Lab 5: Mutation is the Word
Overview
In this lab we will make extensive use of while loops and strings. We will explore
mutation of strings (words, DNA, …) through playing the game Doublets.
Materials
Description
Charles Lutwidge Dodgson (aka
Lewis Carroll), who is most famous for his novel “Alice in Wonderland”,
introduced a puzzle called Doublets in an article for
Vanity Fair. His rules description is as follows:
The rules of the Puzzle are simple enough. Two words are proposed, of the same length;
and the Puzzle consists in linking these together by interposing other words,
each of which shall differ from the next word in one letter only. That is to say,
one letter may be changed in one of the given words, then one letter in the word so
obtained, and so on, till we arrive at the other given word. The letters must not be
interchanged among themselves, but each must keep to its own place. As an example,
the word ‘head’ may be changed into ‘tail’ by interposing the words
‘heal, teal, tell, tall’. I call the given words ‘a Doublet’, the interposed words
‘Links’, and the entire series ‘a Chain’, of which I here append an example:
H E A D
h e a l
t e a l
t e l l
t a l l
T A I L
It is, perhaps, needless to state that it is de rigueur that the links should be
English words, such as might be used in good society.
(More details on the connection between Lewis Carroll and genetics can be found in an article by
David B. Searls entitled
From Jabberwocky to Genome: Lewis Carroll and Computational Biology.)
Our task for this lab is to create a Python program that lets a user play the Doublets game
and enforces all the rules given above by Lewis Carroll.
Step 1 - Develop Pseudocode
Develop an algorithmic solution using
pseudocode. This should correspond to the structure of a
Python program, but is written in English instead of Python.
For example, pseudocode for a guessing-game could look like this:
Generate a number for the human to guess
While the human has not guessed the number:
Request a guess from the human
If it's too high or too low, say so
Congratulate the human on their insight
The point is that this is structured like Python, using indentation
and hinting at loops and conditionals. But the individual pieces are
just English descriptions, which you will later translate into precise
Python code. You do not need to use any functions at this
point—we will do that later.
To demonstrate the eventual behavior of the program, here is a
sample run of the Doublets game. For now, do not worry about what
should happen if the user enters invalid input; assume that the user
will always enter exactly what is expected.
What is the starting word? CAT
What is the ending word? DOG
Start = CAT
Current = CAT
End = DOG
Which character do you want to change? (the first character is 1) 3
What is your new character? B
Current = CAB
End = DOG
Which character do you want to change? (the first character is 1) 2
What is your new character? O
Current = COB
End = DOG
Which character do you want to change? (the first character is 1) 3
What is your new character? G
Current = COG
End = DOG
Which character do you want to change? (the first character is 1) 1
What is your new character? D
Solution path found in 4 steps.
CAT -> CAB -> COB -> COG -> DOG
Create a new Python file named doublets.py and write your pseudocode
in some comments.
Step 2 - Initial Implementation
Step 2.1 - swap_character()
Now begin by writing the swap_character() function:
def swap_character(word: str, where: int, updated: str) -> str:
Given a word, the index of a character in the word to replace, and a new character to
insert, this function will return a new string with the new character in place of the old
character. In keeping with our example transcript above, the where variable will be an
integer between 1 and the length of word.
To implement swap_character():
- Use string slicing and string concatenation.
- Do not use a
while loop.
- Do not use
input. All of the input you need is given by the three parameters (word, where, and updated).
Here are a few examples. Test your implementation of swap_character() in the Python Console
at the bottom of the PyCharm window with these examples:
swap_character("cat", 3, "r") should return "car"swap_character("track", 3, "u") should return "truck"swap_character("talk", 1, "w") should return "walk"
Step 2.2 - Main program
Based on your pseduocode description, implement your program in Python, within
a function named main(). When you have received the user’s input concerning the
character position to change and new character to use, call swap_character() to
build the updated word. Include the printing of the number of solution steps, but
do not include the solution chain until Step 2.3.
The program structure should look something like this:
def main():
...
def swap_character(word: str, where: int, updated: str) -> str:
...
main()
Be sure you can successfully play a game of Doublets with your
program, as shown in the example above, before moving on to the next step.
Step 2.3 - Solution Path
Create a string variable to store the solution chain. It should initially be the starting
word. On each loop step, concatenate " -> " and the current word. Then print it out
at the end of the program.
Step 3 - Error Handling and Function Decomposition
Of course the user might not always enter input in the expected form.
In this step, you will incrementally modify your program to ensure that
the user only enters valid input. Building a program with robust error
handling is much easier if it is decomposed into functions, so we will
introduce error handling and functions into our program together.
Step 3.1 - Starting word validation
Write a function called get_start_word() to get the starting word from the user.
First, create the function header:
def get_start_word() -> str:
In Step 2.2, you wrote an input instruction to request the starting word. Copy it into
get_start_word(). On the next line, add a return statement to send the value
back to the function caller. Then replace the original input statement with a call
to get_start_word().
