why i get the undefined variable message?

I have this code for QAM:

p=(sign(randn(N,1))+2)/2;
for i=1:length(p/2)-1
    if p(i)==0 && p(i+1)==0
        s(i)=1+1i;
    elseif p(i)==0 && p(i+1)==1
        s(i)=1-1i;
    elseif p(i)==1 && p(i+1)==1
        s(i)=-1-1i;
    elseif p(i)==1 && p(i+1)==0
        s(i)=-1+1i;
    end
end
scatterplot(s);

and when i try to run this thing i get the message :undefined variable 's'.

Do you know what should i do to fix this.I dont think its a code fault but its a matlab problem .

Thank you.

[Adam Danz]

If none of your conditions are met, s will never be defined. So when you get to the last line of your code, s is undefined.

The problem is that none of your conditions are being met and 's' has no default value for when that happens.

Answer 1

[KALYAN ACHARJYA]

and when i try to run this thing i get the message :undefined variable 's'.

First:

Because you did not defined N

Second for S, considering N=10 here, whatever you choose the value of N, p reflects numbers of 0.5 and 1.5

Also

p =
    0.5000
    1.5000
    1.5000
    0.5000
    1.5000
    0.5000
    0.5000
    1.5000
    1.5000
    1.5000

Neither the any if condition is true for p value, so "s" value didnot get assign. Beacuse p(i) and p(i+1) ==0 or 1 is not true for any case