Colleagues,
I was surprised today when an if() statement failed to execute as expected. In the code below when parameter 'a' is 0x00 or 0, the if statement should always evaluate as true. It does not, and instead the 'else' statement is evaluated.
I am using uVision v3.53 and executing on an NXP LPC2368 which is ARM7 TDMI.
unsigned char TestStrange ( unsigned char a ) { INT16 i; unsigned char b = 0; for(i=0; i<8; i++) { if( ((a) >> (7 - i)) & 0x01 == 0) b = 0xFF; else b = 0x0F; } return(b); }
A work around is to evaluate the contents of the if() ahead of time, as shown below.
unsigned char TestStrange ( unsigned char a ) { INT16 i; unsigned char c, b = 0; for(i=0; i<8; i++) { c = ((a) >> (7 - i)) & 0x01; if( c == 0) b = 0xFF; else b = 0x0F; } return(b); }
Does anyone know why this is happening?
Thank you,
Yes, you are correct. You have a faulty if statement.
Is there any difference between the following two?
if( ((a) >> (7 - i)) & 0x01 == 0)
and
if( ((a >> (7 - i)) & 0x01) == 0)
I don't know why you think you need parentheses around a variable name - it is already an atomic primitive.
Have you checked the precedence rules for all operators you use in your expression?
Look at this table, and see if something seems a bit strange in relation to your if statement: www.difranco.net/.../op-prec.htm
Notice that == is above & in the table?
To reduce your expression a bit:
if (0 & 0x01 == 0) ...
is evaluated as
if (0 & (0x01 == 0)) ...
which can be reduced to
if (0 & 0) ...
further reduced to
if (0) ...
Per,
The extra () were and artifact of the function I orignially had the problem with.
And yes, it was an operator precedence issue. I didn't realize == evaluated before &.
Thanks,