The Task ahead

In this chapter we look at the various ways that the control of flow statements can be used in a C program, including some statements that haven't been introduced so far. They are almost always used in conjunction with logical expressions to select the next action. Examples of logical expressions that have been seen already are some simple ones used in if or while statements. As you might have expected, you can use expressions more complicated than simple comparison (>, <=, etc.); what may surprise you is the type of the result.

Logical expressions and Relational Operators

All of the examples we have used so far have deliberately avoided using complicated logical expressions in the control of flow statements. We have seen expressions like this

if(a != 100)
exit(EXIT_SUCCESS);
}

Example 3.1

Which produces this on its standard output:

value of i is -10, i == 0 = 0, i > -5 = 0
value of i is -9, i == 0 = 0, i > -5 = 0
value of i is -8, i == 0 = 0, i > -5 = 0
value of i is -7, i == 0 = 0, i > -5 = 0
value of i is -6, i == 0 = 0, i > -5 = 0
value of i is -5, i == 0 = 0, i > -5 = 0
value of i is -4, i == 0 = 0, i > -5 = 1
value of i is -3, i == 0 = 0, i > -5 = 1
value of i is -2, i == 0 = 0, i > -5 = 1
value of i is -1, i == 0 = 0, i > -5 = 1
value of i is 0, i == 0 = 1, i > -5 = 1
value of i is 1, i == 0 = 0, i > -5 = 1
value of i is 2, i == 0 = 0, i > -5 = 1
value of i is 3, i == 0 = 0, i > -5 = 1
value of i is 4, i == 0 = 0, i > -5 = 1
value of i is 5, i == 0 = 0, i > -5 = 1

In this probably mistaken piece of code, what do you think happens?

if(a = b)

The value of b is assigned to a. As you know, the result has the type of a and whatever value was assigned to a. The if will execute the next statement if the value assigned is not zero. If zero is assigned, the next statement is ignored. So now you understand what happens if you confuse the assignment with the equality operator!

In all of the statements that test the value of an expression, the if, while, do, and for statements, the expression is simply tested to see if its value is zero or not.

We will look at each one in turn.