Help with matrix . Error message is being displayed : Subscript indices must either be real positive integers or logicals. Error in implicit (line 26) A(i,0) = a;

function [] = implicit(~)

%first non-linear equation.

err=input('min_error');
n= input('number_of_iterations');
m= input('number_of_points');

%drichlet boundary conditions

a=input('right hand side temp =');
b=input('left hand side temp = ');

A = zeros(m+1,n+1);

for i=1:m+1
    A(i,0) = a;
end

for i=1:n+1
    A(i,n+1) = b;
end

disp(A);

end

Answer 1

[Walter Roberson]

A = zeros(m+1,n+1);
A(:,1) = a;
A(:,end) = b;

[Anshuman S]

Sir, I have checked my code according to your corrections. It was just an indexing problem. I made a mistake in indexing the first column.

This code is working ;

   A = zeros(m+1,n+1);
  for i=1:m+1

      A(i,1) = a;
  end
  for i=1:n+1
      A(i,n+1) = b;

Thank you

[Walter Roberson]

You are missing an "end"

[Jan]

@Anshuman S: It is typical for Matlab that you can replace for loops by "vectorizing": You can move the indexing from the for command directly into the assignment:

for i=1:m+1
    A(i,1) = a;
end

Take the "1:m+1" from for and move it to:

      A(1:m+1,1) = a;

If A is pre-allocated as in Walter's answer, the index can be even omitted and the ":" is used:

      A(:, 1) = a;

This is nicer, leaner and faster than the loop. While the speed might not matter now, leaner code is a benefit, because you have less chances for typos! :-)

Answer 2

[John BG]

MATLAB matrix indexing does not use null or negative indices.

You can define reference vectors as function of a given matrix that take all sorts of values value, positive, negative or null values.

But once the reference value found, the index and not the reference value is the index to apply to the matrix being read.

When retrieving matrix elements through indexing, you have to pass the matrix index that corresponds to the sought elements, and such indices have to be positive and not null.

John BG

<mailto:[e-mail] [e-mail]>

[Anshuman S]

Thanks John; Could you please tell me; what are the changes I have to make in my code ? ( I want to make a m+1Xn+1 matrix and want to set the first and the last col of this matrix as a & b , which are user defined input number.

[John BG]

ok, my understanding of your question is you asking for the following:

clear all;close all

a=input('right hand side temp =');
b=input('left hand side temp = ');
n= input('number_of_iterations');
m= input('number_of_points');

A(1)=1;A(end)=b;

or perhaps you mean this

A([1:1:m])=a;
A([m+1:1:n+1])=b;

I doubt there's need for any for loop at all for the kind of matrix assignment you are asking for

John BG

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