Parentheses in C, how?

Question

I want to write this equation in C but I don't know how to use the right parentheses.

Eq=sqrt(e^(-((T-thr))/T) )   + (1-a)/4

If I've interpreted your formula correctly `eq = sqrt(exp(-1 * (T - thr) / T)) + ((1 - a) / 4)` ought to do it. But I'm not sure - it might be `eq = (sqrt(exp(-1 * (T - thr) / T)) + (1 - a)) / 4`. I'm not sure what you're trying to do with the `/ 4` term - are you trying to divide the entire preceding expression by four, or just the `(1 - a)`? What the heck, it's only software - try 'em both and see what you like better - unless you're building something like a nuclear reactor control system, in which case you might want to review this carefully before implementing anything. Best of luck. — Bob Jarvis - Слава Україні, May 06 '15 at 01:14
What exactly does "I don't know how to use the right parenthesis" mean? — Keith Thompson, May 06 '15 at 01:14
@BobJarvis: Why do you replace the unary `-` with a multiplication by `-1`? — Keith Thompson, May 06 '15 at 01:15
I believe OP meant "...how to use parentheses correctly". YMMV. — Bob Jarvis - Слава Україні, May 06 '15 at 01:15
I thought that by "right parentheses" you mean the `)` character. Did you really mean "how to use parentheses correctly"? — Keith Thompson, May 06 '15 at 01:28

rob mayoff · Accepted Answer · 2015-05-06T01:34:01.030

In C, the ^ operator is not exponentiation. Instead, in C, we write e^x as exp(x). Other than that, your equation is the same in C. I would put spaces around some of the operators, though:

Eq = sqrt(exp(-(T - thr) / T)) + (1 - a) / 4;

I have assumed that your variables (T, thr, and a) are a floating-point type (float or double). If they are integers, you probably want to force the compiler to use floating-point arithmetic, which you can do (for example) like this:

Eq = sqrt(exp(-((double)T - thr) / T)) + (1 - a) / 4.0;

Also... -(T - thr) is the same as (thr - T), so we can simplify:

Eq = sqrt(exp((thr - (double)T) / T)) + (1 - a) / 4.0;

And (a^b)^c = a^{b c}, which we can apply to the square root of the exponential: √(e^x) = (e^x)^1/2 = e^x/2. So we can eliminate the square root:

Eq = exp((thr - (double)T) / (2 * T))) + (1 - a) / 4.0;

Parentheses in C, how?

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