how to preallocate rows or columns of matrix through iteration?

hello, good morning everyone. please help me to preallocate the 'locs', thankyou.

clc; clear all; 
I = zeros(151,151);
W=64;
%locs=zeros(151,1);  
for 
    x = ;
    y = ;	    
 end 

Answer 1

[Walter Roberson]

locs = zeros(length(X), length(Y)) ;

[Walter Roberson]

Use

locs = false(size(I));

[vishnu]

it works now. so many thanks :)