CX 103: Introduction to Computers

Lecture 25







 
 



Comments on Exam 2







7. What is the output of the following?

var Grade = 62;

if ( Grade >= 60 )

document.writeln("You Passed<BR>");

else

document.writeln("Low Grade<BR>");

document.writeln("Study Harder<BR>");

The else clause ends with ("Low Grade<BR>");

The statementdocument.writeln("Study Harder<BR>"); is a separate action.
 
 
 
 

To get both document.writeln statements as part of the else clause:
 
 
 
 

var Grade = 62;

if ( Grade >= 60 )

document.writeln("You Passed<BR>");

else

{

document.writeln("Low Grade<BR>");

document.writeln("Study Harder<BR>");

}

10. Rewrite the following for loop as an equivalent while loop

var i;

for (i = 0; i <= 5; i = i + 1)

document.writeln("Middlebury Rocks!");
 
 

var i;

i = 0; ( initialize i)

while ( i <= 5)

{ document.writeln("Middlebury Rocks!");

i = i + 1 } ( increment i)
 
 
 
 

i must have a value for the first termination test.

We must also change i inside the body of the loop
 
 

11. Given the following declarations, write a loop which determines and reports the product of the numbers in the array.

var i, A = [2, 1.414, 7.8, 1776, 3.14156, 8, 9, 11], Product;

Product = 1;

for ( i = 0; i < A.length ; i = i + 1)

Product = Product * A[i];

document.writeln("The product is " + Product);
 
 
 
i Product
i = 0 1 * A[0] = 1 * 2 = 2
i= 1 (1*2) * A[1] = 1* 2 * 1.414 
i= 2 (1*2*1.414)*A[2] = 1* 2 * 1.414 * 7.8
   
   


 

12. With the declarations below what do the following JavaScript statements produce?

var W = "WORDS";
 
1. function RemoveSomething (A)
2. { var X = "";
3. for (i = 0; i <= A.length-2; i = i+1)
4. X = X + A.charAt(i);
5. W = X;
6. document.writeln(W + "<BR>");
7. }
8.  
9. RemoveSomething(W);
10. RemoveSomething(W);
   

 

Action begins with Line 9:

RemoveSomething(W);

Current Value of W is "WORDS"

The function is called:

A copy of W is put into A

A.length is 5
 
A[0] A[1] A[2] A[3] A[4]   X
W 0 R D S    

 

Line 3: for (i = 0; i <= A.length-2; i = i+1)

X = X + A.charAt(i);

i will take on values 0, 1, 2, 3
 
step i Action
i = 0 X = X + A.charAt(0); 

X = "" + "W" = "W"
i = 1 X = X + A.charAt(1); 

X = "W" + "O" = "WO"
i = 2 X = X + A.charAt(2); 

X = "WO" + "R" = "WOR"
i = 3 X = X + A.charAt(3);

X = "WOR" + "D" = "WORD"

 
 
 

The Loop is now Terminated with X = "WORD"
 
 
 
1. function RemoveSomething (A)
2. { var X = "";
3. for (i = 0; i <= A.length-2; i = i+1)
4. X = X + A.charAt(i);
5. W = X;
6. document.writeln(W + "<BR>");
7. }
8.  
9. RemoveSomething(W);
10. RemoveSomething(W);
   

Now we're ready to do Step 5:

The result is W = "WORD"

Finally, Step 6 displays WORD on the screen.

Thus the effect of calling the function RemoveSomething is to chop off the last character of W and print the remaining shortened string followed by a line break.

We have completed Step 9, the first call of the function.

The result of Step 10 is to chop off the D in

WORD so that W becomes WOR and then to print WOR

Hence the output is:

WORD
WOR
 


 

13. With the declarations of Problem 12, what is the outcome of the following?
 
1. function Twist(A)
2. { L = A.length;
3. document.write( A.charAt(L-1) );
4  if (L != 1) 
5.     { L = A.length;
6.       RemoveSomething(W);
7.       Twist(W);
8.       }
9. }
10. Twist(W);

 
 
 

10. Twist(W) = Twist("WORDS")
 
 
 
    Screen Action
1. function Twist("WORDS")    
2. { L = 5    
3. document.write( A.charAt(4) ); S  
4 . if (L != 1)     
5. {     
6. RemoveSomething(W); SWORD W=WORD
7. Twist(W);    
8.  }    
9. }    

 

