4.13 Increment and Decrement Operators
C++ provides two unary operators (summarized in Fig. 4.16) for adding 1 to or subtracting 1 from the value of a numeric variable. These are the unary increment operator, ++, and the unary decrement operator, --. A program can increment by 1 the value of a variable called c using the increment operator, ++, rather than the expression c=c +1 or c+=1. An increment or decrement operator that’s prefixed to (placed before) a variable is referred to as the prefix increment or prefix decrement operator, respectively. An increment or decrement operator that’s postfixed to (placed after) a variable is referred to as the postfix increment or postfix decrement operator, respectively.
Fig. 4.16 Increment and decrement operators.
| Operator | Operator name | Sample expression | Explanation |
|---|---|---|---|
++ |
prefix increment | ++a |
Increment a by 1, then use the new value of a in the expression in which a resides. |
++ |
postfix increment | a++ |
Use the current value of a in the expression in which a resides, then increment a by 1. |
-- |
prefix decrement | --b |
Decrement b by 1, then use the new value of b in the expression in which b resides. |
-- |
postfix decrement | b-- |
Use the current value of b in the expression in which b resides, then decrement b by 1. |
Using the prefix increment (or decrement) operator to add 1 to (or subtract 1 from) a variable is known as preincrementing (or predecrementing). This causes the variable to be incremented (decremented) by 1; then the new value of the variable is used in the expression in which it appears.
Using the postfix increment (or decrement) operator to add 1 to (or subtract 1 from) a variable is known as postincrementing (or postdecrementing). This causes the current value of the variable to be used in the expression in which it appears; then the variable’s value is incremented (decremented) by 1.
Good Programming Practice 4.4
Unlike binary operators, the unary increment and decrement operators as a matter of style should be placed next to their operands, with no intervening spaces.
Difference Between Prefix Increment and Postfix Increment Operators
Figure 4.17 demonstrates the difference between the prefix increment and postfix increment versions of the ++ increment operator. The decrement operator (--) works similarly.
Fig. 4.17 Prefix increment and postfix increment operators.
Alternate View
1 // Fig. 4.17: Increment.cpp
2 // Prefix increment and postfix increment operators.
3 #include <iostream>
4 using namespace std;
5
6 int main() {
7 // demonstrate postfix increment operator
8 unsigned int c{5}; // initializes c with the value 5
9 cout << "c before postincrement: " << c << endl; // prints 5
10 cout << " postincrementing c: " << c++ << endl; // prints 5
11 cout << " c after postincrement: " << c << endl; // prints 6
12
13 cout << endl; // skip a line
14
15 // demonstrate prefix increment operator
16 c = 5; // assigns 5 to c
17 cout << " c before preincrement: " << c << endl; // prints 5
18 cout << " preincrementing c: " << ++c << endl; // prints 6
19 cout << " c after preincrement: " << c << endl; // prints 6
20 }
c before postincrement: 5
postincrementing c: 5
c after postincrement: 6
c before preincrement: 5
preincrementing c: 6
c after preincrement: 6
Line 8 initializes the variable c to 5, and line 9 outputs c’s initial value. Line 10 outputs the value of the expression c++. This expression postincrements the variable c, so c’s original value (5) is output, then c’s value is incremented (to 6). Thus, line 10 outputs c’s initial value (5) again. Line 11 outputs c’s new value (6) to prove that the variable’s value was indeed incremented in line 10.
Line 16 resets c’s value to 5, and line 17 outputs c’s value. Line 18 outputs the value of the expression ++c. This expression preincrements c, so its value is incremented; then the new value (6) is output. Line 19 outputs c’s value again to show that the value of c is still 6 after line 18 executes.
Simplifying Statements with the Arithmetic Compound Assignment, Increment and Decrement Operators
The arithmetic compound assignment operators and the increment and decrement operators can be used to simplify program statements. For example, the three assignment statements in Fig. 4.14 (lines 21, 24 and 28)
passes = passes + 1;
failures = failures + 1;
studentCounter = studentCounter + 1;
can be written more concisely with compound assignment operators as
passes += 1;
failures += 1;
studentCounter += 1;
with prefix increment operators as
++passes;
++failures;
++studentCounter;
or with postfix increment operators as
passes++;
failures++;
studentCounter++;
When incrementing or decrementing a variable in a statement by itself, the prefix increment and postfix increment forms have the same effect, and the prefix decrement and postfix decrement forms have the same effect. It’s only when a variable appears in the context of a larger expression that preincrementing and postincrementing the variable have different effects (and similarly for predecrementing and postdecrementing).
Common Programming Error 4.4
Attempting to use the increment or decrement operator on an expression other than one to which a value can be assigned is a syntax error. For example, writing ++(x + 1) is a syntax error, because (x + 1) is not a variable.
Operator Precedence and Associativity
Figure 4.18 shows the precedence and associativity of the operators introduced to this point. The operators are shown top-to-bottom in decreasing order of precedence. The second column indicates the associativity of the operators at each level of precedence. Notice that the conditional operator (?:), the unary operators preincrement (++), predecrement (--), plus (+) and minus (-), and the assignment operators =, +=, -=, *=, /= and %= associate from right to left. All other operators in Fig. 4.18 associate from left to right. The third column names the various groups of operators.
Good Programming Practice 4.5
Refer to the operator precedence and associativity chart (Appendix A) when writing expressions containing many operators. Confirm that the operators in the expression are performed in the order you expect. If you’re uncertain about the order of evaluation in a complex expression, break the expression into smaller statements or use parentheses to force the order of evaluation, exactly as you’d do in an algebraic expression. Be sure to observe that some operators such as assignment (=) associate right to left rather than left to right.
Fig. 4.18 Operator precedence for the operators encountered so far in the text.
| Operators | Associativity | Type | |||||
|---|---|---|---|---|---|---|---|
:: |
() |
left to right [See Fig. 2.10’s caution regarding grouping parentheses.] | primary | ||||
++ |
-- |
static_cast<type>() |
left to right | postfix | |||
++ |
-- |
+ |
- |
right to left | unary (prefix) | ||
* |
/ |
% |
left to right | multiplicative | |||
+ |
- |
left to right | additive | ||||
<< |
>> |
left to right | insertion/extraction | ||||
< |
<= |
> |
>= |
left to right | relational | ||
== |
!= |
left to right | equality | ||||
?: |
right to left | conditional | |||||
= |
+= |
-= |
*= |
/= |
%= |
right to left | assignment |