C# Cookbook

3.11. Making Error-Free Expressions

Problem

A complex expression in your code is returning incorrect results. For example, if you wanted to find the average area given two circles, you might write the following expression:

double radius1 = 2;
double radius2 = 4;
double aveArea = .5 * Math.PI * Math.Pow(radius1, 2) + Math.PI * 
                 Math.Pow(radius2, 2);

However, the result is always incorrect.

Complex mathematical and Boolean equations in your code can easily become the source of bugs. You need to write bug-free equations, while at the same time making them easier to read.

Solution

The solution is quite simple: use parentheses to explicitly define the order of operations that will take place in your equation. To fix the expression presented in the Problem section, rewrite it as follows:

double radius1 = 2;
double radius2 = 4;
double aveArea = .5 * (Math.PI * Math.Pow(radius1, 2) + Math.PI * 
                 Math.Pow(radius2, 2));

Notice the addition of the parentheses; these parentheses cause the area of the two circles to be calculated and added together first. Then the total area is multiplied by .5. This is the behavior we are looking for. An additional benefit is that the expression can become easier to read as the parentheses provide clear distinction of what part of the expression is to be evaluated first. This technique works equally well with Boolean equations.

Discussion

Parentheses are key to writing maintainable and bug-free equations. Not only is your intention clearly spelled out, but you ...