ENGR/CS 101 CS Session
Lecture 9
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Log into Windows/ACENET (reboot if in
Linux)
Start Microsoft Visual Studio 2012
Any questions about the exercise from last
time?
Lecture 4
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Outline
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Problem: Encode a single letter using Caesar
shift cipher
Substitution Cipher Project
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Lecture 4
Textboxes
Getting user input
Computing ciphertext letter
Displaying result
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Problem Specification
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Will work our way up to a program to do the
Caesar shift cipher on messages in a file.
Today's program will do the following:
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Lecture 4
Allow the user to enter a shift key letter in
uppercase
Allow the user to enter a plaintext letter in
uppercase to be enciphered
Have a button for the user to click to have the
plaintext letter enciphered
Display the ciphertext letter
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Interface Mockup
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Textboxes to enter
plaintext and shift
key, and to display
ciphertext
Button to encipher
plaintext.
Labels to identify the
textboxes
Lecture 4
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Create the GUI
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Create a new project. Make sure the
language template is Visual C#.
Select "Windows Form Application". Give
the project a name like "SubstitutionCipher".
Make sure the location is on your network
drive.
Click on OK. You will get the GUI designer
with a blank form. Change the Text property
to something like "Substitution Cipher".
Lecture 4
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Textboxes
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A Textbox is a GUI element that can be used
to get text input from a user. It also can be
used to display results. (This is better than a
label for larger amounts of output.)
The text in a Textbox is accessed through the
Text property.
Lecture 4
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In-class Exercise, Part 1
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Create a simple form with three Textboxes for
the plaintext, ciphertext, and shift key, and a
Button for doing the encipher.
For textboxes containing the plaintext and the
ciphertext, change the Multiline property to
True. After this, pull on the handles to make
them as large as you like.
Label and arrange these elements however you
like on the form. Change properties like color
and font as you wish.
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In-class Exercise, Part 1
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Change the Name properties of the GUI
elements as follows:
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Lecture 4
Plaintext textbox is named plaintextBox
Ciphertext textbox is named ciphertextBox
Shift key textbox is named keyBox
Encipher button is named encipherBtn
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Program Analysis & Design
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How will the program accomplish the
specifications?
Identify the data being used
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shift key, plaintext letter, ciphertext letter
Write the steps of an algorithm
1. Get the shift key letter from the user
2. Get the plaintext letter from the user
3. Compute the ciphertext letter
4. Output the ciphertext letter to the screen
Lecture 3
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Application Data
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Double-click on the Encipher button to
create the handler method stub.
Unlike the Dot Chaser program, all of the
action happens within the handler when
the Encipher button is clicked.
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Declare the application data in the handler
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char shiftKey,
// key letter
plainLetter, // user input
cipherLetter; // result
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Handler Design
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The specific design of the handler code for the
Encipher button is as follows
1. Clear the ciphertext box
2. Get the shift key from keyBox.Text
3. Get the plaintext letter from plaintextBox.Text
4. Compute the ciphertext letter
5. Append the cipher letter to the ciphertext box
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Clearing the ciphertext box is simply setting the
Text property of the empty string.
ciphertextBox.Text = "";
Lecture 4
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Getting User Input
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We will get the user data from the keyBox and
plaintextBox using the Text property like so:
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keyBox.Text
plaintextBox.Text
Since the input is in the form of a string, it
must be converted to the appropriate type
before assignment. Each type has a Parse
function for this purpose. E.g.
shiftKey = char.Parse(keyBox.Text);
plainLetter = char.Parse(plaintextBox.Text);
Lecture 4
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Computing the Cipher Letter
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Assume that variable shiftKey holds the key
letter and variable plainLetter holds the letter
to be enciphered.
Since the alphabetic characters have
sequential mapping (i.e., 'A' is first, followed
by 'B', etc.), the number of places to shift is
the key letter minus 'A'. In C# code, this is:
shiftNumber = shiftKey - 'A';
Lecture 3
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Computing the Cipher Letter
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To find the cipher letter, we determine the index of
plaintext letter (i.e., where in the alphabet it is when
we start counting at 0) using a similar method, then
add the shift number.
This will be the index of the ciphertext letter, except
that the number may be greater than 26. To make it
circular, we compute the modulus with respect to
26. In code, this is:
index = (plainLetter - 'A' + shiftNumber) % 26;
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The modulus operator symbol is %
Lecture 3
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Computing the Cipher Letter
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Now we add this new index back to 'A' to find
the ciphertext letter.
However, C# is strict about types and is
unhappy that we are trying to add a number
to a character, so we have to tell the compiler
to treat 'A' as a number, then treat the result
as a character by casting. The code
becomes:
cipherLetter = (char) ((int) 'A' + index);
Lecture 3
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Displaying the Result
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To display the result, it must be placed in the
Text property of the ciphertext box.
In preparation for the next phase of the
project, we will do this by concatenating the
resulting cipher letter to the ciphertext box
Text.
ciphertextBox.Text += cipherLetter;
Lecture 4
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In-class Exercise, Part 2
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Double-click on the Encipher button to get to
the handler function stub, if you haven't done
so already.
Finish writing the code for the Encipher
button handler. All of the code is shown
together on the next two slides.
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Encipher Button Handler Code
// Application data
char shiftKey,
//
plainLetter, //
cipherLetter; //
int shiftNumber,
//
index;
//
key letter
user input
result
# of shift places
of cipher letter
// Clear the ciphertext (result) box
ciphertextBox.Text = "";
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Encipher Button Handler Code
// Get the key letter and a letter to encipher
shiftKey = char.Parse(keyBox.Text);
plainLetter = char.Parse(plaintextBox.Text);
// Compute the corresponding ciphertext letter
shiftNumber = shiftKey - 'A';
index = (plainLetter - 'A' + shiftNumber) % 26;
cipherLetter = (char)((int)'A' + index);
// Display the result
// Append the enciphered letter to ciphertext box
ciphertextBox.Text += cipherLetter;
Lecture 4
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Test the Program
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When you have competed writing the handler
code, try running it with different keys and
input.
What happens if you try to input an entire
word in the plaintext box?
Next class we will enhance the project to
handle encoding an entire word.
Lecture 4
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