c++ switch [Error] 'x' cannot appear in a constant-expression - c++

Question

im trying to execute this script its for my university
int main()
{
int x;
double y;
Provo:
cout<<"Vlera e X: ";
cin>>x;
switch(x)
{
case ((x)<(0.9)):
y=x*x;
break;
case (x==0.9):
y=2*x;
break;
case 'x>0.9':
y=x-3;
break;
}
cout<<"\n\n";
return 0;
}
this is teh code and the error is:
17 10 ....\Untitled1.cpp [Error] 'x' cannot appear in a constant-expression
someone help please?

Answer 1

I think you have misunderstood how to use the switch statement. switch is used to branch your code based on the condition, in your case x, taking different integral values. It is not suitable for use with double values like you do.
A correct switch expression would look like:
switch(x)
{
case 1:
y=x*x;
break;
case 2:
case 3:
case 4:
y=2*x;
break;
case 5:
y=x-3;
break;
}
To accomplish what you want, use if else instead, for example:
if (x < 0.9) {
y=x*x;
else if(x == 0.9) {
y=2*x;
} else {
y=x-3;
}
However, comparing floating point values for equality is a bad idea. It is usually better to do something like:
double epsilon = <some small value>;
if (x < 0.9-epsilon) {
y=x*x;
else if(x > 0.9+epsilon) {
y=x-3;
} else {
y=2*x;
}