7. Twist(W) = Twist("WORD")
 
    Screen Action
1. function Twist("WORD")    
2. { L = 4    
3. document.write( A.charAt(3) ); SWORD

D

  
4 . if (L != 1)     
5. {     
6. RemoveSomething(W); SWORD

DWOR

 W=WOR
7. Twist(W);    
8.  }    
9. }    

 

Each call of Twist writes the last character of the current value of W, chops off that character fom W, and then writes the remainder of W on the same line and finally causes a line break:
 
 

Thus the output is

SWORD

DWOR

RWO

OW

W

14. What is the output of the following JavaScript program if X and Y are positive integers?

<HTML>

<HEAD>

<SCRIPT LANGUAGE = "JavaScript" >

function Mystery(A,B)

{

if ( B = = 1) return A;

else return A + Mystery(A, B-1);

}

X = parseInt(window.prompt("Enter a positive integer", "") );

Y = parseInt(window.prompt("Enter a positive integer", "") );

document.writeln( Mystery(X,Y) ) ;

 </SCRIPT> </HEAD> <BODY> </BODY> </HTML>
 
Y   What is written
1 Mystery(X,1) X
2 Mystery(X,2) X + Mystery(X,1) = X + X = 2X
3 Mystery(X,3) X + Mystery(X,2) = X + 2X = 3X
4 Mystery(X,4) X + Mystery(X,3) = X + 3X = 4X
     

Mystery(X,Y) = X * Y

Where have we seen this function before:

function Mystery(A,B)

{

if ( B = = 1) return A;

else return A * Mystery(A, B-1);

}
 
 

function Power(Base, Exponent)

{

if ( Exponent = = 1) return Base;

else return Base * Power(Base, Exponent-1);

}
 
 
 
 

Optional Extra Credit Programming Assignments

Due: Friday, May 11

JavaScript should be fully annotated. Submit program and printout of sufficiently ample runs of the program to show that it functions properly. You must also provide an electronic version of the program or a URL for locating it.
 
 

From Deitel, Deitel & Nieto


11.32/33 Guess the Number
11.39 Enhanced CAI
11.43 Craps with Wagers
12.15/16 More Shooting Craps
12.21 Turtle Graphics
12.22 Knight's Tour
12.27 Sieve of Eratosthenes
12.28 Bucket Sort
12.37 Quicksort
12.38-40 Mazes
12.41 Tortoise and the Hare

 

CX 103 Computer Dating Questionnaire





Name
 
 

Answer Each of the following Questions with a "YES" or a "NO"

You may check the appropriate box:
 
 
 
 
 
Question Yes No
Do you prefer raw carrots to cooked carrots?    
Do you prefer black jelly beans to other colors?    
Do you like Burger King more than McDonald's?    
Do you prefer to squeeze toothpaste from the bottom of the tube?    
Do you like the morning more than the evening?    
Do you prefer cats to dogs?    
Do you prefer the Red Sox to the Yankees?    
Do you prefer jazz to opera?    
Do you prefer pickup trucks to SUV's?    
Do you enjoy sleeping with the window open?    


Multiple-Subscripted Arrays

Arrays and Computer Dating Services
 
  #1 #2 #3 #4 #5 #6 #7 #8 #9 #10
Andrew Yes No No Yes Yes Yes Yes No No No
Annalise Yes Yes Yes No No Yes No No Yes Yes
Beth Yes Yes No No Yes No Yes Yes Yes Yes
Bob No Yes Yes Yes No No No Yes No No
Brendan Yes No Yes Yes Yes No No Yes Yes Yes
Casey Yes No Yes Yes No Yes Yes No Yes Yes
Charles M Yes No No Yes Yes No Yes Yes No Yes
Charles R No Yes No Yes Yes No No Yes No No
Colin Yes Yes Yes No No No Yes Yes Yes Yes
David M Yes No Yes Yes No Yes No Yes Yes No
David R No No No No No Yes Yes Yes Yes No
David S Yes No No Yes No Yes No No No Yes
Eliza Yes Yes No Yes Yes No No Yes Yes Yes
John No Yes No Yes Yes No Yes No No Yes
Jordan Yes Yes No Yes Yes No Yes No Yes No
Kate Yes No No No No No Yes Yes No Yes
Keenen Yes Yes No No No No No Yes No Yes
Liz No Yes No Yes Yes No No No Yes Yes
Louisa Yes No No Yes No No Yes Yes No No
Matt No Yes No No Yes Yes No Yes Yes No
Megan Yes No No Yes Yes Yes Yes No Yes No
Missy No No No Yes No No No Yes No No
Nathaniel Yes No No No Yes No No Yes No Yes
Rada No No Yes Yes Yes No Yes Yes Yes Yes
Rebecca No Yes No Yes No No No Yes No Yes
Ryan B Yes No Yes Yes Yes Yes No No Yes No
Ryan P Yes Yes No No Yes No No No Yes Yes
Ted Yes No Yes Yes No Yes Yes Yes Yes Yes
Tejas Yes No Yes Yes No No Yes Yes Yes No
Trevor No Yes No Yes No Yes Yes No No Yes
Tyler No Yes No No No No Yes Yes Yes No
Veljko No Yes Yes Yes No No Yes No No Yes
                     

