This message is one of several periodic postings to DevShed's Perl forum intended to make it easier for Perl programmers to find answers to common questions. The core of this message represents an excerpt from the documentation provided with every Standard Distribution of Perl.

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Why am I getting long decimals (eg, 19.9499999999999) instead of the numbers I should be getting (eg, 19.95)?

The infinite set that a mathematician thinks of as the real numbers can

only be approximate on a computer, since the computer only has a finite

number of bits to store an infinite number of, um, numbers.

Internally, your computer represents floating-point numbers in binary.

Floating-point numbers read in from a file or appearing as literals in

your program are converted from their decimal floating-point

representation (eg, 19.95) to the internal binary representation.

However, 19.95 can't be precisely represented as a binary floating-point

number, just like 1/3 can't be exactly represented as a decimal

floating-point number. The computer's binary representation of 19.95,

therefore, isn't exactly 19.95.

When a floating-point number gets printed, the binary floating-point

representation is converted back to decimal. These decimal numbers are

displayed in either the format you specify with printf(), or the current

output format for numbers (see the section on "$#" in the perlvar manpage

if you use print. `$#' has a different default value in Perl5 than it did

in Perl4. Changing `$#' yourself is deprecated.)

This affects all computer languages that represent decimal floating-point

numbers in binary, not just Perl. Perl provides arbitrary-precision

decimal numbers with the Math::BigFloat module (part of the standard Perl

distribution), but mathematical operations are consequently slower.

To get rid of the superfluous digits, just use a format (eg,

`printf("%.2f", 19.95)') to get the required precision. See the section on

"Floating-point Arithmetic" in the perlop manpage.

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