C - Unexpected output with #define

Question

When i compile the following program I get the output as 47. I expected the output to be 144

#include<stdio.h>
#define FIRST_PART 7
#define LAST_PART 5
#define ALL_PARTS FIRST_PART + LAST_PART
int main() {
printf ("The Square of all parts is %d", ALL_PARTS * ALL_PARTS) ;
return(0);
}

Welcome to SO! Try minimize you example, and make the title more precise: unexpected how? — Ciro Santilli OurBigBook.com, Oct 23 '15 at 11:44
Use your compiler settings to output the preprocessed file. You will find that your expectations are wrong, as this is correctly calculated as `7 + 5*7 + 5`, which indeed is `47`. — Jongware, Oct 23 '15 at 11:46

score 1 · Accepted Answer · answered Oct 23 '15 at 11:45

The preprocessor works before the compiler. It is a simple 'stupid' text-replace mechanism. Therefore:

ALL_PARTS gets converted to 7 + 5
ALL_PARTS * ALL_PARTS gets converted to 7 + 5 * 7 + 5.

Because of this mechanism, it is recommended to surround the parameters of macros with parentheses and also surround the whole macro with parentheses, such as:

#define ALL_PARTS (FIRST_PART + LAST_PART)

score 1 · Answer 2 · answered Oct 23 '15 at 11:45

FIRST_PART + LAST_PART will evaluate as 7 + 5 (It's no more than a text substitution).

You expression is then 7 + 5 * 7 + 5 which is 7 + 35 + 5 (as multiplication has a higher precedence than addition). The value of 7 + 35 + 5 is, of course, 47.

A remedy is to use (FIRST_PART + LAST_PART) as your definition.

score 0 · Answer 3 · answered Oct 23 '15 at 11:45

ALL_PARTS * ALL_PARTS

Will become

FIRST_PART + LAST_PART * FIRST_PART + LAST_PART

Which is

7 + 5 * 7 + 5 = 7 + 35 + 5 = 47

So 47 is the expected answer.

If you want to see your expected result, you need to add () to your define

#define ALL_PARTS (FIRST_PART + LAST_PART)

ashiquzzaman33 · Answer 4 · 2015-10-23T11:51:23.773

0

Your Preprocessor evaluated as

printf ("The Square of all parts is %d", ALL_PARTS * ALL_PARTS) ;

then

printf ("The Square of all parts is %d", FIRST_PART + LAST_PART * FIRST_PART + LAST_PART) ;

and then

printf ("The Square of all parts is %d", 7 + 5 * 7 + 5) ;

So your final result is 47

To get desired result you can use

#define ALL_PARTS (FIRST_PART + LAST_PART)

edited Oct 23 '15 at 11:51

answered Oct 23 '15 at 11:46

ashiquzzaman33

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score 0 · Answer 5 · answered Oct 23 '15 at 11:46

0

Just need to change this:

#define ALL_PARTS FIRST_PART + LAST_PART

To this:

#define ALL_PARTS (FIRST_PART + LAST_PAR)

answered Oct 23 '15 at 11:46

Hayden

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score 0 · Answer 6 · answered Oct 23 '15 at 11:47

0

FIRST_PART is 7 LAST_PART is 5

FIRST_PART + LAST_PART is 7 + 5

7 + 5 * 7 + 5 = 7 + 35 + 5 = 47

To fix, do

#define ALL_PARTS ( FIRST_PART + LAST_PART )

answered Oct 23 '15 at 11:47

Magisch

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score 0 · Answer 7 · answered Oct 23 '15 at 11:47

0

Your code is expanded to 7+5*7+5 = 7+35+5 = 47. This is a very common failure.

answered Oct 23 '15 at 11:47

johnjohnlys

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C - Unexpected output with #define

7 Answers7