Jasmine
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Jan 19, 2001, 2:29 PM
Post #1 of 1
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Why am I getting long decimals (eg, 19.94999999999
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(From the Perl FAQ)
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 $# 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 Floating-point Arithmetic.
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