Converting a Matlab program to Octave

[momozilla76]

Im new to Octave and I am using a previously made program to learn how to use
octave but I am having troubles with the function.

function [o] = adaptiveThres(a,W,noShow); 
%Adaptive thresholding is performed by segmenting image a 
[w,h] = size(a); 
o = zeros(w,h); 
%seperate it to W block 
%step to w with step length W 
for i=1:W:w 
for j=1:W:h 
mean_thres = 0; 
if i+W-1 <= w & j+W-1 <= h 
 mean_thres = mean2(a(i:i+W-1,j:j+W-1)); 
 mean_thres = 0.8*mean_thres;  57
 
 o(i:i+W-1,j:j+W-1) = a(i:i+W-1,j:j+W-1) < mean_thres; 
 end; 
 end; 
end; 
if nargin == 2 
imagesc(o); 
colormap(gray); 
end; 


output:
error: 'a' undefined near line 3 column 13
error: evaluating argument list element number 1

can someone please help me 



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