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2.4 Rounding Floating-Point Numbers
NN 2, IE 3
2.4.1 Problem
You want to round a
floating-point value to the nearest whole number.
2.4.2 Solution
Use the Math.round(
) method on the value:
var roundedVal = Math.round(floatingPointValue);
The operation does not disturb the original value. Capture the
rounded result in a variable.
2.4.3 Discussion
The algorithm that the Math.round( ) method uses
is that any floating-point value that is greater than or equal to
x.5 is rounded up to
x+1; otherwise, the
returned value is x.
JavaScript's Math object contains
some other useful methods for trimming floating-point numbers of
their fractional parts. Math.floor(
) and Math.ceil(
) return the next lowest and next highest
integer values, respectively. For example,
Math.floor(3.25) returns 3,
while Math.ceil(3.25) returns
4. With negative values, the rules still apply,
but the results seem backward at first glance:
Math.floor(-3.25) returns the next lowest integer,
-4; Math.ceil(-3.25) returns
-3. For positive values, you can use the
Math.floor( ) method as a substitute for what some
other languages treat as the integer of a number.
Anytime a floating-point number evaluates to a number equal to an
integer value, the decimal and digits to the right of the decimal go
away. A variable can hold a floating-point number in one statement
and be modified to an integer in the next. This drives some
programmers crazy because they were indoctrinated by other languages
to treat each type of number as a different data type.
2.4.4 See Also
Section 2.0.2 in the
introduction of this chapter.
