Generate "characters" based on a probability

Question

I need a little help with my tetris game that I am coding in C++ , here is my problem:

The list of tetris blocks are 7 types: {'I', 'J', 'L', 'S', 'Z', 'O', 'T'}

and i need to pick ONE of the above character such that S and Z are selected with probability 1/12 each, and the other blocks are selected with probability 1/6 each.

What is my best way to generate the blocks according to these probabilities?

possible duplicate of [Weighted random numbers](http://stackoverflow.com/questions/1761626/weighted-random-numbers) — Hasturkun, Dec 01 '11 at 14:06

score 7 · Answer 1 · answered Dec 01 '11 at 14:03

7

Declare and define a twelve-item array with a single occurrence of S and Z; two occurrences each of the rest; and pick a random element from that.

answered Dec 01 '11 at 14:03

Pontus Gagge

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score 4 · Accepted Answer · answered Dec 01 '11 at 14:04

4

Create an array like so:

const char* block_distribution = {'I', 'I', 'J', 'J', 'L', 'L', 'S', 
                                  'Z', 'O', 'O', 'T', 'T'};

Then pick one element from that array using a uniform distribution.

answered Dec 01 '11 at 14:04

Björn Pollex

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Thankyou, I have made the array, now I just have to generate a random numbber from [0,11] , do you know exactly how I can do that? I am not familiar with random number generation in c++ – Shehryar Farooq Dec 01 '11 at 14:07
@Shehryar This should be an exercise left for the reader. This is easy enough to look up in pretty much every reference manual. – Manny D Dec 01 '11 at 14:10
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@ShehryarFarooq: Make sure your answer isn't `rand() % 12`, which doesn't do what you asked! – Kerrek SB Dec 01 '11 at 14:51

score 1 · Answer 3 · answered Dec 01 '11 at 14:08

1

put those characters in a character array and generate random values from 0 to 6 with srand() than you can get a random

char myArray[] = {'I', 'J', 'L', 'S', 'Z', 'O', 'T'};

and then get values with

myArray[ (rand()%5)+1 ]

answered Dec 01 '11 at 14:08

znarf

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Hi johnny, i use this method if I want them to be generated with EQUAL probability correct? – Shehryar Farooq Dec 01 '11 at 14:10
yes, initial srand() with the time in the main function to prevent always the same "random" value – znarf Dec 01 '11 at 14:14

score 0 · Answer 4 · answered Dec 01 '11 at 14:09

The easiest solution I can think of is to use numeric ranges and pseudo-random numbers.

So, first assign ranges to each letter:

I: 0 to 1
J: 1 to 2
L: 2 to 3
S: 3 to 3.5
Z: 3.5 to 4
O: 4 to 5
T: 5 to 6

then generate pseudo-random numbers within the range 0 to 6. Whichever part of the range that the number falls within, that's the letter you choose.

Unfortunately I do not know C++ so I cannot provide you with code, but I don't think it should be too difficult to implement.

Generate "characters" based on a probability

4 Answers4