Show column which starts with certain numbers

Hi, i have matrix size 500x500. And i need to find column which starts with vector [1 2]'. Only one column in whole matrix starts with these two certains numbers. The matrix looks something like this

[1 1 1 1 1]
[1 2 3 4 5]
[1 2 3 4 5]

I need function that tells me column number two starts with 1 and second value is 2.

Thanks for help!

Answer 1

[Cedric]

If your matrix is named M, you can do

 col = M(1,:) == 1 & M(2,:) == 2 ;

to get a vector of logicals indicating the relevant column, and

 find(col)

to get the column number.

[Youssef Khmou]

efficient

Answer 2

[Youssef Khmou]

hi, try :

 a=rand(500);

 a(1:2,312)=[1 2]; % for example the column 312 starts with  [1 2] 
 b=a(1:2,:);
 test=[1 2]';
 for n=1:length(b)
  diff(n)=norm(b(:,n)-test);
 end
 Index=find(diff==0);

Index is the number of the column,