how do you define a matrix and its variable

I want to create a matrix , by i * j, in matlab, and then let matlab run this. I need to tell matlab what is i and what is j before i define the matrix, right? Then when I define the matrix, can I write aMatrix = [i, j]?

I know how to do matrix like: aMatrix = [1 3 7 ; 5 6 7], but now I want to know how to make a general matrix by giving i and j in matlab.

Answer 1

[Adam Danz]

Learn about preallocation.

Options:

i = 4; 
j = 3;
M = nan(i,j)

M = 4×3

NaN NaN NaN NaN NaN NaN NaN NaN NaN NaN NaN NaN

M = ones(i,j)

M = 4×3

1 1 1 1 1 1 1 1 1 1 1 1

M = zeros(i,j)

M = 4×3

0 0 0 0 0 0 0 0 0 0 0 0

[Wenchen Liu]

Hi! can i do like this:

a = -1-i;

b = 1+i;

M = zeros(a,b)

since i want to create a general matrix. But matlab shows me:

Error using zeros

Size vector must be a row vector with real elements.

Error in Assignment (line 3)

M = zeros(a,b)

so I am not sure how to fix the code. Thank you!

[Adam Danz]

'a' and 'b' are the size of the matrix of 0s. Unless you've defined 'i', i is an imaginary unit. As the error message indicates, 'a' and' 'b' should be real numbers.

[Wenchen Liu]

but how could I define i？and should I define it before a and b?

[Adam Danz]

Yes, just like in my demo.