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;

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Anshuman S
Anshuman S 2017-7-20
评论： John BG 2017-7-23
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

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采纳的回答

Walter Roberson
Walter Roberson 2017-7-22
A = zeros(m+1,n+1);
A(:,1) = a;
A(:,end) = b;
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Jan
Jan 2017-7-22
@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! :-)

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更多回答（1 个）

John BG
John BG 2017-7-20
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:jgb2012@sky.com jgb2012@sky.com>
  2 个评论
Anshuman S
Anshuman S 2017-7-20
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
John BG 2017-7-23
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
<mailto:jgb2012@sky.com jgb2012@sky.com>

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