How to simply convert a float to a string in c?

Question

FILE * fPointer;
float amount = 3.1415;
fPointer =fopen("vending.txt","w");
fprintf(fPointer ,amount);
printf("The file has been created for the first time and we added the value %f" , amount);
fclose(fPointer);

I am trying to save a float number to a text file but when i try to run this code it triggers a compiling errors because the function fprintf expects the second parameter to be an array of characters so how can i convert my float to a string so i can pass it , i come from a c# background where something like .toString() is possible so is there any thing like that in c to directly cast a float to a string ?

it was a mistake but somebody already deleted them for me , thanks . — Fady Sadek, Dec 30 '16 at 09:51
No this question is in c while the link you have pasted is a solution to a problem to c++ — Fady Sadek, Dec 30 '16 at 10:07
Voting to re-open as post was mis-tagged, tags corrected, yet dupe refers to another language's answer that is irrelevant to his question. — chux - Reinstate Monica, Dec 30 '16 at 12:50

René Vogt · Accepted Answer · 2019-06-12T14:09:26.530

8

The second parameter is the format string after which the format arguments follow:

fprintf(fPointer, "%f", amount);

%f tells fprintf to write this argument (amount) as string representation of the float value.

A list of possible format specifiers can (for example) be found here.

edited Jun 12 '19 at 14:09

answered Dec 30 '16 at 09:51

René Vogt

41,709
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Suggest `"%e"` instead of `"%f"`. The latter will print nearly half of all `float` as `"0.000000'` or `"-0.000000'`. – chux - Reinstate Monica Dec 30 '16 at 12:37

score 6 · Answer 2 · edited Jan 19 '22 at 17:41

If you can use C99 standard, then the best way is to use snprintf function. On first call you can pass it a zero-length (null) buffer and it will then return the length required to convert the floating-point value into a string. Then allocate the required memory according to what it returned and then convert safely.

This addresses the problem with sprintf that were discussed here.

Example:

int len = snprintf(NULL, 0, "%f", amount);
char *result = malloc(len + 1);
snprintf(result, len + 1, "%f", amount);
// do stuff with result
free(result);

score 4 · Answer 3 · answered Dec 30 '16 at 10:00

By using sprintf() we can convert from float to string in c language for better understanding see the below code

#include <stdio.h>
int main()
{
   float f = 1.123456789;
   char c[50]; //size of the number
    sprintf(c, "%g", f);
    printf(c);
    printf("\n");
}

Hope this will help you.

score 1 · Answer 4 · answered Dec 30 '16 at 12:58

// OP's code           v---- Format string expected here. 
// fprintf(fPointer, amount);

To print different text for each float, use the format string "%.*e".
FLT_DECIMAL_DIG - 1 is the number of digits needed to print each float value uniquely without undue precision.

#include <float.h>
#include <stdio.h>

fprintf(fPointer, "%.*e", FLT_DECIMAL_DIG - 1, amount);

Using "%f" will print nearly half of small float like 0.000000001f and -1.2345e-10 as 0.000000 or -0.000000.

Using "%f" will print large float like FLT_MAX with verbose text like "340282346638528859811704183484516925440.000000".

How to simply convert a float to a string in c?

4 Answers4