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, Oct 07 '15 at 05:19
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, Oct 07 '15 at 06:15
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, Oct 07 '15 at 07:11

rst · Answer 1 · 2015-10-07T07:55:50.490

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 07 '15 at 07:55

answered Oct 07 '15 at 05:15

rst

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I can't use any MATLAB functions. – William Diuguid Oct 07 '15 at 05:17
2

in case `zeros` is also a forbidden function, you could use `A(3,3) = 0` for pre-allocation. – Robert Seifert Oct 07 '15 at 06:18
@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. – patrik Oct 07 '15 at 07:14
@patrik [**Here is an extensive article answering your question.**](http://undocumentedmatlab.com/blog/preallocation-performance) – Robert Seifert Oct 07 '15 at 08:06

score 3 · Answer 2 · edited May 23 '17 at 11:58

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.

Convert row vector to matrix

2 Answers2