# Array operations in Matlab?

1. Dec 5, 2004

### ichbinschneller

Array operations in Matlab?????

Is it possible to apply operations to the rows of a matrix as an array? Such as I have a matix A which is 30 rows by 1 column and I want to treat each row as a vector such that I can solve for the polynomial roots of each row using the roots comand (treating each row as its own equation and the matrix A a system of eqautions)?

Also would it be possible to take the square roots of each row of a same size matrix. Meaning say I had the same size matrix and I want to take the square root of each row and have the output be a matrix of the same size just with the values of those square roots?

2. Dec 6, 2004

### enigma

Staff Emeritus
Hi, Iamfaster

Welcome to PF!

If you need to perform a function on each row of a matrix like

function(A), where A is a 30x1 vector, I think you'll have to break it up and write a for loop

Code (Text):

for i=[1:length(A)]
function(A(i));
end

or if A is 30x2 (or more)
Code (Text):

for i=[1:length(A)]
function(A(i,:));
end

if you want to take the square root, you can find it using the .^ operator, which raises each element to a power as opposed to the ^ which performs matrix multiplication

Code (Text):

A.^(1/2)

3. Dec 6, 2004

### ichbinschneller

Thanks, I am getting closer now but I still cant seem to enter the code in the right form.
Say I have a matrix

g1=
5 9 8
4 7 6
1 2 8

and I want a matrix g2= roots of each row as if each row was a vector consisting of the coeficients of a polynomial. I have tried using code such as

for i=[1:length(g1)]
g2(i)=roots(g1(i));
end

but I keep getting errors. What am I doing wrong? What code would yield the results I desrire?

4. Dec 6, 2004

### enigma

Staff Emeritus
You're not indexing the entire row.

g1(1) doesn't have any meaning for a m by n matrix when n>1.

If you want to reference a single term, you need to put in the entire address lke so:

g1(1,2)

to reference the first row, second column.

If you want to reference the entire row, you need to put in the : operator like so:

g1(1,:)

Just using : by itself is "shorthand" for explicitly defining the starting and ending conditions.

If your matrix g is 4x4

Code (Text):

g=[1,2,3,4
6,7,8,9
10,11,12,13
14,15,16,17]

typing

a=g(2,2:3) will give the result a=[7,8]
b=g(:,1) will give the result b=[1;6;10;14]
c=g(3,1:4) will give the result c=[10,11,12,13]
and
d=g(3,:) will give the same result as c. d=[10,11,12,13]

Hope that helps,

5. Dec 7, 2004

### gerben

also, lose the [ and the ]
like this:

for i = 1:length(g1)
g2(i, :) = roots(g1(i, ;
end

6. Dec 7, 2004

### enigma

Staff Emeritus
Works with or without. I usually leave them in.

7. Dec 9, 2004

### gerben

yes it works because [1:5] evaluates to: 1, 2, 3, 4, 5 (you could say: for i = [1,4,18])
but you can tell the loop that you want "it" done for the numbers 1 through 5 instead of telling it that you want "it" done for the numbers 1, 2, 3, 4 and 5.

(I guess you leave them in because of similarity with other programming languages that usually have brackets there, like in C/C++: for(i=1; i<5; i++) )