*Learning objectives*

The class random_source |

(Pseudo-)random numbers play an important role in many simulations, games, and last but not least in randomized algorithms, that is in algorithms whose efficiency is based on random decisions.

When we speak of *pseudo-random numbers*,
we thereby take the circumstance into account that a
deterministic computing machine, as our computer is, can never
create something really random. It merely consists of a finite
number of bits, therefore it has a finite number of
configurations only, and therefore it must reach a configuration
again, after a finite number of computing steps, in which it was
some time before.^{[25]}
Its output repeats as of this time;
the machine, therefore, is not “random” but deterministic.

Furthermore, it is very hard to define with mathematical exactness what “chance” really is. We do not want to go into this discussion here. The interested reader can look this up in [6].

Therefore, in the following, if we speak of *random numbers*, we actually
mean pseudo-random numbers by that. We do not ask of these that
they be random in the mathematical sense; rather, they should
meet certain statistical criteria, for example that every
(integer) number from a given interval be chosen from this
interval with equal frequency.

LEDA has a class * random_source*
for creating random numbers particularly comfortably. We will
get to know this class in this section. It serves for creating
random integer numbers from a certain interval (“integer
mode”) and random bit strings of a length defined before
(“bit mode”).

^{[25] }Here we assume a finite
input which is defined in advance. The user may not specify
random input at runtime.