Convert row vector to matrix

Question

I need to write the code that turns a row vector into a matrix. For example, if I had a = 1 2 3 4 5 6 7 8 9, I want the matrix to be:

m = [1 2 3; 4 5 6; 7 8 9]

I have this, which doesn't work. Can anyone please assist me?

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

Moreover, I am not allowed to use any of Matlab's built-in functions (such as reshape).

If your vector was 1x8, would the matrix be 2x4, or 4x2? Or is it only for 3x3? If you create a matrix of zeros of the size you want the result to be (for example, A=zeros(3,3)) then you can do A(:)=a. You will have to check whether the elements go to the right places in the matrix. — David
– David, Commented Oct 7, 2015 at 5:19
Posting homework assignments is usually not well received on SO, but as you showed what you have tried, +1 =) Have a look at Shai's edit to your question (you can click the "edited x minutes ago" link). Your original loop was not pretty =) — Stewie Griffin
– Stewie Griffin, Commented Oct 7, 2015 at 6:15
A note here, for loops automatically steps up the loop index. The loop statement for i= 1:length(a) in Matlab is similar to what in java or c++ is for (int i=1; i<=length(a); i++). However, one can assume that the implementation is more sophisticated than that since for i=a would perform a similar behaviour to for (T t : a), where T is its type. So the point is that you do not need to increment i in the loop. It is done implicitly in a for loop. — patrik
– patrik, Commented Oct 7, 2015 at 7:11

rst · Accepted Answer · 2015-10-07 07:55:50Z

4

Use reshape

a = [1 2 3 4 5 6 7 8 9]; A = reshape(a, 3, [])'

where the third argument is taken automatically (the number of cols)

Or if you cannot use any MATLAB build in function use this here

A = zeros(3,3); %// or A(3,3) = 0 by thewaywewalk (thank you) A(:) = a; A = A';

edited Oct 7, 2015 at 7:55

answered Oct 7, 2015 at 5:15

rst

2,7644 gold badges24 silver badges57 bronze badges

Sign up to request clarification or add additional context in comments.

4 Comments

William Diuguid Over a year ago

I can't use any MATLAB functions.

Robert Seifert Over a year ago

in case zeros is also a forbidden function, you could use A(3,3) = 0 for pre-allocation.

patrik Over a year ago

@thewaywewalk Are there any case where zeros(N,M) actually is preferred over A(N,M) = 0;? zeros is of course safer, since this will not modify an existing variable. But still if you are in this situation you are pretty much screwed anyway.

Robert Seifert Over a year ago

@patrik Here is an extensive article answering your question.

Community · Accepted Answer · 2017-05-23 11:58:22Z

Of course, using reshape is the right way to convert a vector into a matrix.
However, since you do not want to use any Matlab function, you can use a loop

A = zeros(3,3); %// preallocate for ii=1:3, A(ii,:) = a( (ii-1)*3 + 1:3 ); %// put one row into place end

Or, more generally, if you want to reshape a H*W vector into a H-by-W matrix:

A = zeros(H,W); for ii=1:H A( ii, : ) = a( (ii-1)*W + 1:W ); end

PS,
Note that it is best not to use i as a variable name in Matlab.

Collectives™ on Stack Overflow

Convert row vector to matrix

2 Answers 2

4 Comments

Comments

Linked

Hot Network Questions

Collectives™ on Stack Overflow

2 Answers 2

4 Comments

Comments

Linked

Related