Best way to delete first row from matrix in matlab

Question

I plan on deleting the first row of a matrix multiple times and was wondering what the best/most efficient way of doing this would be.

I know I can do something like this

  M(1,:)=[]

or

M = M(2:end)

but I am not sure which is the best way or if there is another better way.

Answer 1

Hey just tested those 2 methods with tic and toc This is the code I used:

A=rand(100,100000);
tic
a=A(2:end,:);
t1=toc
tic
A(1,:)=[];
t2=toc

and this is the result:

t1 =

    0.0603    

t2 =

    0.0744

If you use longer columns it gets even more obvious:

A=rand(10000,100);
t1 =

    0.0083  

t2 =

    0.0124

So saving the columns you want to keep seems to be faster.

Edit

It was commented that tic and toc are not "trustworthy" in the millisecond domain so it was recommended to use loops to run the code multiple times. But the result doesn't change .

A=rand(100,100000);
size_A=size(A);
tic
for k=1:1:100
    A1=A;
    A1=A1(2:end,:);
end
t1=toc
tic
for k=1:1:100
    A1=A;
    A1(1,:)=[];
end
t2=toc 

this results in:

t1 =

    7.5237

t2 =

   15.2234

Answer 2

Generally it might be faster to keep what you want. Depending on the dimensions of your matrix however results may vary. Consider the following test case where two matrices are generated, A1, and B1, of dimension 100x100000 and 100000x100. The results are obtained from the profile viewer but tic toc measurements confirmed these results.

A1=rand(100,100000);

for ii=1:100
    A=A1;
    A=A(2:end,:);
end

for ii=1:100
    A=A1;
    A(1,:)=[];
end

B1=rand(100000,100);

for ii=1:100
    B=B1;
    B=B(2:end,:);
end

for ii=1:100
    B=B1;
    B(1,:)=[];
end

The results clearly show that the first case (keeping what you want on a matrix with lots of columns) is very slow actually.

There is no clear this or that is faster though. You should try to time for your situation!