What does this C expression mean?

Question

Looking for some code to reuse in my C app I came across this expression:

MDO = ((output_data & 0x80) ? 1 : 0);

I understand what is between brackets but what does ? 1 : 0 mean?

score 4 · Accepted Answer · answered Nov 11 '13 at 08:08

It is shorthand for if-else, called the ternary operator.

In your case it is equal to:

if (output_data & 0x80) {
   MDO = 1;
} else {
   MDO = 0;
}

And a little word of advice, don't use it for complicated if constructs, it hinders readability. Only use it in cases like this, where it can be understood immediately.

score 3 · Answer 2 · answered Nov 11 '13 at 08:09

'?' is the ternary operator, it's a short-hand for

if ((output_data & 0x80) != 0) { MDO = 1; } else { MDO = 0; }

which will assign 1 to MDO if output_data has bit 8 set (0x80 = 128 = bit 8), otherwise MDO gets the value of 0

A simpler example: There are 10 beers on Friday, otherwise there's only 2.

int beers = (day == Friday) ? 10 : 2;

score 1 · Answer 3 · answered Nov 11 '13 at 08:08

1

it means if the expression is true than MDO will have the value of 1 else 0

answered Nov 11 '13 at 08:08

baliman

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odedsh · Answer 4 · 2013-11-11T08:43:58.813

1

output_data& 0x80 is a bitwise binary and. So it returns 0x80 if that particualr bit is on out_putdata otherwise it returns 0

MDO= ? :

Is the same as

if (expression)
    MDO = 1    
else
    MDO = 0

so MDO will be 1 if the bit is on on output_data otherwise it is 0

edited Nov 11 '13 at 08:43

answered Nov 11 '13 at 08:10

odedsh

2,564
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It is not the same. The ternary operator will not `return`. – Bart Friederichs Nov 11 '13 at 08:12
I mean return the expression no 'return' from the function.. fixing to make clearer – odedsh Nov 11 '13 at 08:43

What does this C expression mean?

4 Answers4