"lvalue required asleft opperand of assignment"

[MatthewK]

Hey, I have a code that uses the atan function but I want it to be able to say what the angle is given what quadrant you are in but I keep getting an error message (title) when I do it.

This is the part of the code that is getting the error

if(xRand < 0 && (yRand < 0 || yRand > 0))
atan(xRand/yRand) += pi;

I have defined pi = 4*atan(1) and am using #include<math.h>

[firedraco]

atan(xRand/yRand) += pi;

Tell me what this line is trying to do.

[MatthewK]

If the x component is negative (which I guess I can get rid of yRand because it can be either positive or negative) then the angle will be off by pi so I want to add pi to the resulting angle to fix it.

[MiiNiPaa]

And where will it be stored?
Even if would work, you wold discard resulting angle immideatly.

angle will be off by pi so I want to add pi to the resulting angle to fix it.

There is atan2() function for that:
http://en.cppreference.com/w/cpp/numeric/math/atan2

[MatthewK]

Ooh, okay. I didn't know that. But what if I wanted the angles to be between 0 and 2pi, not -pi to pi? I just tried that same thing kinda but with:

if(atan2(xRand,yRand) < 0)
atan2(xRand,yRand) += (2*pi);


And got the same error. What can I do to fix it? I don't care about the old atan2 value in my program.

[MiiNiPaa]

Ok, assume it was succesfull.
How will you get this value?

[MatthewK]

I use it in a function in an if statement right after. Will the value be deleted? Should I make it it's own function above my main one and call it?

[MiiNiPaa]

No, it is not deleted. it is not stored anywhere.
What you doing is essentually this:
7 += 3;
You should assign resul of atan to a value first.
Like: double x = atan2(xRand,yRand);
Actually you are gerring wrong result here: it is atan2(yRand, xRand)
Yoiu can do something like:

double x = atan2(xRand,yRand);
x < 0? x+= 2*pi:x;

[MatthewK]

Ooohh, okay. That makes a lot of sense. I got it to work :). I appreciate this MiiNiPaa!