use of #define in c

Question

#include<stdio.h>
#include<stdlib.h>

#define d 10+10

int main()
{
    printf("%d",d*d);
return 0;
}

I am new to the concept of macros.I found the output for the above program to be 120.What is the logic behind it?

Thanks.

Use cpp source.c to see how is expanded

David Ranieri
– David Ranieri

2013-01-15 20:03:50 +00:00
Commented Jan 15, 2013 at 20:03 — David Ranieri
– David Ranieri, Commented Jan 15, 2013 at 20:03

ouah · Accepted Answer · 2013-01-15 19:57:20Z

5

10+10*10+10 == 10 + 100 + 10

Is that clear?

answered Jan 15, 2013 at 19:57

ouah

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Comments

Kerrek SB · Accepted Answer · 2013-01-15 19:57:59Z

3

Macros are replaced literally. Think of search/replace. The compiler sees your code as 10+10*10+10.

It is common practice to enclose macro replacement texts in parentheses for that reason:

#define d (10 + 10)

This is even more important when your macro is a function-like macro:

#define SQ(x) ((x) * (x))

Think of SQ(a + b)...

answered Jan 15, 2013 at 19:57

Kerrek SB

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Comments

Ryan Cavanaugh · Accepted Answer · 2013-01-15 19:58:12Z

1

d*d expands into 10+10*10+10. Multiplication comes before addition, so 10 + 100 + 10 = 120.

In general, #define expressions should always be parenthesized: #define d (10+10)

answered Jan 15, 2013 at 19:58

Ryan Cavanaugh

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Comments

James M · Accepted Answer · 2013-01-15 19:58:36Z

1

A macro is a nothing more than a simple text replacement, so your line:

printf("%d",d*d);

becomes

printf("%d",10+10*10+10);

You could use a const variable for more reliable behaviour:

const int d = 10+10;

answered Jan 15, 2013 at 19:58

James M

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Comments

pmg · Accepted Answer · 2013-01-15 19:58:48Z

1

The macro is expanded as is. Your program becomes

/* declarations and definitions from headers */

int main()
{
    printf("%d",10+10*10+10);
return 0;
}

and the calculation is interpreted as

10 + (10 * 10) + 10

Always use parenthesis around macros (and their arguments when you have them)

#define d (10 + 10)

answered Jan 15, 2013 at 19:58

pmg

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Comments

Paolo · Accepted Answer · 2013-01-15 19:59:02Z

1

#define

preprocessor directive substitute the first element with the second element.

Just like a "find and replace"

answered Jan 15, 2013 at 19:59

Paolo

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Comments

Matthew Blott · Accepted Answer · 2013-01-15 20:00:12Z

1

I'm not sure about #include but in C# #define is used at the top to define a symbol. This allows the coder to do things like

#define DEBUG

string connStr = "myProductionDatabase";

#if DEBUG
    connStr = "myTestDatabase"
#edif

answered Jan 15, 2013 at 20:00

Matthew Blott

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Comments

Adam Sznajder · Accepted Answer · 2013-01-15 19:57:27Z

0

10+10*10+10 = 20 + 100 = 120

Simple math ;)

Macro doesn't evaluate the value (it doesn't add 10 + 10) but simply replaces all it's occurences with the specified expression.

answered Jan 15, 2013 at 19:57

Adam Sznajder

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