# Demonstrating Basic Probability with Ruby

Apr 25, 2017I’m not sure I can explain the following code I wrote clearly. But I’ll try.

What’s the probability that a specific number is drawn out of 10 numbers? Say ‘7’ out of 1 to 10? It’s 10% right?

If so, then on 10 repeated draws, at least once (or exactly once?) ‘7’ should show up right? Is it guaranteed? Not always. It can be more or less than 10.

But how to **make sure** it is close to the ideal 10% ?

We’ll have to increase the number of the draws, aka the experiments. This code draws only once, through 10 crore times. The `variance`

tells about how close the actual selection was to the ideal 10%.

The code demonstrates that, as the draws (or experiments) increase, the accuracy increases. The variance approaches 0 faster in higher values than in the initial low draw counts.

```
dice_heads = 10
success_count = 0
selection = 7
ideal = 10.0
experiments = [
{ trials: 1, result: 0, variance: 0 },
{ trials: 10, result: 0, variance: 0 },
{ trials: 100, result: 0, variance: 0 },
{ trials: 1000, result: 0, variance: 0 },
{ trials: 10000, result: 0, variance: 0 },
{ trials: 100000, result: 0, variance: 0 },
{ trials: 1000000, result: 0, variance: 0 },
{ trials: 10000000, result: 0, variance: 0 },
{ trials: 100000000, result: 0, variance: 0 },
]
experiments.each do |experiment|
experiment[:trials].times do
success_count += 1 if rand(1..dice_heads) == selection
end
experiment[:result] = (success_count.to_f/experiment[:trials]) * 100
experiment[:variance] = (ideal - experiment[:result]).abs.round(5)
success_count = 0
end
puts experiments
```

Here are the results of the code:

```
{:trials=>1, :result=>0.0, :variant=>10.0}
{:trials=>10, :result=>20.0, :variant=>10.0}
{:trials=>100, :result=>11.0, :variant=>1.0}
{:trials=>1000, :result=>10.2, :variant=>0.2}
{:trials=>10000, :result=>10.040000000000001, :variant=>0.04}
{:trials=>100000, :result=>10.105, :variant=>0.105}
{:trials=>1000000, :result=>10.051400000000001, :variant=>0.0514}
{:trials=>10000000, :result=>9.97998, :variant=>0.02002}
{:trials=>100000000, :result=>9.992652, :variant=>0.00735}
```