- By John Sharp
Incrementing and decrementing variables
If you want to add 1 to a variable, you can use the + operator, as demonstrated here:
count = count + 1;
However, adding 1 to a variable is so common that C# provides its own operator just for this purpose: the ++ operator. To increment the variable count by 1, you can write the following statement:
Similarly, C# provides the -- operator that you can use to subtract 1 from a variable, like this:
The ++ and -- operators are unary operators, meaning that they take only a single operand. They share the same precedence and are both left associative.
Prefix and postfix
The increment (++) and decrement (--) operators are unusual in that you can place them either before or after the variable. Placing the operator symbol before the variable is called the prefix form of the operator, and using the operator symbol after the variable is called the postfix form. Here are examples:
count++; // postfix increment ++count; // prefix increment count--; // postfix decrement --count; // prefix decrement
Whether you use the prefix or postfix form of the ++ or -- operator makes no difference to the variable being incremented or decremented. For example, if you write count++, the value of count increases by 1, and if you write ++count, the value of count also increases by 1. Knowing this, you’re probably wondering why there are two ways to write the same thing. To understand the answer, you must remember that ++ and -- are operators and that all operators are used to evaluate an expression that has a value. The value returned by count++ is the value of count before the increment takes place, whereas the value returned by ++count is the value of count after the increment takes place. Here is an example:
int x; x = 42; Console.WriteLine(x++); // x is now 43, 42 written out x = 42; Console.WriteLine(++x); // x is now 43, 43 written out
The way to remember which operand does what is to look at the order of the elements (the operand and the operator) in a prefix or postfix expression. In the expression x++, the variable x occurs first, so its value is used as the value of the expression before x is incremented. In the expression ++x, the operator occurs first, so its operation is performed before the value of x is evaluated as the result.
These operators are most commonly used in while and do statements, which are presented in Chapter 5, “Using compound assignment and iteration statements.” If you are using the increment and decrement operators in isolation, stick to the postfix form and be consistent.