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

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?

Answer 1

[Voss]

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

[Reza Lashani]

Thank you. but as I sayed this method doesn't create the exact interval I want. Is'nt there any other method?

[Voss]

Yes it does.

What do you want? 5,8,11,14,17,20,23,26,29,32, and 34? 34 is not 3 more than 32.

If you want 34, please explain how you can achieve that by starting at 5 and going up by steps of 3.

Maybe you meant to say 35?

5:3:35

ans = 1×11

5 8 11 14 17 20 23 26 29 32 35

If you meant to say 35, set b = 35 in my answer, and it will work:

a = 5;
b = 35;
step = 3;
offset = 2;

n_levels = 1+floor((b-a)/step)

n_levels = 11

% for your reference, showing the min and max 
% random values that will be generated:
offset + step*[1 n_levels]

ans = 1×2

5 35

% required size of resulting random matrix:
m = 10;
n = 12;

% generate the matrix using randi,
% [1 n_levels] maps to [5 35]
result = offset + step*randi([1 n_levels],m,n)

result = 10×12

32 23 32 29 35 20 14 8 8 23 26 35 11 29 32 23 5 17 35 23 17 23 17 14 5 14 5 32 14 35 26 26 11 14 5 8 35 5 11 17 14 35 5 29 29 8 23 23 32 17 8 20 14 11 35 5 20 29 29 8 20 5 26 20 17 23 26 14 14 11 23 5 29 20 14 23 35 17 20 26 11 26 35 23 29 32 5 11 26 23 35 35 17 17 29 32 17 11 23 17 35 5 5 20 17 17 26 23 23 35 14 26 17 20 23 23 20 11 29 26

[Reza Lashani]

You are correct. Thank you so much 👌🙏

[Voss]

You're welcome!

Answer 2

[John D'Errico]

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]

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

Answer 3

[Jonas]

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]

Thank you so much 👌

Answer 4

[Image Analyst]

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]

It is a very nice method. Thank you