Ranges of floating point datatype in C?

c floating-point ieee-754
ipkiss picture ipkiss · Apr 11, 2012 · Viewed 105.6k times · Source

I am reading a C book, talking about ranges of floating point, the author gave the table:

Type     Smallest Positive Value  Largest value      Precision
====     =======================  =============      =========
float    1.17549 x 10^-38         3.40282 x 10^38    6 digits
double   2.22507 x 10^-308        1.79769 x 10^308   15 digits

I dont know where the numbers in the columns Smallest Positive and Largest Value come from.

Answer

Andreas Brinck picture Andreas Brinck · Apr 11, 2012

A 32 bit floating point number has 23 + 1 bits of mantissa and an 8 bit exponent (-126 to 127 is used though) so the largest number you can represent is:

(1 + 1 / 2 + ... 1 / (2 ^ 23)) * (2 ^ 127) = 
(2 ^ 23 + 2 ^ 23 + .... 1) * (2 ^ (127 - 23)) = 
(2 ^ 24 - 1) * (2 ^ 104) ~= 3.4e38

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