Looping past the end of an array - I don't see how?

When I run this code, it runs K times and bombs (depending on how the array was randomized) due to an array index out of bounds for the while loop on line 19. But I thought I prevented that with the preceeding 4 statements. ??

k=0;
while 1==1
novel=1:80;
novel=novel(randperm(80));
targ=ones(80)*81;
freq=ones(480)*82;
stim=1:640;
stim(1:80)=novel(1:80);
stim(81:160)=targ(1:80) ;
stim(161:640)=freq(1:480);
stim=stim(randperm(640)); %  Is this truely a random perm ?

% Scan looking for 2 n/t in a row, move the 2nd further along
% and replace with a frequent.
si=1;
while si<639
    if (stim(si)<81) && (stim(si+1)<81) && (si<639)
        i=si+2;
        while (stim(i)<82) && (i<641) i=i+1; end
        if (i<641) && (stim(i)==82)
            stim(i)=stim(si+1); %push the right  one right
            stim(si+1)=82; % replace with a frequent
            si=si+2;
        end
    end
    si=si+1;
end
% Now repeat the process scanning in the opposite direction
% in case n/t pairs were bunched up at the end of the vector.
si=640;
while si>2
    if (stim(si)<81) && (stim(si-1)<81)  && (si>2)
        i=si-2;
        while( stim(i)<82) && (i>2) i=i-1; end
        if (i>0) && (stim(i)==82)
            stim(i)=stim(si-1); % push the left one left
            stim(si-1)=82; % replace with a frequent
            si=si-2;
        end
    end
    si=si-1;
end
display('First 40');
stim(1:40)
display('Last 40');
stim(601:640)
k=k+1;
k
end

Answer 1

[Rick Rosson]

Please format your code. If you don't know how, please review the following:

Markup help

Thanks!

Rick

Answer 2

[Steve D]

Is the problem that the array indexes from 0 instead of 1?

Answer 3

[Rick Rosson]

Big improvement. Thanks!

Answer 4

[Rick Rosson]

Have you tried setting a break-point on the offending line of code, and then examining the values of the relevant variables in debug mode?

[Steve D]

I never did figure out how to copy in code without losing the indentation.

On the breakpoints, It fails after a couple hundred runs. That would take hours.

For an array of 640 elements, does the index run valid from 0 to 639 or from 1 to 640?

[bym]

1 to 640

[Fangjun Jiang]

MATLAB uses 1-based index. change your number from 641 to 640 avoided the error but not sure if the algorithm is correct.

while (stim(i)<82) && (i<640)

Answer 5

[Steve D]

Thanks for your suggestions. It looks like matlab evaluates the while conditions in order such that the i that is over the limit, (stim(i)<82), is used before it is checked, && (i<640).

I reversed the order, and the problem went away.

3 Cheers, Steve