Halstead complexity measures

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Halstead complexity measures

Halstead complexity measures are software metrics introduced by Maurice Howard Halstead in 1977. These metrics are computed statically, without program execution.

Calculation

First we need to compute the following numbers, given the program source code:

$n 1$ = the number of distinct operators
$n 2$ = the number of distinct operands
$N 1$ = the total number of operators
$N 2$ = the total number of operands

From these numbers, five measures can be calculated:

Program length: $N = N1 + N2 \,$
Program vocabulary: $n = n1 + n2 \,$
Volume: $V= N \times \log_2 n \,$
Difficulty : $D= { n_1 \over 2 } \times { N_2 \over n_2 } \,$
Effort: $E= D * V \,$

The difficulty measure is related to the difficulty of the program to write or understand, e.g. when doing code review.