910);

    puts(b);

    free(b);
}

Upvotes: 2

Goswin von Brederlow
Goswin von Brederlow

Reputation: 12332

The general C way to do this is to pass in a char * buf and size_t len. Then return how many chars have been or would have been written to the buffer.

But if you want to write a function that allocates a buffer of the right size you have 2 choices:

1) always allocate the maximum size for "%2f". Given the size that's probably the best. It's not worth the CPU overhead to compute the right size to save a byte here or there.

2) find out the right length using:

char c;
int len = snprintf(&c, 1, "%2f, k);
assert(len >= 0);
++len;

or output to a fixed sized buffer and then use strdup(). Sort of a combination of 1 and 2.

Upvotes: 0

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