Boolean parameter with check up

Posted on 2014-11-26
Medium Priority
Last Modified: 2014-11-28
I try to find out how the logic of this code is: msFrameSpan /= upsampleFPS ? 2 : 1;
So I want to learn how to use the questionmark to give a boolena parameter. Where I can find information about this?
Question by:Ingo Foerster
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LVL 86

Expert Comment

ID: 40467550
This is called the 'ternary operator', you will find a clear and concise description at http://www.cprogramming.com/reference/operators/ternary-operator.html or http://www.cplusplus.com/articles/1AUq5Di1/ (as well as http://en.wikipedia.org/wiki/%3F: - WP covers other languages as well. The scoop is

(expression 1) ? expression 2 : expression 3

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If expression 1 evaluates to true, then expression 2 is evaluated.

If expression 1 evaluates to false, then expression 3 is evaluated instead.

LVL 86

Expert Comment

ID: 40467690
Or, to explain that using your own code snippet as an example:

msFrameSpan /= upsampleFPS ? 2 : 1;

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If 'upsampleFPS is non-zero, 'msFrameSpan' will be divided by 2, otherwise it will be divided by 1 - i.e. stay the same. Which also illustrates that this operator can be misused, since a
 if (upsampleFPS) msFrameSpan /= 2;

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would achieve the same, omitting an unnecessary division by 1. Not sure if every compiler is able to optimize that code to leave out that operation.

Author Comment

by:Ingo Foerster
ID: 40467795
So as I understand, if I have a boolean

x = upsampleFPS ==true ? false

So when upsampleFPS is true it will be true else false?

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LVL 86

Expert Comment

ID: 40467876
That will depend on what type 'upsampleFPS' is. If it is 'bool', 'true' or 'false' are the two options, but an expression in the sense of C/C++ is considered 'true' when it is non-null. 'false' is the equivalent to 'equal to 0'.

Author Comment

by:Ingo Foerster
ID: 40467890
upsampleFPS is a simple integer. It contain pal or ntsc.  

So bool x = upsampleFPS==25 ? true: false

So if upsampleFPS is 25 it will set x to true else to false?
LVL 86

Accepted Solution

jkr earned 2000 total points
ID: 40467915
Yes, that's it. To make the above a bit more readable, I'd add parenthesis like

bool x = upsampleFPS==25 ? true: false;
// or, just a bit nicer to read
bool x = (upsampleFPS==25) ? true: false;

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Author Closing Comment

by:Ingo Foerster
ID: 40470125
Thank you.

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