Python Intro Continued

Still working on this CS50 assignment. I’ve finished porting to Python some of the first programs we wrote in C. There are some really cool things Python does out of the gate that made me a little bit jealous! For example:

Iterating Through A String

We did a couple of cipher programs to change a message from the user into a coded message. To write this in C it requires a for loop to manually go letter by letter:

// prompt user for word to encode
printf("plaintext: ");
string plain = get_string();

// for each letter, shift by key and output cipher value
printf("ciphertext: ");
for (int i = 0, n = strlen(plain); i < n; i++)
{
	// do the shifting
}

The way that loops are written in Python makes it so much simpler! And being able to do this for a string, not just an array or object, is pretty cool if you ask me:

# prompt user for word to encode
print("plaintext: ", end="")
plain = cs50.get_string()

# for each letter, shift by key and output cipher value
print("ciphertext: ", end="")
for c in plain:
	# do the shifting

But…ASCII Values

One thing that I like in C that doesn’t work in Python is the fact that characters (letters, numbers, etc.) are equivalent to their ASCII value, so a == 97, A == 65, etc. This made it pretty straightforward in the cipher exercises because you can just do simple addition or subtraction to shift the letters. In Python you have to explicitly change characters into numbers with ord(char), do the manipulation, and then change it back to a letter with chr(num). I think it’s because Python forces type without needing the programmer to explicitly define type…so with characters they must be stored as strings? Anyway not the end of the world but I thought this was interesting.

Sentiment Analyzer

Next up we are writing a program to analyze the sentiment of someone’s tweets using the Natural Language Toolkit and the Twitter API. Pretty big jump from our little cipher programs but I’m on it!

Python Class Syntax

I’m getting familiar with the syntax of writing objects in Python, for example:

class Student():
	def __init__(self, name, id):
		self.name = name
		self.id = id

	def changeID(self, id):
		self.id = id

	def print(self):
		print("{ } - { }" .format(self.name, self.id))

Classes require an initialization function so will always start with the __init__ method. They use the self parameter so that methods can be called on the class; self will always be the first parameter, and there will always be at least this one parameter.

Data Structures

I also learned the difference between a list, dict, and set in figuring out how to load the positive and negative word lists being used to analyze the tweets:

List: Items are kept in order as values
Dict: Items are a key-value pair
Set: Values only, order is not maintained, and duplicates are not allowed.

So in this case set is best, and has the added benefit of a speedier search since it’s not necessary to read through the whole list—it will just stop when the relevant word is found.