CODEWITHSUNDEEP
iosobjective-cpreprocessor-directive
Mikayil Abdullayev
Mikayil Abdullayev

Reputation: 12367

Getting different result with the same number

I have defined to preprocessor directives:

#define PTM_RATIO 32
#define PTM_RATIO_2 isIPad?64:32

Then inside a method I use the above directives like this:

NSLog(@"PTM_RATIO %d",PTM_RATIO);
NSLog(@"PTM_RATIO_2 %d",PTM_RATIO_2);

float32 test=100*PTM_RATIO;
float32 test2=100*PTM_RATIO_2;

NSLog(@"Test %f",test);
NSLog(@"Test2 %f",test2);

Here's the output of the above code:

2013-03-25 23:43:57.011 Box2d[3625:15203] PTM_RATIO 32
2013-03-25 23:43:57.019 Box2d[3625:15203] PTM_RATIO_2 32
2013-03-25 23:43:57.020 Box2d[3625:15203] Test 3200.000000
2013-03-25 23:43:57.021 Box2d[3625:15203] Test2 32.000000

As you can see, both PTM_RATIO and PTM_RATIO_2 are 32. Then why do I get 3200.000000 when 100 is multiplied by PTM_RATIO, but 32.000000 when it's multiplied by PTM_RATIO_2?

Upvotes: 1

Views: 54

Answers (2)

trumpetlicks
trumpetlicks

Reputation: 7065

The PTM_RATIO_2 is evaluated as follows in real code

float32 test2 = (100*isIPad)?64:32;

You are assuming that the MACRO will evaluate prior to live code calculation.

Upvotes: 1

Eric Postpischil
Eric Postpischil

Reputation: 222244

Preprocessor macros are largely text substitutions. Change:

#define PTM_RATIO_2 isIPad?64:32

to:

#define PTM_RATIO_2 (isIPad?64:32)

Without that, your source code:

100*PTM_RATIO_2

expanded to:

100*isIPad?64:32

which is then parsed as:

(100*isIPAD) ? 64 : 32

Upvotes: 3

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