I want to get child_fitness=0 i got some eror please any one figure it out let me know thanks?

clear;close all;clc;
GENSIZE=20;
target  = input('Enter a string (max 30 chars in length): ', 's');
genepool=[];
pop= char(randi(127,GENSIZE,length(target)))
for i=1:GENSIZE
  dna = pop(i,:);
  fitness = calc_fitness(dna, target)
  genepool(i,:)=(fitness)
  end
while fitness ~=0
  parent1 = get_parent(genepool);
  parent2 = get_parent(genepool);
  child = mutate(parent1, parent2);
 if fitness < genepool(end).fitness
      genepool(end) = child
  end
end
function [fitval]= calc_fitness(source, target)%def calc_fitness;
fitval=0;
for i = 1 : length(source)
  fitval= fitval + (double(target(i)) - double(source(i))) ^ 2;
end
endfunction [child_dna,fitness]=mutate(parent1,parent2)
child_dna =parent1(1,:);
start=randi(size(parent2(1,:)),1,1);
stop=randi(size(parent2(1,:)),1,1);
if(start>stop)
  tmp=start;
  start=stop;
  stop=start;
end
child_dna(start:stop)=parent2(1,start:stop);
charpos=randi(size(child_dna),1,1);
child_dna(charpos)=char(uint8(child_dna(charpos))+randi(3,1,1)-2);
fitness = calc_fitness(child_dna, target)
end
function [x] = get_parent(genepool , GENSIZE)
wRndNr = rand() * rand() * ( GENSIZE );
wRndNr =  floor(wRndNr);
if  wRndNr ==0
  wRndNr =1;
end
x = genepool(wRndNr,:);
end