How to create a matrix using nested for loops

t=linspace(0,2,100);
x=linspace(0,1,50);
n=linspace(1,5,5000);
sum=0;

   for x1=0:50

      for t1=0:100

          for  n1=1:5000

          sum(x1,t1,n1)= sum(x1,t1,n1) + x.^n/factorial(n)+ t.^n/(50*factorial(n));

          end 

      end 
  end

This is my code. Essential I'm trying to create a matrix of the solutions to the sum so I can later use the functions surf. However once I run the code I get "Subscript indices must either be real positive integers or logicals." What I'm doing wrong and can be done to fix my code? Thank you.

[Stephen23]

@jesus escareno: you should NOT call any variable sum, as this is the name of an extremely important inbuilt function sum. When you define your variable with the same name ("shadowing" the inbuilt fucntion) it will stop lots of code from working. DO NOT USE SUM as a variable name!

Answer 1

[Star Strider]

The problem, simply, are:

   for x1=0:50

      for t1=0:100

Zero is not a positive integer. In MATLAB, all indices and index references begin with 1. If you want to use the indices with zero values in your calculations, subtract 1 from the index values, but always define indices that are positive integers (integers greater than or equal to 1).

[jesus escareno]

yea that took care of that problem......but I'm getting Matrix dimensions must agree.

[Star Strider]

First — call your variable something other than ‘sum’ since that is the name of a (very important) built-in MATLAB function, and it will cause problems for you if you later want to use it in your code. This is called ‘overshadowing’, and is to be avoided at all costs!

Second — You have created ‘t’, ‘x’ and ‘n’ as vectors, so you have to refer to them element-wise (that is to say element-by-element) in your loop calculation as you have defined it. Referring to them as vectors (although correct MATLAB syntax when using vectorised calculations), will cause the errors you are getting when you specify element-by-element calculations as you do in your code. You need to subscript each of them appropriately for your code to work.

I am not certain what you are doing, but there is nothing inherently wrong with your code as you have written it, other than using ‘sum’ as a variable name. You simply have to do element-wise addressing for your variables, and it should create the 3D matrix you want.