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Increment and Decrement Operators

There are some operations that occur so frequently in writing assignment statements that C++ has shorthand methods for writing them.

One common situation is that of incrementing or decrementing an integer variable. For example:

n = n + 1;
n = n - 1;

C++ has an increment operator ++ and a decrement operator --. Thus

n++; can be used instead of n = n + 1;
n--; can be used instead of n = n - 1;

The ++ and -- operators here have been written after the variable they apply to, in which case they are called the postincrement and postdecrement operators. There are also identical preincrement and predecrement operators which are written before the variable to which they apply. Thus

++n; can be used instead of n = n + 1;
--n; can be used instead of n = n - 1;

Both the pre- and post- versions of these operators appear to be the same from the above, and in fact it does not matter whether n++ or ++n is used if all that is required is to increment the variable n. However both versions of the increment and decrement operators have a side effect which means that they are not equivalent in all cases. These operators as well as incrementing or decrementing the variable also return a value. Thus it is possible to write

i = n++;

What value does i take? Should it take the old value of n before it is incremented or the new value after it is incremented? The rule is that a postincrement or postdecrement operator delivers the old value of the variable before incrementing or decrementing the variable. A preincrement or predecrement operator carries out the incrementation first and then delivers the new value. For example if n has the value 5 then

i = n++;
would set i to the original value of n i.e. 5 and would then increment n to 6. Whereas
i = ++n;
would increment n to 6 and then set i to 6.

For the moment this notation will only be used as a shorthand method of incrementing or decrementing a variable.


next up previous
Next: Specialised Assignment Statements Up: Further Assignment Statements & Previous: Further Assignment Statements &
Peter JB King
1999-08-31