Floating point numbers

Modern computers use binary, not decimal, number representations for numbers with a decimal part. The significant digits and exponents are represented in binary rather than decimal and raise 2 rather than 10 to the exponent.


Nearly all modern programming languages use the same binary representation for floating point numbers, which was set by the IEEE 754 standard. The most popular floating point number sizes are usually referred to as single precision and double precision floating points, these numbers use either 32 bits or 64 bits to represent their values.


Single precision floating point numbers (32 bits) according to the IEEE 754 standard, can represent values from 1.2e-38 to 3.4e+38. Double precision floating point numbers of the same standard may represent values from 5.0e-324 to 1.8e+308.

Example of an exact representation

Some numbers may be represented exactly. For instance, the number 0.625 (5/8) would be represented as the pair (101, -11); because 5 is 101 in binary and -11 is the binary representation of -3, the pair (101, -11) stands for 5 ยท 2-3 = 5 / 8 = 0.625.

Example of an approximation

What about the decimal fraction 1/10, which we write as 0.1? The best we can do with four significant binary digits is (0011, -101). This is equivalent to 3/32, i.e., 0.09375. If we had five significant binary digits, we would represent 0.1 as (11001, -1000), which is equivalent to 25/256, i.e., 0.09765625. How many significant digits would we need to get an exact floating-point representation of 0.1? An infinite number of digits. There do not exist integers sig and exp such that sig * 2 ^ -exp equals 0.1. Therefore, no matter how many bits the language chooses to use to represent floating point numbers, it will be able to represent only an approximation of 0.1. In many implementations, there are 53 bits of precision available for floating point numbers, so the significant digits stored for the decimal number 0.1 will be


This allows for the approximation of the decimal number


Which is close to 1/10, but not 1/10.