This sets up the structure for validating our input. By handling the input validation
in a function, we keep the main() code straightforward and uncluttered.
For English word validation, download the text file
english3.txt
and the Python module
spellcheck.py files, and
make sure they are in the same folder where you will put your lab. At the top of your
Python program, you should put
which will allow you to use the functions in spellcheck.py.
The spellcheck module contains a function
valid_word(word, file), which will
return True if the word is found in the file and False
otherwise. You can call the function by writing something like
spellcheck.valid_word(some_word, 'english3.txt')
Handling erroneous input requires using a loop. The following example shows
a loop structure that you may find useful in completing
get_start_word(). This is just an example; instead of checking
whether the word has four letters, you should check whether it is a
valid English word.
valid = False
while not valid:
word = input("Enter a word that is at least four letters long: ")
if len(word) < 4:
print(f"{word} is less than four letters long. Please try again.")
else:
valid = True
Test your program in the Python Console before incorporating it into the rest of the program.
When you test it in the console, give it a few invalid words followed by a valid word to ensure
it behaves correctly.
Here is an example of how the program would behave with the error checking in place:
What is the starting word? lig
lig is not a word. Please try again.
What is the starting word? log
What is the ending word?
Step 3.2 - Ending word validation
Write a function to validate the ending word. This will be a slightly more complex
variation of the function you wrote in Step 3.1, as it must ensure not only that
the word is valid, it must also ensure that the word is of the same length as the
starting word. Here is the function definition header:
def get_end_word(start_word_length: int) -> str:
Replace your original input statement with a call to get_end_word() to ensure
that the ending word is valid and of the correct length. Test the function in the
Python Console first. Make sure it only allows words of the specified word length.
For example, if you test it as get_end_word(5), it should only allow you to enter
words of length 5.
Here is a continuation of our earlier example to show how the program as a whole
should behave once you have incorporated get_end_word():
What is the starting word? log
What is the ending word? worm
The lengths are not equal. Please try again.
What is the ending word? sku
sku is not a word. Please try again.
What is the ending word? bug
Start = LOG
Current = LOG
End = BUG
Which character do you want to change? (the first character is 1)
Step 3.3 - Character index validation
Write a function to validate the user’s input of a character index to change.
It should return an integer between 1 and the length of the current word. Any
non-integer or any integer outside that range should result in a message to
the user requesting suitable input. Here is the function declaration header:
def get_index(current_word_len: int) -> int:
Replace your original input statement with a call to get_index() to ensure valid
input. Try get_index(5) in the Python Console. It should only allow you to enter numbers
from 1 to 5.
Here is a continuation of our earlier example to show how the program should behave:
Start = LOG
Current = LOG
End = BUG
Which character do you want to change? (the first character is 1) 0
0 is out of range. Please try again.
Which character do you want to change? (the first character is 1) 4
4 is out of range. Please try again.
Which character do you want to change? (the first character is 1) x
x is not a number. Please try again.
Which character do you want to change? (the first character is 1) 1
What is your new character?
Step 3.4 - Character validation
Write a function to ensure that the user inputs a single character. Here is
the function declaration header:
def get_one_character() -> str:
Replace your original input statement with a call to get_one_character().
Test it in the Python Console. Make sure it only allows single-character answers.
Try testing it by typing the Enter key without any input: it should ask the user
to try again in that case as well.
Here is a continuation of our earlier example to show how the program should
behave once you are using your function:
Which character do you want to change? (the first character is 1) 1
What is your new character? bb
bb is more than one character. Please try again.
What is your new character? b
Current = BOG
End = BUG
Which character do you want to change? (the first character is 1)
Step 3.5 - Word validation
Unlike the other error-handling steps, this step is straightforward to
implement without writing an additional function. Before committing
to the new word with the new character, make sure the new word obtained from calling
swap_character() is a valid word, again using the spellcheck module. If it is a
valid word, update your current word, the number of steps, and the solution path.
If not, print an error message. In either case, the program will resume
at the top of the while loop.
Here is a continuation of our earlier example with this error-checking
in place:
Current = BOG
End = BUG
Which character do you want to change? (the first character is 1) 2
What is your new character? 4
B4G is not a valid word
Current = BOG
End = BUG
What character do you want to change? (the first character is 1) 2
What is your new character? u
solution path found in 2 steps.
LOG -> BOG -> BUG
Step 4 - Play
Once your program is working, use it to create at least two puzzles,
with solutions. One puzzle should be between two four-letter words,
requiring at least four steps; another puzzle should be between two
five-letter words, requring at least five steps. If you would like a
challenge, you can try solving some of the puzzles below, and include
your solutions:
- FIRE – HEAT
- SLEEP – DREAM
- APE – MAN
- WHITE – BLACK
However, you are also welcome to create your own puzzles.