Binary search is one of the basic algorithms of computer sciences. It goes like this:

Given a sorted(!) array of strings and a single string, what is the fastest way to find the location of the string?

As we would like to look at the algorithms and the complexity of algorithms, we will avoid using the built in index function and the first_index of List::MoreUtils.

## Linear search

We can go over element by element, comparing each element to the string we are looking for. The linear_search function is simple to implement. Given N elements in the array, it will take on average N/2 iterations and in the worst case, when the value we are looking for is the last element, it will take N iterations.

examples/linear_search_in_array.pl

use 5.010;
use strict;
use warnings;

my @planets = qw(
Ceres
Charon
Earth
Jupiter
Mars
Mercury
Neptune
Pluto
Saturn
Uranus
Venus
);

my \$name = shift or die "Usage: \$0 PLANET";
my \$res = linear_search(\$name, \@planets);
if (not defined \$res) {
} else {
say "\$name found at \$res";
}

sub linear_search {
my (\$str, \$list) = @_;
for my \$i (0 .. @\$list -1) {
if (\$list->[\$i] eq \$str) {
return \$i;
}
}
return;
}

## Binary search

The binary_search function is a log more complex, but in the worst case it will take log2(N) iterations. Just two compare the two

N    log2(N)
1     0     (we know the answer instantly)
2     1     (1 iteration)
10     3.3
100     6.6
1000     9.9
10000    13.3

As you can see as N growth the difference is dramatic.

examples/binary_search_in_array.pl

use 5.010;
use strict;
use warnings;

my @planets = qw(
Ceres
Charon
Earth
Jupiter
Mars
Mercury
Neptune
Pluto
Saturn
Uranus
Venus
);

my \$name = shift or die "Usage: \$0 PLANET";
my \$res = binary_search(\$name, \@planets);
if (not defined \$res) {
} else {
say "\$name found at \$res";
}

sub binary_search {
my (\$str, \$list) = @_;

my \$min = 0;
my \$max = @\$list - 1;

while (\$min <= \$max) {
my \$middle = int((\$max+\$min) / 2);
# say "\$min - \$max (\$middle)";
if (\$name lt \$list->[\$middle]) {
\$max = \$middle-1;
next;
}
if (\$name gt \$list->[\$middle]) {
\$min = \$middle+1;
next;
}
return \$middle;
}

return;
}

## How does the binary search work?

We maintain two variables \$min and \$max that hold two indices of the array. We assume the value we are looking for can be found between the two. At first we set \$min to 0 and \$max to the largest index of the array. We call this the window of search. Surely if the element can be found it is somewhere there.

Then at every iteration we calculate the index of the element exactly half way between the current minimum and maximum. (We se int to round the result of the division as we would like to get a whole number as index.) We comare the value at the selected index.

If is smaller than the value we are looking for then the real location must be above it, in the upper part of the window. We move the \$min slightly above the selected element.

If is bigger than the value we are looking for then the real location must be below it, in the lower part of the window. We move the \$max slightly below the selected element.

In either case we made the window smaller.

If the current value is neither smaller nor bigger than the value we are looking for then we found the location and return the middle value which is the index we were looking for.

If the window becomes empty (the \$min is bigger than the \$max) then we can conclude the element we were looking for is not in the array.

I've left in a commented say statement in case you'd like to see the values a search tries.