Need Help with For Loops

I need help with this lab assignment. Dont see where I am messing up. Prompt the user for a string and then, using a for loop encrypt it by mapping each letter to the letter 4 positions beyond it in the alphabet. That is, a-->e, b-->f, c-->g, etc. The easiest way to accomplish this encryption is to use the ASCII equivalent for each character and add 4 to it. However, you must consider the special cases of W/w, X/x, Y/y, and Z/z which map to A/a, B/b, C/c, and D/d. Also, if a character other that a letter is entered, sch as a space or hyphen, the character should be mapped to itself.

Teacher then asked in class:

1. Read in a sentence
2. Echo sentence one letter at a time
3. Add 4 to each letter and echo
4. Use switch/if to account for special cases.

flag=false;

message=input('Enter a message to encode','s');

code='';

for i=1:1:length(message)

%fprintf('%s',message(i),13);

code(i)=int8(message(i))+4;

 if code(i)>=65&&code(i)<=86||code(i)>=97&&code(i)<=118

      code(i)=code(i)+4;

elseif code(i)==87;

      code(i)=65;

elseif code(i)==88;

      code(i)=66;

elseif code(i)==89;

      code(i)=67;

elseif code(i)==90;

      code(i)=68;

elseif code(i)==119;

      code(i)=97;

elseif code(i)==120;

      code(i)=98;

elseif code(i)==121;

      code(i)=99;

elseif code(i)==122;

      code(i)=100;

 else

    code(i)=code(i)+4;

 end

end

fprintf('%s',code);

[Jan]

Please explain the problems you get. We can guess them, but this is not efficient. And in many cases, an exact description of the problem involves a strategy to obtain a solution already.

[Sean]

We have to input a string to encode:After all, tomorrow is another day.

After all, tomorrow is another day is encrypted as Ejxiv epp, exsqsvvsa mw ersxliv hec. My code has to to the first statement and encode it into the second statement

[Jan]

@Sean: Ok. What is the problem of your program? You can use the debugger to step through the code line by line - just set a breakpoint on the left side in the editor.

Answer 1

[Jan]

qwk = mrtyx('Irxiv e qiwweki xs irgshi: ', 'w');
jsv m = 1:pirkxl(qwk)
   mj mwpixxiv(qwk(m))
      qwk(m) = qwk(m) + 4;
      mj ~mwpixxiv(qwk(m))
         qwk(m) = qwk(m) - 26;
      irh
   irh
irh

PS. Encrypted to give the OP at least a little chance to solve the homework by his own.

[Thomas]

Awesome response... Though the key can be guessed pretty easily..

[Fangjun Jiang]

+1. Post of the day!

[Jan]

The message *is* the key. :-)

Answer 2

[Fangjun Jiang]

At the beginning of the for-loop, you add an incremental of 4 right away. That is not right.

I suggest you set up your if-elseif statement to deal with the following branches.

1. 'a' to 'v'
2. 'w' to 'z'
3. 'A' to 'V'
4. 'W' to 'Z'
5. All others

This will make your logic very clear and then probably you can combine some together because the operation is the same.

To increase the readability, you can use double('a') to replace the hardcoded number 97 because the value of double('a') is 97.

[Jan]

You can use 'a' directly without casting it to DOUBLE.

[Fangjun Jiang]

Indeed, you can use code(i)>'a' !

[Sean]

Im confused on how you would write that in the script. I understand what you are saying about doubles, but i tried it and an error message came up

[Fangjun Jiang]

I'll give you a hint. This is not really a hard task. Type doc if to see examples.

If code(i)>='a' && code(i)<='v'

code(i)=code(i)+4;

elseif

...

end

[Sean]

It still is displaying the wrong characters. Instead of displaying Ejxiv epp, xsqsvvsa mw ersxliv hec it is displaying

(Inbmz(itt4 (bwuwzzw(qa(ivwbpmz(li

[Jan]

+1: Your assistance is more helpful than posting the solution.

Answer 3

[Jan]

The lookup table approach is more compact:

qwk = mrtyx('Irxiv e qiwweki xs irgshi: ', 'w');
wlmjx                     = divsw(1, 256);
wlmjx(['e':'z', 'E':'Z']) = 4;
wlmjx(['a':'d', 'A':'D']) = -22;
qwk = glev(qwk + wlmjx(qwk));

[Jan]

Irxiv e qiwweki!

Reminds me of "atte katte nuwa, emisa demisa dulla misa de".

Answer 4

[Naz]

message=input('Enter a message to encode','s');

for i=1:length(message)

     if (message(i)>=65 && message(i)<=90)
        t=message(i)+4;
             if t>90
                message(i)=t-90+64;
             else
                message(i)=t;
             end
     elseif (message(i)>=97 && message(i)<=122)
             t=message(i)+4;
             if t>122
                message(i)=t-122+96;
             else
                message(i)=t;
             end

     end

end

FINALLY!

[Sean]

Thanks for the help. Much appreciated

[Jan]

@Sean: If you submit this as solution of your homework, be sure to mention Naz as author. Your teacher knows this forum, too.

[Fangjun Jiang]

@Naz, we usually don't provide complete solution to simple homework assignment.