Functions returning non-integer values in C

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Many numerical functions like sqrt, sin, and cos return double; other specialized functions return other types than int. To illustrate how to deal with this, let us write and use the function atof(s), which converts the string s to its double-precision floating-point equivalent. It handles an optional sign and decimal point, and the presence or absence of either part or fractional part. Our version is not a high-quality input conversion routine; that would take more space than we care to use. The standard library includes an atof; the header <stdlib.h> declares it.

First, atof itself must declare the type of value it returns, since it is not int. The type name precedes the function name:

   #include <ctype.h>

   /* atof:  convert string s to double */
   double atof(char s[])
   {
       double val, power;
       int i, sign;

       for (i = 0; isspace(s[i]); i++)  /* skip white space */
           ;
       sign = (s[i] == '-') ? -1 : 1;
       if (s[i] == '+' || s[i] == '-')
           i++;
       for (val = 0.0; isdigit(s[i]); i++)
           val = 10.0 * val + (s[i] - '0');
       if (s[i] == '.')
           i++;
       for (power = 1.0; isdigit(s[i]); i++) {
           val = 10.0 * val + (s[i] - '0');
           power *= 10;
       }
       return sign * val / power;
   }
Second, and just as important, the calling routine must know that atof returns a non-int value. One way to ensure this is to declare atof explicitly in the calling routine. The declaration is shown in this primitive calculator (barely adequate for check-book balancing), which reads one number per line, optionally preceded with a sign, and adds them up, printing the running sum after each input:
   #include <stdio.h>

   #define MAXLINE 100

   /* rudimentary calculator */
   main()
   {
       double sum, atof(char []);
       char line[MAXLINE];
       int getline(char line[], int max);

       sum = 0;
       while (getline(line, MAXLINE) > 0)
           printf("\t%g\n", sum += atof(line));
       return 0;
   }
The declaration
   double sum, atof(char []);
says that sum is a double variable, and that atof is a function that takes one char[] argument and returns a double.

The function atof must be declared and defined consistently. If atof itself and the call to it in main have inconsistent types in the same source file, the error will be detected by the compiler. But if (as is more likely) atof were compiled separately, the mismatch would not be detected, atof would return a double that main would treat as an int, and meaningless answers would result.

In the light of what we have said about how declarations must match definitions, this might seem surprising. The reason a mismatch can happen is that if there is no function prototype, a function is implicitly declared by its first appearance in an expression, such as

   sum += atof(line)
If a name that has not been previously declared occurs in an expression and is followed by a left parentheses, it is declared by context to be a function name, the function is assumed to return an int, and nothing is assumed about its arguments. Furthermore, if a function declaration does not include arguments, as in
   double atof();
that too is taken to mean that nothing is to be assumed about the arguments of atof; all parameter checking is turned off. This special meaning of the empty argument list is intended to permit older C programs to compile with new compilers. But it's a bad idea to use it with new C programs. If the function takes arguments, declare them; if it takes no arguments, use void.

Given atof, properly declared, we could write atoi (convert a string to int) in terms of it:

   /* atoi:  convert string s to integer using atof */
   int atoi(char s[])
   {
       double atof(char s[]);

       return (int) atof(s);
   }
Notice the structure of the declarations and the return statement. The value of the expression in
   return expression;
is converted to the type of the function before the return is taken. Therefore, the value of atof, a double, is converted automatically to int when it appears in this return, since the function atoi returns an int. This operation does potentionally discard information, however, so some compilers warn of it. The cast states explicitly that the operation is intended, and suppresses any warning.

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