if condition for Matrix

I have Matrix for example

M = ones(10,10)

how I make ( if condition) s.t

M(i-tau,j)=0 if i-tau <0,

where i =1:10 and j =1:10

Thanks

[Torsten]

M(i-tau,j) does not exist if (i-tau)<0. So you can't set it to a specified value ( 0 in this case ).

Answer 1

[Steven Lord]

M(i-tau,j)=0 if i-tau <0,

There's no such thing as row -1 of a matrix in MATLAB. Depending on what you're trying to do you could limit the values of i over which you iterate to those that make i-tau a positive integer value or you could detect the condition where i-tau is negative (actually you'd want to detect when it's non-positive since there's no such thing as row 0 in a matrix in MATLAB either. The first row in a matrix in MATLAB is row 1.) and skip attempting to index into M entirely.