How to create random matrix with specified step in interval [a, b]

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Reza Lashani
Reza Lashani on 22 May 2022
Edited: Reza Lashani on 23 May 2022
Hello everyone. I want to create a matrix with size (m×n) and I want its entities be between 5 and 34 (or in interval [5,34]) with a step of 3. Which means it's only allowed to have 5, 8, 11, 14, ... . How can I create this matrix?

Accepted Answer

Voss
Voss on 22 May 2022
Edited: Voss on 22 May 2022
a = 5;
b = 34;
step = 3;
offset = 2;
n_levels = 1+floor((b-a)/step)
n_levels = 10
% for your reference, showing the min and max
% random values that will be generated:
offset + step*[1 n_levels]
ans = 1×2
5 32
% required size of resulting random matrix:
m = 10;
n = 12;
% generate the matrix using randi,
% [1 n_levels] maps to [5 32]
result = offset + step*randi([1 n_levels],m,n)
result = 10×12
8 8 11 23 26 23 8 20 20 32 32 26 5 26 11 20 8 14 11 29 32 23 23 11 32 20 20 11 23 17 11 17 29 5 23 11 17 5 11 32 26 14 20 8 11 20 23 20 26 8 17 20 20 14 8 14 32 14 8 14 29 32 32 23 17 14 23 8 8 26 29 14 23 17 23 11 29 8 5 11 26 26 23 26 17 26 29 17 29 23 17 14 11 20 29 23 32 8 29 32 20 14 32 26 5 8 17 17 11 11 5 32 23 14 8 8 20 8 17 17
  4 Comments

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More Answers (3)

John D'Errico
John D'Errico on 22 May 2022
Edited: John D'Errico on 22 May 2022
You can also use indexing. The idea is to generate random indexes.
ValidRandomSet = 5:3:34
ValidRandomSet = 1×10
5 8 11 14 17 20 23 26 29 32
m = 7;
n = 6;
ind = randi(numel(ValidRandomSet),[m,n])
ind = 7×6
3 10 4 5 6 7 5 4 2 10 7 6 6 10 2 10 8 6 3 5 1 10 8 10 7 7 9 6 4 6 3 3 2 5 9 10 1 8 9 10 7 3
X = ValidRandomSet(ind)
X = 7×6
11 32 14 17 20 23 17 14 8 32 23 20 20 32 8 32 26 20 11 17 5 32 26 32 23 23 29 20 14 20 11 11 8 17 29 32 5 26 29 32 23 11
The virtue of such a scheme is it works for any target set of numbers.
  1 Comment
Reza Lashani
Reza Lashani on 23 May 2022
very nice. I could write this by using a foor loop. but your way to doing it, is much better. thank you for your help.
This is what I wrote:
function out = myfun(Step, a, b, m, n)
out = zeros(m, n);
ValidSet = a: Step: b;
for i = 1: m*n
ind = randi(length(ValidSet));
out(i) = ValidSet(ind);
end
end

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Jonas
Jonas on 22 May 2022
Edited: Jonas on 22 May 2022
generate a matrix with random integer values using randi. The spacing will be 1, you can then multiply the values by 3 to get spacing of 3. then shift your values towards your interval by addition
randi(10, 20,30)*3+2
max value will be 32 and minimum value 5

Image Analyst
Image Analyst on 22 May 2022
Here's another way. You can create your random numbers as floating point then use discretize them into numbers in the set you specify:
m = 6
n = 18
% Get the random numbers
r = 5 + (35-5) * rand(m, n)
% Discretize into nearest integer that is in the set
% [5 8 11 14 17 20 23 26 29 32]
edges = 5:3:35
values = edges(1:end-1)
r2 = discretize(r, edges, values)

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