Otsu’s segmentation : mat-lab code automatic Threshold value

I understand everything in the code expect one major think why did he choose the k to be 80 (manual Threshold )

Shouldn't the algorithm find the value automatically ??

 a=imread(‘cameraman.tif‘);
[r c]=size(a);
L = 256;
k = 80;

   p=zeros(1,L);

   %—–Step 1———-Computing Histogram-————————–
   for level=1:1:L
   for i=1:1:r
   for j=1:1:c
   if(a(i,j)==level)
   p(level)=p(level)+1;
   end
   end
   p(level) = p(level);
   end
   end
   p1 = zeros(1,L);
   p2 = zeros(1,L);
   m1 = zeros(1,L);
   m2 = zeros(1,L);
   var = zeros(1,L);
   temp=0;
   temp2 = 0;
   temp3 = 0;
   temp4 = 0;
   mg = 0;
   %—–Step 2—————-p1 calculation-————————–
   for thresh=1:k
   for ite= 1:thresh
   temp=temp+p(ite);
   end
   p1(thresh)=temp;
   temp=0;
   end

   %—–Step 2————–p2 calculation—————————–
   for thresh2 = k+1: L
   for i = thresh2:L

   temp2 = temp2 + p(i);
   end
   p2(thresh2)=temp2;
   temp2=0;
   end

   %——-Step 3————-m1 calculation—————————-
   for thresh3 =1:k
   for i =1:thresh3
   temp3 = temp3 + i* p(i);
   end
   m1(thresh3) = temp3/p1(thresh3);
   end

   %——-Step 3———-m2 calculation———————————–
   for thresh4 =k+1:L
   for i =thresh4:L
   temp4 = temp4 + i* p(i);
   end
   m2(thresh4) = temp4/p2(thresh4);
   end

    %——–Step 4———Global mean value calculation ———————

    mg = p1(k)*m1(k)+p2(k)*m2(k);
    %———-Step 5—–Between Class Variance ——————————
for v =1:L
var(v) = ( p1(v)*(m1(v)-mg)^2 )+( p2(v)*(m2(v)-mg)^2 );
end

    %—–Step 6——–finidng max var(k*)—————————–

 [maxNum IndexofMaxNum] = max(var)
output=im2bw(a,IndexofMaxNum/255);

% mat-lab built in otso
T2=graythresh(a)
output_m=im2bw(a,T2);

   subplot(1,3,1)
imshow(a);
title(‘Orignal Image’);
subplot(1,4,2)
stem(p);
title(‘Histogram‘);
subplot(1,4,3)
title(‘Output Image‘);
imshow(output);
subplot(1,4,4)
title('Output Image by matlab command');
imshow(output_m);