Comments on: MATLAB Basics video: Storing data in a matrix from a loop

By: Doug

Doug — Tue, 10 Nov 2009 21:15:25 +0000

@Ashok

First, please do not use variable names that are MATLAB commands (std and mean).

Second, p(j) should be p(i)

Third, look at this line of code:

p(j) = mean(i) + std(i)*randn(100,1);

This says

The jth element of the matrix p should be the sum of a scalar(mean(i)) and a vector(std(i)*randn(100,1)).

This does not make sense to put a vector into the element of a matrix.

Fourth, usually asking for user input like that is slow, and causes trouble since people can type anything.

Enjoy,
Doug

By: Ashok

Ashok — Thu, 05 Nov 2009 08:58:33 +0000

how to store 10 or more random number in a loop a loop
for i = 1:n
mean(i) = input(’enter the mean value of distribution = ‘)
std(i) = input(’enter the std value of distribution = ‘)
p(j) = mean(i) + std(i)*randn(100,1);
end
how to make it work becoz there is string mismatch. Please tell me

By: Doug

Doug — Tue, 27 Oct 2009 14:09:09 +0000

@Leo,

Here is the “English version” of that code.

“vec = []” makes an empty variable named ‘vec’.

“vec = [vec val]” makes the variable ‘vec’ equal to the variable ‘vec’ and variable ‘val’ in a row. (This would fail if ‘vec’ was not already defined, thus the above line.) This was done in a loop, so the empty ‘vec’ is there for the first iteration really.

-Doug

By: leo

leo — Tue, 27 Oct 2009 07:10:10 +0000

Dear Doug
I have a question in your code ‘October 9th, 2009 at 13:53′

vec = [];
vec = [vec val];

I don’t know the meaning of these statements.
I just started the matlab.
So, i don’t know well about matlab.
Could you tell me the meaning?

By: Doug

Doug — Mon, 19 Oct 2009 18:08:32 +0000

@michael,

Without your code, I can not debug this. Please post the shortest version of your code that shows the behavior.

Dou

By: Michael

Michael — Thu, 15 Oct 2009 21:28:56 +0000

Hi, I think my problem is simple but its been bugging me. Basically I want to save each step in the loop as a single column vector. Inside the loop is a matrix times a column vector which should result in a colunm vector. In the end I will build a matrix from my collection of vectors i get from the loop. How can I get the loop to save each iteration as a column vector. thanks alot

I’m getting this error message below
??? In an assignment A(I) = B, the number of elements in B and I must be the same.

By: Doug

Doug — Fri, 09 Oct 2009 17:00:37 +0000

Steve,

I can see your point. I think this comes down to “religious issues” I started with something like yours, but I did not like the repeated “val >= 0″ statement. I can see that my code obscures the exit condition though.

Thanks for contributing!
Doug

By: Steve L

Steve L — Fri, 09 Oct 2009 14:57:01 +0000

Normally in a loop I’d preallocate the vector into which I’m going to insert data; however, in this case the limiting factor on the speed of the loop is not going to be reallocating the memory for the growing vector but waiting for the user to enter the value so it’s not as big a concern. I would do things slightly differently from Doug’s answer, to avoid the need for the infinite WHILE loop and the BREAK:

val = Inf;
vec = [];
while val >= 0
    val = input('Next value (negative number to quit): ');
    if val >= 0
        vec = [vec val];
    end
end

or, slightly sneakier:

val = Inf;
vec = [];
while val >= 0
    val = input('Next value (negative number to quit): ');
    vec = [vec val];
end
vec(end) = []; % Delete the negative value at the end of vec

I find it easier to read the code if condition is in the WHILE statement rather than somewhere inside the loop, but that’s just my personal preference.

By: Doug

Doug — Fri, 09 Oct 2009 13:53:54 +0000


vec = [];

while 1
    val = input('Next value (negative to quit): ');
    if val >= 0
        vec = [vec val];
    else
        break
    end
end

Enjoy, Doug

By: Eta

Eta — Thu, 08 Oct 2009 16:06:22 +0000

i am trying to create function that a while loop that prompts the user for a number. if the number is 0 or more, the value is stored in a vector and the user is prompted to enter another number and so on but if the number is negative, the function stops and returns the vector of the previously stored numbers.
this is my code but i know i am doing something wrong in the section where i store the value in a vector and when displaying this vector.

% x is input data value
function [] = whileInput()
x = input(’Please enter a value: ‘);
while x >= 0
% store value in vector
vector(x) = 1:length(x);
% ask for new value
x = input(’Please enter your next value: ‘);
end
if x < 0
disp(vector);
else
end

please help. thank you