How can I debug this?

clc;clear
C1 = input('Please enter a code to break: ' , 's');
if length(C1) ~= 6
    disp('Decoy message: Not a six-digit number.');
else
    A = mod(sum(C1 - '0'),2);
    if A == 1
        disp('Decoy message: Sum is odd.')
    else
        C2 = (C1(1) - '0') * (C1(2) - '0') - (C1(3) - '0');
        failed = false;
        switch C2
            case 1  
                Day = 'Rescue on Monday';
            case 2
                Day = 'Rescue on Tuesday';
            case 3
                Day = 'Rescue on Wednesday';
            case 4 
                Day = 'Rescue on Thursday';
            case 5
                Day = 'Rescue on Friday';
            case 6 
                Day = 'Rescue on Saturday';
            case 7
                Day = 'Rescue on Sunday';
            otherwise
                disp('Decoy message: Invalid rescue day.');
                failed = true;
        end
        F1 = str2double(C1(4));
        F2 = str2double(C1(5));
        F3 = str2double(C1(6));
        if mod(F1,3) == 0
            B = F2 - F3;
        else
            B = F3 - F2;
        end
        if ~failed
            switch(B)
                case 1
                    Position = ' at the bridge.';
                case 2
                    Position = ' at the library.';
                case 3
                    Position = ' at the river crossing.';
                case 4
                    Position = ' at the airport.';
                case 5
                    Position = ' at the bus terminal.';
                case 6 
                    Position = ' at the hospital.'; 
                case 7
                    Position = ' at St. Petes Church.';
                otherwise
                    disp('Decoy message: Invalid rendezvous point.');
            end
            disp([Day , Position])
        end
    end
end

If I enter 918990， I do get what I want, it will show Decoy message: Invalid rendezvous point. But there will appear an error message, how I can debug this?

Answer 1

[Dana]

The error is because if you end up in the "otherwise" case of your switch(B), the variable Position doesn't ever get defined, so when you try to do disp([Day , Position]) it causes an error. To avoid this, you can just put a return statement:

.
.
.
            otherwise
                disp('Decoy message: Invalid rendezvous point.');
                return
            end
            disp([Day , Position])
        end
    end
end

[Minhao Wang]

thank you so much!

[Minhao Wang]

sry, I have another question, I just realize this homework does not allowed us to use 'return' statement. Any other suggestion?

[Dana]

You could set up a flag that tracks whether you ended up in the "otherwise" case (essentially what you did with your failed variable), and use that to decide whether the disp([Day , Position]) step gets executed.

[Minhao Wang]

Can you please show me how is it done? I am a bit lost to set up a flag in this situation....

[Minhao Wang]

oh, okay i got it ! thank you !

[Dana]

You already did it once with your failed variable: you set it equal to false initially, and then if you ended up in your "otherwise" case you switched it to true, and then you only proceeded to the switch(B) statement if failed=false. It's exactly the same logic in this case.