Implementation of atoi and atof

atof:- double my_atof(const char *s) function converts the string argument str to a floating-point number
of (type double).

atoi:- int my_atoi(const char *s) function converts the string argument str to a number of type integer.

#include <stdio.h>
#include <string.h>
 
int find_pos(char *);
int my_atoi(const char *);
double my_atof(const char *);
 
int main()
{
    int i;
    char s[20]={0,};
    double d;
     
    printf("Enter the float string : ");
    gets(s);
     
    for(i=0;s[i];i++)
    {
        if(isalpha(s[i]))
        {
            printf("Invalid data\n");
            return 0;
        } 
    }
 
     
    d = my_atof(s);
    printf("Floating point Number = %lf\n",d);
     
    return 0;
}
 
double my_atof(const char *s)
{
    float f,f1,f2;
    int b,m,i,j,k;
    char s1[10]={0,},s2[10]={0,};
     
    m = find_pos(s);
    j = k = 0;
     
    for(i=0;i<strlen(s);i++)
    {
        if (i != m)
        {
            if(i<m)
                s1[j++]=s[i];
            else
                s2[k++]=s[i];
        }
    }
    s1[j]='\0';
    s2[k]='\0';
     
    for(b=1,i=0;i<strlen(s2);i++,b=b*10);
         
    f1 = (float)my_atoi(s1);
    f2 = (float)my_atoi(s2);
   
    f=f1+(f2/b);
    return (double)f;
     
}
 
int find_pos(char *s)
{
    int i;
    for(i=0;i<strlen(s);i++)
    {
        if (s[i] == '.')
            return i;
    }
}
 
int my_atoi(const char *s)
{
    int n=0;
    while(*s)
    {
        n = 10*n+((*s)-48);
        s++;
    }
    return n;
}

Output

Enter the float string : 44
Floating point Number = 44.000000

Enter the float string : wr45
Invalid data