atof() and strtod() Functions in C

Question: Which String Conversion Functions in Standard C Library Return Type “double” Values?

Answer: Following two functions take string as arguments and convert them into double values.

    double atof(char const *string);
    double strtod(char const *string, char **ptr2loc);

These are defined in ‘stdlib.h’ header. Notice that if second argument to ‘strtod()’ function is not NULL then it points to location where is saved pointer-to-first-character in non-converted part of string. If string argument doesn’t contain valid numeric value, both functions return ZERO. Also, any leading white space characters in the string argument are skipped by each function. Let’s see an example C program below

/* fp_str_con.c -- string conversion functions return double values */
/* double atof(char const *string) */
/* double strtod(char const *string, char **p2cs) */
#include <stdio.h>
#include <stdlib.h>
#define SIZE 100
 
int main(void)
{
    char string[SIZE];
    double ret;
    char **p2unused;
 
    /* read some string from the user */
    printf("User, type in some string with legal numeric value "
           "within it...\n");
    gets(string);
 
    /* atof() takes just string argument */
    ret = atof(string);
    printf("String \"%s\" converted to: %lf\n", string, ret);
 
    /* call strtod() to convert string to double value */
    ret = strtod(string, p2unused);
    printf("String \"%s\" converted to: %lf\n", string, ret);
    printf("Part of \"%s\" not converted:\"%s\"\n", string, *p2unused);
 
    return 0;
}

Output for some arbitrary values as follows

User, type in some string with legal numeric value...
       23.9 hello dear
String "       23.9 hello dear" converted to: 23.900000
String "       23.9 hello dear" converted to: 23.900000
    return 0;
}
 
Output for some arbitrary values as follows
 
<pre lang="C" cssfile="hk1_style">
User, type in some string with legal numeric value...
       23.9 hello dear
String "       23.9 hello dear" converted to: 23.900000
String "       23.9 hello dear" converted to: 23.900000
Part of "       23.9 hello dear" not converted: " hello dear"
 
User, type in some string with legal numeric value...
where are you? 123
String "where are you? 123" converted to: 0.000000
String "where are you? 123" converted to: 0.000000
Part of "where are you? 123" not converted: "where are you? 123"
 
User, type in some string with legal numeric value...
23e-05 why this?
String "23e-05 why this?" converted to: 0.000230
String "23e-05 why this?" converted to: 0.000230
Part of "23e-05 why this?" not converted: " why this?"

Notice that any leading white space characters were skipped while trailing illegal characters after valid numeric value were ignored.

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