 
 
 

Students is an array with 33 elements with elements 1 through 32 corresponding to the individual students in the class. The element in cell 0 is the instructor, but his answers to the questions are not known.

Each element Student[i] is itself an array with 12 cells. In cell 0 is the student's name. In cells 1 - 10 are the "Yes" or "No" answers to each question.

Cell 11 is reserved for saving the number of matches with some other prescribed student
 
 
 
Rebecca No Yes No Yes No No No Yes No Yes  
David S Yes No No Yes No Yes No No No Yes 6

 
 
 
 
Rebecca No Yes No Yes No No No Yes No Yes  
Tyler No Yes No No No No Yes Yes Yes No 6

 
 
 
 
Rebecca No Yes No Yes No No No Yes No Yes  
Veljko No Yes Yes Yes No No Yes No No Yes 7


 
 

ENCRYPTION ASSIGNMENT

Encryption.html

Original:

Break message into pairs of consecutive characters.

Replace each pair by 4 digit number made up of their ASCII values

Add 7 % 10 to each digit.

Reverse the digits.
 
 
 
1. <HTML>
2. <HEAD>
3. <TITLE>Stage One of Encryption</TITLE>
4. <SCRIPT LANGUAGE = "Javascript">
5.  
6. var W, i, S;
7.  
8.  
9. function Begin()
10. { var i, A,B,S, T, Number, Length, M, First, Second, Third, Fourth;
11. W = document.EncryptForm.Phrase.value;
12.  
13. Length = W.length;
14.  
15. S = "It has " + Length + " characters.";
16. Append = "That is an odd number. I will append an X to the end and

proceed with the encryption."
17.  
18.  
19.  
20. if (W.length % 2 == 1)
21. { S = S + Append;
22. W = W + "X"; 
23. Length = Length + 1 }
24.  
25. var Originals = new Array(Length/2);
26.  
27. document.EncryptForm.Comment.value = S;
28.  
29. T = "Its original numerical representation is "; 
30. document.EncryptForm.Digits.value = T;
31.  
32. for (i = 0; i < Length ; i = i + 2)
33. { A = W.charCodeAt(i) ;
34. B = W.charCodeAt(i+1) ; 
35. Number = 100*A + B;
36. Originals[i/2] = Number;
37. }
38.  
39. document.EncryptForm.plain.value = Originals.join(" ");;
40.  
41. var Scrambleds = new Array (Length/2);
42. var Digits = new Array(4)
43.  
44. function power(k)
45. { var i, N;
46. N = 1;
47. for (i = 1; i <= k; i = i+ 1) N = N * 10;
48. return N;
49. }
50.  
51. for (i = 0; i < Length/2; i = i + 1)
52. {
53. M = Originals[i];
54. for (j = 0; j <= 3; j = j+ 1)
55. { Digits[j] = Math.floor(M/power(j));
56. Digits[j] = ( Digits[j] + 7) % 10; }
57. M = 0;
58. for (j = 0; j <= 3; j = j+ 1)
59. M = M + Digits[j] * power(3-j);
60.  
61.  
62. Scrambleds[i] = M 
63. }
64.  
65. document.EncryptForm.Scrambled.value = Scrambleds.join(" "); 
66.  
67.  
68. }
69.  
70.
71. </SCRIPT>
72. </HEAD>
73. <BODY BGCOLOR = "white">
74. <FORM NAME = "EncryptForm">
75. Enter a word or phrase <INPUT NAME = "Phrase" SIZE = 80 TYPE = "text">
76. <BR>
77. <INPUT TYPE = "button" VALUE = "Click to Encrypt" SIZE = "20"
78. ONCLICK = "Begin()" > 
79. <BR>
80. <TEXTAREA NAME = "Comment" ROWS =4 COLS = 50 TYPE = 

