I tried \"x = y ** e\", but that didn\'t work.
To add to what Evan said: C does not have a built-in operator for exponentiation, because it is not a primitive operation for most CPUs. Thus, it's implemented as a library function.
Also, for computing the function e^x, you can use the exp(double), expf(float), and expl(long double) functions.
Note that you do not want to use the ^ operator, which is the bitwise exclusive OR operator.