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Subsection 3.10.1 Motivation for Randomness

We often want to use random numbers in programs. Here are a few typical uses:

To play a game of chance where the computer needs to throw some dice, pick a number, or flip a coin,

To shuffle a deck of playing cards randomly,

To randomly allow a new enemy spaceship to appear and shoot at you,

To simulate possible rainfall when we make a computerized model for estimating the environmental impact of building a dam,

For encrypting your banking session on the Internet.

Python provides a module `random`

that helps with tasks like this. You can take a look at it in the documentation. Here are the key things we can do with it.

Press the run button a number of times. Note that the values change each time. These are random numbers.

The `randrange`

function generates an integer between its lower and upper argument where the lower bound is included, but the upper bound is excluded. So, `randrange(1,7)`

will include numbers from 1-6. If you omit the first parameter it is assumed to be 0 so `randrange(10)`

will give you numbers from 0-9. All the values have an equal probability of occurring (i.e. the results are *uniformly* distributed).

The `random()`

function returns a floating point number in the range [0.0, 1.0) — the square bracket means “closed interval on the left” and the round parenthesis means “open interval on the right”. In other words, 0.0 is possible, but all returned numbers will be strictly less than 1.0. It is usual to *scale* the results after calling this method, to get them into a range suitable for your application.

In the case shown below, we’ve converted the result of the random() method call to a number in the range [0.0, 5.0). Once more, these are uniformly distributed numbers — numbers close to 0 are just as likely to occur as numbers close to 3, or numbers close to 5. If you continue to press the run button you will see random values between 0.0 and up to but not including 5.0.

It is important to note that random number generators are based on a deterministic algorithm — repeatable and predictable. So they’re called pseudo-random generators — they are not genuinely random. They start with a *seed* value. Each time you ask for another random number, you’ll get one based on the current seed attribute, and the state of the seed (which is one of the attributes of the generator) will be updated. The good news is that each time you run your program, the seed value is likely to be different meaning that even though the random numbers are being created algorithmically, you will likely get random behavior each time you execute.

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Note 3.10.1.

Another useful function is `random.choice()`

which requires a sequence type variable as a parameter. Run the activecode below to randomly choose a colour from the list and a character from a string. You will see this function again in class assignments!