Functions returning non-integer values in C

By: Baski Emailed: 1728 times Printed: 2338 times    

Latest comments
By: rohit kumar - how this program is work
By: Kirti - Hi..thx for the hadoop in
By: Spijker - I have altered the code a
By: ali mohammed - why we use the java in ne
By: ali mohammed - why we use the java in ne
By: mizhelle - when I exported the data
By: raul - no output as well, i'm ge
By: Rajesh - thanx very much...
By: Suindu De - Suppose we are executing

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.

C Home | All C Tutorials | Latest C Tutorials

Sponsored Links

If this tutorial doesn't answer your question, or you have a specific question, just ask an expert here. Post your question to get a direct answer.



Bookmark and Share

Comments(0)


Be the first one to add a comment

Your name (required):


Your email(required, will not be shown to the public):


Your sites URL (optional):


Your comments:



More Tutorials by Baski
Compiling multiple source files and specifying classpath using javac
The equals() Method example in Java
Default Values for Data Types in Java
JSP Example to connect to MS SQL database using Tomcat Connection Pool
<convertNumber> and <convertDateTime> in JSF
Enable/Disable Scripting Elements in JSP
Using calloc() Function in C
lseek() sample program in C
Arrays of Structures example program in C
The Basic Syntax Expression Language in JSP
Initialization of Pointer Arrays in C
Functions returning non-integer values in C
Increment and Decrement Operators in C
Getting Started with C
What is JSF (JavaServer Faces)?

More Tutorials in C
Sum of the elements of an array in C
Printing a simple histogram in C
Sorting an integer array in C
Find square and square root for a given number in C
Simple arithmetic calculations in C
Command-line arguments in C
Calculator in C
Passing double value to a function in C
Passing pointer to a function in C
Infix to Prefix And Postfix in C
while, do while and for loops in C
Unicode and UTF-8 in C
Formatting with printf in C
if, if...else and switch statements in C with samples
Statements in C

More Latest News
Most Viewed Articles (in C )
Using memset(), memcpy(), and memmove() in C
scanf and sscanf sample program in C
UNIX read and write system calls sample program in C
perror() Function - example program in C
Formatting with printf in C
Printing a simple histogram in C
lseek() sample program in C
Unicode and UTF-8 in C
Using free() Function in C
assert() Function Example program in C
Symbolic Constants using #define in C
External Variables and Scope in C
Functions returning non-integer values in C
Macro Substitution using #define in C
Pointers and Arrays in C
Most Emailed Articles (in C)
Listing Files and Directories sample program in C
Increment and Decrement Operators in C
Error Handling - Stderr and Exit - sample program in C
switch in C
register Variables in C
Arrays sample program in C
Character Arrays in C
Arithmetic Operators in C
A C program similar to grep command in UNIX
Variable Scope and functions in C
Static Variables in C
Recursion in C
Pointers and Arrays in C
Character Pointers and Functions in C
Pointers to Structures example program in C