CODEWITHSUNDEEP
cprintf
Fady Sadek
Fady Sadek

Reputation: 1140

How to simply convert a float to a string in order to write it to a text file in C?

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 error 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 have a C# background where something like .toString() is possible, is there anything like that in C to directly convert a float to a string?

Upvotes: 11

Views: 112472

Answers (4)

chux
chux

Reputation: 153517

to save a float number to a text file

Use "%e", "%g", "%a"

To print unique text for each float, use the format string "%.*e", "%.*g" or "%a" with sufficient precision.

FLT_DECIMAL_DIG - 1 is the number of digits needed with "%.*e"
(FLT_DECIMAL_DIG is the number of digits needed ,with "%.*g")
to print each float value uniquely without undue precision.

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

fprintf(fPointer, "%.*e", FLT_DECIMAL_DIG - 1, amount);
// or 
fprintf(fPointer, "%.*g", FLT_DECIMAL_DIG, amount);
// or 
fprintf(fPointer, "%a", amount); // Hexadecimal significand

What is nice about this is that the upper bound of characters needed can be pre-calculated if one wants to print to a string (typically about 18).

//                     -   d   .      dddddddd             e   -  expo \0
#defined FLT_STR_SIZE (1 + 1 + 1 + (FLT_DECIMAL_DIG - 1) + 1 + 1 + 4 + 1
char buffer[FLT_STR_SIZE];
snprintf(buffer, sizeof buffer, "%.*e", FLT_DECIMAL_DIG - 1, amount);

"%f" weaknesses

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".

OP's coding error's include:

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

Upvotes: 3

kumar
kumar

Reputation: 89

By using sprintf() we can convert from float to string in C. 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");
}

Upvotes: 3

Nikola Novak
Nikola Novak

Reputation: 4462

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);

Upvotes: 15

Ren&#233; Vogt
Ren&#233; Vogt

Reputation: 43886

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.

Upvotes: 8

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