Text to floating point: how does it work

Question

Say, I write down in a programming language the characters 5.4 and store it into a double variable. How exactly does a computer decide what floating point representation (mantissa and exponent) is the closest to that number?

Lovely question, but I think it might belong to a different SE site. — Ramon Snir, Sep 08 '11 at 08:55
the question is asked often: see for example http://stackoverflow.com/questions/85223/how-to-manually-parse-a-floating-point-number-from-a-string/85526#85526 — Matteo, Sep 08 '11 at 09:04

score 1 · Answer 1 · answered Sep 08 '11 at 09:08

Well, the compiler or interpreter of the programming language uses an algorithm to convert from a decimal string representation to a (binary) floating point representation.

One implementation is David Gay's strtod() at http://www.netlib.org/fp/dtoa.c .

Note that while Gay's code is the basis for many implementations of strtod() and snprintf() in widely used C libraries, the code as such suffers from a number of issues and should not be used as such. But, it's instructive as a self-contained example.

score 1 · Accepted Answer · edited May 23 '17 at 11:55

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I once described this, in a simple, easily understood way, for the value 5.2, see this SO article.

edited May 23 '17 at 11:55

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answered Sep 09 '11 at 00:04

Rudy Velthuis

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Text to floating point: how does it work

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