# Random Numbers in Python

Python provides the ability to create randomness by generating random numbers or picking randomly from a list. To utilize random numbers in Python, we must `import`

a new library:

`# Import the random library, allowing the use of functions that generate random numbers`

import random

## Generating Random Numbers Using `random.randint`

One of the primary ways we generate random numbers in Python is to generate a random integer (whole number) within a specified range. For example, if I want to generate a number to simulate the roll of a six-sided die, I need to generate a number in the range 1-6 (including the endpoints 1 and 6). With the `random`

library, this is possible:

`# Randomly generates a number in the range 1-6, including the end points:`

random.randint(1, 6)

A **new random number** will be generated **every time this code runs**. This means that you might get `2`

and then another `2`

(just like it's possible to roll `2`

twice in a row), but it's more likely that the second number you get will not be a `2`

.

The `random.randint`

function will always **generate numbers with equal probability** for each number within the range. This means that the probability of getting any specific number when running `random.randint(1, 10)`

is only 10% -- since each of the numbers 1-10 are each 10% likely to show up.

`# Randomly generates a number in the range 1-10, including the end points:`

random.randint(1, 10)

When you generate a generate many random numbers, you'll expect to see a random distribution of numbers and some areas where the same number appears many times in a row. Here's the output of running `random.randint(1, 6)`

to generate a number in the range 1-6 a total of 100 times:

1 # Our very first roll was a 1! :) 4 3 6 6 # This is the first time with two numbers in a row... 3 3 3 # ...followed by three 3s in a row! 5 3 5 4 6 2 # It took 14 rolls before our first 2 was rolled. 4 3 2 5 # We rolled a 5 here on the 18th roll... 4 1 6 3 4 1 3 2 4 1 6 6 3 6 2 4 2 3 3 1 3 1 3 3 5 # ...and then we don't roll another 5 for 25 rolls! 2 4 2 3 5 # 5 # This is the longest sequence in the whole set (four 5s). 5 # ...for any set of 100 dice rolls, we expect a run of four 5 # to happen about once. (We can simulate this later!) 2 4 1 # However, it is not as rare to have runs of exactly three. 1 # (A run of exactly three occurred twice this set of 100 numbers: 1 # 3s on rolls {7,8,9} and then these 1s on rolls {54, 55, 56}.) 2 6 6 1 6 3 5 4 1 2 5 1 4 1 5 5 3 3 2 4 4 5 2 5 2 6 3 6 5 2 5 2 5 1 # The last roll of a 1 -- there is 14 rolls of a 1. 6 # The last roll of a 6 -- there is only 13 rolls of a 6. 5 3 3 # The last roll of a 3, the most popular number -- rolled 21 times! 2 5 5 # The last roll of a 5 -- there is 20 rolls of a 5. 4 # The last roll of a 4 -- there is 14 rolls of a 4. 2 2 # The last roll of a 2 -- there is 18 rolls of a 2.

## Generating Random Numbers Using `random.choice`

When we need more control over the random number generation, `random.choice`

requires a list to be specified and Python will **randomly choose one value from the list**. For example, we can still simulate rolling a six sided die:

`# Randomly generates a number in the range 1-6, including the end points:`

random.choice( [1, 2, 3, 4, 5, 6] )

However, suppose we want to **cheat**! What if we designed a dice what is **twice as likely to land on a six than normal**. To account for this, we can add a second six to `random.choice`

:

`# An unfair die, twice as likely to roll a 6 than any other value:`

random.choice( [1, 2, 3, 4, 5, 6, 6] )

# ^^- An extra 6 was added!

### Generating Random Strings Using `random.choice`

Python will pull any element from the list when using `random.choice`

, so it does not always have to be a number! For example, if we want Python to flip a coin that has a `"heads"`

and `"tails"`

side, we can still use `random.choice`

:

`random.choice( ["heads", "tails"] )`

Remember that Python is equally likely to pull any element form the list -- so this would be a fair coin, as there will be a 50% chance for "heads" and 50% for "tails".

# Example Walk-Throughs with Worksheets

### Video 1: Random Numbers in Python

# Practice Questions

**Q1**: You want to simulate spinning a color wheel with red, orange, yellow, and green sections. How would you do this is Python?

**Q2**: Is the probability of outputting a three equal in the following two Python commands? "random.choice( [1, 2, 3, 4] )", "random.randint(1, 4)"

**Q3**: Given the Python command "x = random.randint(1, 100)", what is the probability that x will be equal to 100?

**Q4**: In order to be able to call the random.choice() and random.randint() functions in Python what line of code must you run first?

**Q5**: You want to simulate the outcome of rolling an eight-sided die that is weighted more heavily on the fourth side. How would you do this in Python?