Assignment Has Low Precedence

Assignments often occur in conditions. Because assignment has relatively low precedence, we usually must parenthesize the assignment for the condition to work properly. To see why assignment in a condition is useful, consider the following loop. We want to call a function until it returns a desired value—say, 42:

// a verbose and therefore more error-prone way to write this loop
int i = get_value();  // get the first value
while (i != 42) {
    // do something ...
    i = get_value();  // get remaining values
}

Here we start by calling get_value followed by a loop whose condition uses the value returned from that call. The last statement in this loop makes another call to get_value, and the loop repeats. We can write this code more directly as

int i;
// a better way to write our loop---what the condition does is now clearer
while ((i = get_value()) != 42) {
    // do something ...
}

The condition now more clearly expresses our intent: We want to continue until get_value returns 42. The condition executes by assigning the result returned by get_value to i and then comparing the result of that assignment with 42.

Without the parentheses, the operands to != would be the value returned from get_value and 42. The true or false result of that test would be assigned to i—clearly not what we intended!