Setting break points to find errors in for loops

Three minute video shows a debugging session for a tricky error

Description

clear
num = [1 1 1 1 1 1 1];
den = [1 2 3 0 5 6 7];

for i = 1: numel(den)
    if den(i) == 0
        a(i) = 1
    else
        a(i) = num(i)/den(i)
    end
end

plot(a,'o-')
                    
The above code does not behave as expected. The values in the middle of the vector are zeroed out. This debugging sesion figures out why this happens.
                    
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• MATLAB release: MATLAB 7.7 (R2008b)
• HTML Files: Video from http://blogs.mathworks.com/pick

Other Files

141 Basics debuging example.flv,
141 Basics debuging example.js,
141 Basics debuging example_config.xml,
141 Basics debuging example_controller.swf,
141 Basics debuging example_nofp_bg.gif,
141 Basics debuging example_preload.swf,
FlashTemplate.css,
mCode/141 Basics debuging example.camrec,
mCode/main.asv,
mCode/main.m,
ProductionInfo.xml,
swfobject.js