"text"> </TEXTAREA>
81. <BR>
82. <INPUT NAME = "Digits" SIZE = 50 TYPE = "text">
83. <BR>
84. <TEXTAREA NAME = "plain" ROWS = 5 COLS = 50 TYPE = "text"> </TEXTAREA>
85. <BR>
86. <H2>The encrypted message is:
87. <BR>
88. <TEXTAREA NAME = "Scrambled" ROWS = 5 COLS = 50 

TYPE = "text"> 
89. </TEXTAREA>
90. </FORM>
91. </BODY>
92. </HTML>

How secure is this method?

How can we make it more secure?

(1) Use a different number other than 7 to add mod 10. Keep that digit secret.

(2) Try some other rearrangment of the digits. (also secret)

How many possibilities are there?

( 10 numbers ) * (4! = 24 rearrangements) = 240

Could we "crack" a secret message by brute force?
 
 
 
 
 
 

(3) Use a different number to add to each digit

Example: Add 2 % 10 to First Digit

Add 9 % 10 to Second Digit

Add 5 % 10 to Third Digit

Add 7 % 10 to Fourth Digit
 
 

How many possibilites are there now?

10 * 10 * 10 * 10 * 4! = 240, 000
 
 
 
 
 
 
 

(4) Use triples of characters to make up six digit numbers.
 
 

How many possibilites are there now?

( 10 * 10 * 10 * 10 * 10 * 10) * 6! = 720,000,000
 


We are a little ahead of the game.

For now, let's continue to add 7 % 10 to each digit and reverse the digits when we are done, but we'll work with 6 digit numbers instead of 4 digit numbers:

Example:

MIDDLEBURY_COLLEGE
 
Triple ASCII + 7 % 10 Reverse
MID 777368 444035 530444
DLE 687669 354336 633453
BUR 668582   952533 
Y_C 893267   439065
OLL 797676   343464
EGE 697160   638463
       

 
 
M I  D D L E B U R Y   C O L L E G E
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17
77 73 68 68 76 69 66 85 82 89 32 67 79 76 76 69 71 69

 
 
 

Programming Issues:

(1)

W is "plaintext" character string

Length of W is W.length

If W.length is not a multiple of 3, we need to pad on extra X's at the end.

How many X's ?
 
W.length% 3 Number of X's
0 0
1 2
2 1

Extensions: If we plan to break up message into blocks of k letters, then length must be a multiple of k.

Example: k = 6
 
W.length % 6 Number of X's  
0 0  
1 5 6 - 1
2 4 6 - 2
3 3 6 - 3
4 2 6 - 4
5 1 6 - 5

 

Issue (2): Getting the six digit number
 
32. for (i = 0; i < Length ; i = i + 2)
33. { A = W.charCodeAt(i) ;
34. B = W.charCodeAt(i+1) ; 
35. Number = 100*A + B;
36. Originals[i/2] = Number;

 

for (i = 0; i < Length ; i = i + 3)

{ A = W.charCodeAt(i) ;

B = W.charCodeAt(i+1);

C = W.charCodeAt(i+2);

Number = 10000*A + 100*B+ C;

Originals[i/3] = Number; }

Questions:

What if we use blocks of 6 consecutive characters?

k consecutive characters where k is specified by user when program runs?

Issue (3) Getting the individual digits:
 
51. for (i = 0; i < Length/2; i = i + 1)
52. {
53. M = Originals[i];
54. for (j = 0; j <= 3; j = j+ 1)
55. { Digits[j] = Math.floor(M/power(j));
56. Digits[j] = ( Digits[j] + 7) % 10; }
57. M = 0;
58. for (j = 0; j <= 3; j = j+ 1)
59. M = M + Digits[j] * power(3-j);

 
 
 
 
 

Issue (4) reporting the results:
 
41. var Scrambleds = new Array (Length/2);
62. Scrambleds[i] = M 
63. }
64.  
65. document.EncryptForm.Scrambled.value = Scrambleds.join(" "); 
86. <H2>The encrypted message is:
87. <BR>
88. <TEXTAREA NAME = "Scrambled" ROWS = 5 COLS = 50 

TYPE = "text">