# and ## Operators in C

In C, # and ## operators are preprocessor operators using in macros for token manipulation. They are known as stringizing and token pasting operators and are used in macro definition with #define preprocessor. In this article, we will learn about these operators and how to use them in C programs.

Stringizing Operator (#)

The stringizing operator (#) is a preprocessor operator that causes the corresponding actual argument to be enclosed in double quotation marks. When the # operator is placed before an argument in a macro definition, it converts that argument into a string literal. It is generally used with macros in C.

Syntax

#define MACRO_NAME(arg) #arg

Example

#include <stdio.h>

// Macro definition using stringification (#)
// convert the argument x into a string
#define PRINT_STRING(x) printf(#x)

int main() {

  	// Using the macro
    PRINT_STRING(Hello Geeks);
    
    return 0;
}

Hello Geeks

Explanation: The # operator in the macro PRINT_STRING(x) converts the argument x into a string by enclosing it into double quotes. The statement PRINT_STRING(Hello Geeks) is hence converted into printf("Hello Geeks").

Note: This operator should not be confused with the # operator used for defining preprocessor directive.

Token Pasting Operator (##)

The token-pasting operator (##) allows tokens used as actual arguments to be concatenated to form other tokens. It is often useful to merge two tokens into one while expanding macros. This is called token pasting or token concatenation.

The ## pre-processing operator performs token pasting. When a macro is expanded, the two tokens on either side of each ‘##’ operator are combined into a single token, which then replaces the ‘##’ and the two original tokens in the macro expansion.

Syntax

#define MACRO_NAME(arg1, arg2) arg1 ## arg2

Examples:

#include <stdio.h>

// Macro definition using the Token-pasting operator
#define concat(a, b) a##b
int main(void) {
    int xy = 30;

    // Printing the concatenated value of x and y
    printf("%d", concat(x, y));
    return 0;
}

30

Explanation: The ## operator in the macro concat(a, b) is the token-pasting operator. It concatenates two tokens (a and b) into a single token. In this example, concat(x, y) concatenates x and y to form xy, which refers to the variable int xy = 30;. Therefore, printf("%d", concat(x, y)); prints the value of xy, which is 30.

Application of Token-pasting operator (##)

The ## provides a way to concatenate actual arguments during macro expansion. If a parameter in the replacement text is adjacent to a ##, the parameter is replaced by the actual argument, the ## and surrounding white space are removed, and the result is re-scanned.

Predict the output of the following program:

#include <stdio.h>

#define combine(a, b, c, d, e, f, g) c ## d ## e ## f
#define start combine(m, a, i, n, p, r, o)

int start()
{
    printf("Did you figure it out?\n");
    return 0;
}