matlab - how to covert 1/4 elements of a matrix to zero -

i have 512x512 matrix .i want convert 1/4 elements of matrix 0/how can can help.my program folows

clc; clear all; close all; a=imread('images.jpg'); b=rgb2gray(a); figure,imshow(b); c=double(b); figure,imshow(a); figure,imshow(c); d=rand(512,512); e=exp(2*pi*d); f=c.*e; %figure,imshow(f); g=fft2(f); h=rand(512,512); i=exp(2*pi*h); j=g.*i; k=fft2(j) 

%here k matrix of order 512x512.in next step want chanage 1/4 elements ero can help

if want points chosen randomly:

if have 512 x 512 array:

v=randperm(512^2); v=v(1:512*128); k(v)=0; 

the above can abbreviated in recent matlab versions

k(randperm(512^2,512*64))=0; 

edit

more generally, image array "k"

(case size(k) nc x nr)

ns = numel(k); v=randperm(ns);    v=v(1:round(ns/4)); k(v) = 0; 

or in recent version of matlab

ns = numel(k); k(randperm(ns,round(ns/4)))=0;  

(case size(k) nc x nr x 3)

ns = numel(k)/3; v=randperm(ns);    v=v(1:round(ns/4)); k(v)=0; k(v+ns)=0; k(v+ns*2)=0;