Demonstration of Prefix and Postfix operators in C++
By: Emiley J
Both the increment operator (++) and the decrement operator(--) come in two varieties: prefix and postfix. The prefix variety is written before the variable name (++myAge); the postfix variety is written after (myAge++).
In a simple statement, it doesn't much matter which you use, but in a complex statement, when you are incrementing (or decrementing) a variable and then assigning the result to another variable, it matters very much. The prefix operator is evaluated before the assignment, the postfix is evaluated after.
The semantics of prefix is this: Increment the value and then fetch it. The semantics of postfix is different: Fetch the value and then increment the original.
This can be confusing at first, but if x is an integer whose value is 5 and you write
int a = ++x;
you have told the compiler to increment x (making it 6) and then fetch that value and assign it to a. Thus, a is now 6 and x is now 6.
If, after doing this, you write
int b = x++;
you have now told the compiler to fetch the value in x (6) and assign it to b, and then go back and increment x. Thus, b is now 6, but x is now 7. Listing below shows the use and implications of both types.
A demonstration of prefix and postfix operators.
1: // Listing 4.3 - demonstrates use of 2: // prefix and postfix increment and 3: // decrement operators 4: #include <iostream.h> 5: int main() 6: { 7: int myAge = 39; // initialize two integers 8: int yourAge = 39; 9: cout << "I am: " << myAge << " years old.\n"; 10: cout << "You are: " << yourAge << " years old\n"; 11: myAge++; // postfix increment 12: ++yourAge; // prefix increment 13: cout << "One year passes...\n"; 14: cout << "I am: " << myAge << " years old.\n"; 15: cout << "You are: " << yourAge << " years old\n"; 16: cout << "Another year passes\n"; 17: cout << "I am: " << myAge++ << " years old.\n"; 18: cout << "You are: " << ++yourAge << " years old\n"; 19: cout << "Let's print it again.\n"; 20: cout << "I am: " << myAge << " years old.\n"; 21: cout << "You are: " << yourAge << " years old\n"; 22: return 0; 23: } Output: I am 39 years old You are 39 years old One year passes I am 40 years old You are 40 years old Another year passes I am 40 years old You are 41 years old Let's print it again I am 41 years old You are 41 years old
Analysis: On lines 7 and 8, two integer
variables are declared, and each is initialized with the value 39.
Their values are printed on lines 9 and 10.
On line 11, myAge is incremented using the postfix increment operator,
and on line 12, yourAge is incremented using the prefix increment
operator. The results are printed on lines 14 and 15, and they are identical
(both 40).
On line 17, myAge is incremented as part of the printing statement, using the postfix increment operator. Because it is postfix, the increment happens after the print, and so the value 40 is printed again. In contrast, on line 18, yourAge is incremented using the prefix increment operator. Thus, it is incremented before being printed, and the value displays as 41.
Finally, on lines 20 and 21, the values are printed again. Because the increment statement has completed, the value in myAge is now 41, as is the value in yourAge.
Archived Comments
1. please sent four program for increament and decreament
View Tutorial By: saurabh seth at 2012-07-27 17:36:20
2. Amit, at your first question, what the preprocessor will do with that define is to put in the code (
View Tutorial By: spiriad at 2011-01-08 14:27:50
3. what will be the output of 'X' in following code and how ?
x=(i++ + ++i + i++ + i++ +
View Tutorial By: Amit Goyal at 2009-09-25 05:48:45
4. Thanks for the above description, but pls let me know the output of the following program and how it
View Tutorial By: Amit Goyal at 2009-09-25 05:33:10
5. ur prog sucks
View Tutorial By: ramkumar at 2008-09-09 00:20:29
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