# Random Generators: What is a seed?

Computers can’t by themselves generate truly random numbers. It’s therefore common to use *pseudorandom* number generators.

A pseudorandom number generator generates the **same number sequence** over and over. It’s in fact semantically equivalent to

```
// Seemingly random, but fixed, list of numbers
numbers ← { 8651, 1932, 18626, 77, 8524 }
while true
for n in numbers
print n
```

In practice `numbers`

obviously has more numbers, and the implementation doesn’t store them explicitly.

The **seed** is simply the **index** at which the generator starts reading numbers.

To get *truly* random numbers, the computer needs some source of entropy. Examples of such sources are keyboard/mouse readings or the system clock.

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