Modification operations

Modification that is also called compound assignment allows combining within one operator arithmetic or bitwise operations with normal assignment.

These operators execute the relevant action for operands e1 and e2, whereupon the result is stored in e1.

An expression like e1 @= e2 where @ is any operator from the table is approximately equivalent to e1 = e1 @ e2. The word "approximately" emphasizes the presence of some subtle aspects.

First, if the place of e2 is occupied by an expression with an operator having a lower priority than that of @, e2 is still calculated before that. That is, if the priority is marked with parentheses, we will get e1 = e1 @ (e2).

Second, if there are side modifications of variables in expression e1, they are made only once. The following example demonstrates this.

Here, arrays a and b contain identical elements and are processed using index variables i and j. At the same time, the expression for array a uses operation '*=', while that for array b uses the equivalent. Results are not equal: Both index variables and arrays differ.

Other operators will be useful in problems with bit-level manipulations. Thus, the following expression can be used to set a specific bit into 1:

Here, shift 1 ('0000 0000 0000 0001') is made by 10 bits to the left, which results in obtaining a number with one set 10th bit ('0000 0100 0000 0000'). Bitwise OR operation copies this bit into variable x.

To reset the same bit, we will write:

Here, the inversion operation is applied to 1 shifted by 10 bits to the left (which we saw in the preceding expression), which results in all bits changing their value: '1111 1011 1111 1111'. Bitwise AND operation resets the zeroed bits (in this case, one) in variable x, while all other bits in x remain unchanged.