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

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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?

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
Voss on 22 May 2022
You're welcome!

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.
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

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
Reza Lashani on 22 May 2022
Thank you so much 👌

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)
Reza Lashani on 22 May 2022
It is a very nice method. Thank you