1) For which value of 'a' are you trying to plot the stem graph? If for all the values from a=0:0.01:11 then shift the plot outside first for loop like this:
for a=0:0.01:11
for n=1:nn
x(1)=0.4;
y(1)=2.9;
x(n+1)=x(n)+(d*((a*x(n))*(1-x(n))-((b*x(n)*y(n))/(x(n)+y(n)))));
y(n+1)=y(n)+(d*(((b*x(n)*y(n))/(x(n)+y(n)))-y(n)));
end
stem(y,x)
hold on
end
If you want to plot for a particular value of 'a' then the first for loop makes no sense.
2) what is the use of 't'?
3) Use of close all, clear all etc are not recommended. See here.
