How to write down a probabilistic function in Wolfram Language?

Question

How does one write a Wolfram function which has a 25% chance of returning 1, a 50% chance of returning 2, and a 25% chance of returning 3?

Here is how the data structure might look like:

D = {(0.25, 1), (0.5, 2), (0.25, 3)}

So how do we write an

f[D_] := ...

with the property above? (Or if this is not the best/most idiomatic data structure for this type of problem, then feel free to dismiss it).

Answer 1

For a one-off call:

RandomChoice[{0.25, 0.5, 0.25} -> {1, 2, 3}]

To make a function as you request:

f[d_] := RandomChoice[d[[All, 1]] -> d[[All, 2]]]
f[{{0.25, 1}, {0.5, 2}, {0.25, 3}}]

Note a couple things: 1) D is a reserved keyword (for derivative). In general, avoid using capitalized variables. 2) A list needs to be created with braces {}, not parentheses ().