The # and ## operators are preprocessing operators in C. The # (stringize) operator encloses the actual argument in double quotes when a macro is expanded. The ## (token pasting) operator concatenates two tokens used as actual arguments to form a new token when a macro is expanded. Examples are given demonstrating how each operator works in macro definitions and function calls.

1. # and ## Operators in C

2. Stringizing operator (#)
• This operator causes the corresponding actual argument to be enclosed in double
quotation marks. The # operator, which is generally called the stringize operator,
turns the argument it precedes into a quoted string. For more on pre-processor
directives – refer this
Examples :
• The following preprocessor turns the line printf(mkstr(geeksforgeeks));into
printf(“geeksforgeeks”);
• // CPP program to illustrate (#) operator
• #include <stdio.h>
• #define mkstr(s) #s
• int main(void)
• {
• printf(mkstr(geeksforgeeks));
• return 0;
• }
• Output:
• geeksforgeeks

3. In this program, value of a is replaced
by macro.
• // CPP program to illustrate (#) operator
• #include <iostream>
• using namespace std;
• #define a 8.3297
• int main()
• {
• cout << "Value of a is " << a << endl;
• return 0;
• }
• Output:
• Value of a is 8.3297

4. This program finds out maximum out of two numbers
using macro
• // CPP program to illustrate (#) operator
• #include <iostream>
• using namespace std;
• #define MAX(i, j) (((i) > (j)) ? i : j)
• int main()
• {
• int a, b;
• a = 250;
• b = 25;
• cout << "The maximum is " << MAX(a, b) << endl;
• return 0;
• }
• Output:
•
The maximum is 250

5. 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.
Examples :
• The preprocessor transforms printf(“%d”, concat(x, y));
into printf(“%d”, xy);

6. The preprocessor transforms printf(“%d”, concat(x, y));
into printf(“%d”, xy);
• // CPP program to illustrate (##) operator
• #include <stdio.h>
• #define concat(a, b) a##b
• int main(void)
• {
• int xy = 30;
• printf("%d", concat(x, y));
• return 0;
• }
• Output:
• 30

7. • Application: 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.

8. # and ## Operators in C ?
• In this section we will see what are the Stringize operator(#) and
Token Pasting operator(##) in C. The Stringize operator is a
preprocessor operator. It sends commands to compiler to convert a
token into string. We use this operator at the macro definition.
• Using stringize operator we can convert some text into string
without using any quotes.
• Example
• #include<stdio.h>
• #define STR_PRINT(x) #x
• main()
• {
• printf(STR_PRINT(This is a string without double quotes));
• }
• Output
• This is a string without double quotes

9. • The Token Pasting operator is a preprocessor operator.
It sends commands to compiler to add or concatenate
two tokens into one string. We use this operator at the
macro definition.
• Example
• Live Demo
• #include<stdio.h>
• #define STR_CONCAT(x, y) x##y
• main()
• {
• printf("%d", STR_CONCAT(20, 50));
• }
• Output
• 2050