The Comma Operator
Rarely used, the comma,
operator ( ,
)
allows us to evaluate two expressions where a single expression is
expected. It takes the general form:
operand1
,operand2
Both operands must be legal expressions. When a comma operation
executes, operand1
is evaluated, then
operand2
is evaluated, and then the
resolved value of operand2
is returned. In
practice, the return value of the comma operator is usually ignored.
The comma operator is primarily used in
for
loops that initialize multiple variables
as shown in Example 5.6. Note that in the first line
of the example, the expressions i=0
and
j=10
are both operands of the comma operator.
(These expressions cause i
and
j
to be initialized to
and 10.) The expressions i++
and
j--
are both operands of a second comma operator.
(These expressions cause i
to be incremented and
j
to be decremented each time the loop executes.)
The comma operator is used in order to squeeze multiple expressions
within the confines of the for
loop’s
syntax.
Example 5-6. Using the Comma Operator in a for Loop
for (var i=0, j=10; i!=j; i++, j--) { trace ("i: " + i + " j: " + j); } // Which produces this output... i: 0 j: 10 i: 1 j: 9 i: 2 j: 8 i: 3 j: 7 i: 4 j: 6
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