Hi list,
I was looking at the gr_pwr_squelch_cc- code, and there is a thing I
just want to check the meaning of. It is not as much that I don’t
believe the code works correctly, it’s just that some lines of code
confuse and scare me. 
The lines are 59 and 64,
57: case ST_MUTED:
58: if (!mute())
58: d_state = d_ramp ? ST_ATTACK : ST_UNMUTED; // If not ramping, go
straight to unmuted
60: break;
61:
62: case ST_UNMUTED:
63: if (mute())
64: d_state = d_ramp ? ST_DECAY : ST_MUTED; // If not ramping, go
straight to muted
65: break;
It might be that I don’t fully understand the “?”- construct, but if I
try to translate line 64 into an if- statement, I would do it (with my
understanding) to something like:
if(d_state = d_ramp){
d_state = ST_DECAY;
}
else{
d_state = ST_MUTED;
}
But this can’t be the way since this will always set d_state = ST_DECAY
(?) Or it could be the way, but in that case… Why the crazy ?-
construct?
As I said earlier, I’m scared and confused, since the code does it’s job
but it looks very… Suspicious
BR
Mattias
On Thu, Apr 29, 2010 at 01:55, Mattias K. [email protected] wrote:
57: case ST_MUTED:
58: if (!mute())
58: d_state = d_ramp ? ST_ATTACK : ST_UNMUTED; // If not ramping, go straight to unmuted
60: break;
The ‘condition ? expr1 : expr2’ construct is an expression that
evaluations to ‘expr1’ if ‘condition’ is true, otherwise it evaluates
to ‘expr2’.
Here, ‘condition’ is d_ramp, ‘expr1’ is ST_ATTACK, and ‘expr2’ is
ST_UNMUTED.
Thus, line 58 says:
“Assign to d_state the value of ‘ST_ATTACK’ if d_ramp is true,
otherwise assign to d_state the value of ‘ST_UNMUTED’.”
In other words, the variable d_state is the left side of an
assignment, and the right side of the assignment evaluates to either
ST_ATTACK or ST_UNMUTED, depending on whether d_ramp is true.
It may help to mentally parse line 58 with parenthesis just before
‘d_ramp’ and after ‘ST_UNMUTED’.
Specifically, this is not a test of the value of d_state.
Hope this helps with your fear and confusion 
Johnathan
On 04/29/2010 04:55 AM, Mattias K. wrote:
understanding) to something like:
?- construct?
As I said earlier, I’m scared and confused, since the code does it’s
job but it looks very… Suspicious
BR
Mattias
Ah, conditional expressions.
Let’s deal with line 64:
d_state = d_ramp ? ST_DECAY : ST_MUTED; // If not ramping, go
straight to muted
Translated into “conventional” conditionals:
if (d_ramp != 0)
{
d_state = ST_DECAY;
}
else
{
d_state = ST_MUTED;
}
In C, you can have a condition expression that just mentions the
variable name, without any explicit
comparision operator:
if (foo)
{
…
}
Which in another language would be:
if (foo not-equal 0)
{
}
So, the “?” operator isn’t all that crazy after all:
(condition) ? (clause-to-execute-if-true) :
(clause-to-execute-if-false)
–
Marcus L.
Principal Investigator
Shirleys Bay Radio Astronomy Consortium
