Question

Write a C function str_to_float() that takes a numeric string as input and converts it to a floating
point number. The function should return the floating point number by reference. If the
conversion is successful, the function should return 0, and -1 otherwise (e.g. the string does not
contain a valid number). The prototype of the function is given below
int str_to_float(char * str, float * number);

Answer 1

Solution

#include <stdio.h>

int str_to_float(char * str, float * number)
{
int i =0;
float f=0.0;
int res;
while(str[i]!='\0')
{
if(str[i]<48 || str[i]>57)
{
return -1;
}
int x= (int)str[i]-48; /* To get the digit from the character of the string */
f = f*10 + x; /* f is the converted floating point number */
i++;
}
*number=f; /* number is a pointer to float so store f at the address stored in number */
return 0;
}

int main()
{
char string[100];
float num=0.0;
int result;
printf("Enter a numeric string ");
scanf("%s", string);
result = str_to_float(string,&num);
if(result==-1)
{
printf("Entered string was not a valid numeric string");
}
if(result==0)
{
printf("The converted floating point number is %f",num);
}
return 0;
}

Explanation

The ASCII value of digits 0 to 9 is given below:

Digit	ASCII Value
0	48
1	49
2	50
3	51
4	52
5	53
6	54
7	55
8	56
9	57

So we retrieve a character one by one from the string typecast it to integer(We get the ASCII value in this step). Next convert the ASCII value to numeric digit 0 to 9 by subtracting 48 from the ASCII value.

Screenshot

Output

Example of valid number

Example of invalid number