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Lecture 6 - CS50x 2023

Welcome!​

  • In previous weeks, you were introduced to the fundamental building blocks of programming.
  • You learned about programming in a lower-level programming language called C.
  • Today, we are going to work with a higher-level programming language called Python.
  • As you learn this new language, you’re going to find that you are going to be more able to teach yourself new programming languages.

Python​

  • Humans, over the decades, have seen how previous design decisions could be improved upon.
  • Python is a programming language that builds upon what you have already learned in C.

Hello​

  • Up until this point, the code has looked like this:

    // A program that says hello to the world

    #include <stdio.h>

    int main(void)
    {
        printf("hello, world\n");
    }

  • Today, you’ll find that the process of writing and compiling code has been simplified.

  • For example, the above code will be rendered in Python as:

    # A program that says hello to the world

    print("hello, world")

    Notice that the semicolon is gone.

  • In C, you might remember this code:

    // get_string and printf with %s

    #include <cs50.h>
    #include <stdio.h>

    int main(void)
    {
        string answer = get_string("What's your name? ");
        printf("hello, %s\n", answer);
    }

  • This code is transformed in Python to:

    # get_string and print, with concatenation

    from cs50 import get_string

    answer = get_string("What's your name? ")
    print("hello, " + answer)

    You can write this code by executing code hello.py in the terminal window. Then, you can execute this code by running python hello.py. Notice how the + sign concatenates "hello, " and answer.

  • Similarly, you could implement the above code as:

    # get_string and print, with format strings

    from cs50 import get_string

    answer = get_string("What's your name? ")
    print(f"hello, {answer}")

    Notice how the curly braces allow for the print function to interpolate the answer such that answer appears within.

Types​

  • Data types in Python do not need to be explicitly declared. For example, you saw how answer above is a string, but we did not have to tell the interpreter this was the case: It knew on its own.

  • In Python, commonly used types include:

    Notice that long and double are missing. Python will handle what data type should be used for larger and smaller numbers.

  • Some other data types in Python include:

      range
      list
      tuple
      dict
      set

  • Each of these data types can be implemented in C, but in Python they can be implemented more simply.

Speller​

  • To illustrate this simplicity, let’s type ‘code dictionary.py’ in the terminal window and write code as follows:

    # Words in dictionary
    words = set()
def check(word):
    """Return true if word is in dictionary else false"""
    if word.lower() in words:
        return True
    else:
        return False


def load(dictionary):
    """Load dictionary into memory, returning true if successful else false"""
    file = open(dictionary, "r")
    for line in file:
        word = line.rstrip()
        words.add(word)
    file.close()
    return True


def size():
    """Returns number of words in dictionary if loaded else 0 if not yet loaded"""
    return len(words)


def unload():
    """Unloads dictionary from memory, returning true if successful else false"""
    return True

```

Notice that there are four functions above. In the `check` function, if a `word` is in `words`, it returns `True`. So much easier than an implementation in C! Similarly, in the `load` function the dictionary file is opened. For each line in that file, we add that line to `words`. Using `rstrip`, the trailing new line is removed from the added word. `size` simply returns the `len` or length of `words`. `unload` only needs to return `True` because Python handles memory management on its own.
  • The above code illustrates why higher-level languages exist: To simplify and allow you to write code more easily.
  • However, speed is a tradeoff. Because C allows you, the programmer, to make decisions about memory management, it may run faster than Python – depending on your code. While C only runs your lines of code, Python runs all the code that comes under the hood with it when you call Python’s built-in functions.
  • You can learn more about functions in the Python documentation

Image Recognition​

  • Numerous libraries have been written by contributors to Python.
  • You can utilize these libraries in your own code.
  • For example, you could simply import facial recognition utilizing a Python library like PIL.
  • David provided a demo of facial recognition utilizing Python and third-party libraries.

CS50 Library​

  • As with C, the CS50 library can be utilized within Python.

  • The following functions will be of particular use:

      get_float
      get_int
      get_string

  • You also have the option of importing only specific functions from the CS50 library as follows:

    from cs50 import get_float, get_int, get_string

Conditionals​

  • In C, you might remember a program like this:

    // Conditionals, Boolean expressions, relational operators

    #include <cs50.h>
    #include <stdio.h>

    int main(void)
    {
        // Prompt user for integers
        int x = get_int("What's x? ");
        int y = get_int("What's y? ");

        // Compare integers
        if (x < y)
        {
            printf("x is less than y\n");
        }
        else if (x > y)
        {
            printf("x is greater than y\n");
        }
        else
        {
            printf("x is equal to y\n");
        }
    }

  • In Python, it would appear as follows:

    # Conditionals, Boolean expressions, relational operators

    from cs50 import get_int

    # Prompt user for integers
    x = get_int("What's x? ")
    y = get_int("What's y? ")

    # Compare integers
    if x < y:
        print("x is less than y")
    elif x > y:
        print("x is greater than y")
    else:
        print("x is equal to y")

    Notice that there are no more curly braces. Instead, indentations are utilized. Second, a colon is utilized in the if statement. Further, elif replaces else if. Parentheses are also no longer required in the if and elif statements.

Variables​

  • Variable declaration is simplified too. In C, you might have int counter = 1;. In Python, this same line would read counter = 1. You need not declare the type of the variable.
  • Python favors counter += 1 to increment by one, losing the ability found in C to type counter++.

Loops​

  • Loops in Python are very similar to C. You may recall the following code in C:

    // Demonstrates while loop

    #include <stdio.h>

    int main(void)
    {
        int i = 0;
        while (i < 3)
        {
            printf("meow\n");
            i++;
        }
    }

  • In Python, this code appears as:

    # Demonstrates while loop

    i = 0
    while i < 3:
        print("meow")
        i += 1

  • for loops can be implemented in Python as follows:

    # Better design

    for i in range(3):
        print("meow")

  • Similarly, one could express the above code as:

    # Abstraction with parameterization

    def main():
        meow(3)
# Meow some number of times
def meow(n):
    for i in range(n):
        print("meow")


main()

```

Notice that a function is utilized to abstract away the meowing.

Calculator​

  • We can implement a simple calculator just as we did within C. Type code calculator.py into the terminal window and write code as follows:

    # Addition with int [using get_int]

    from cs50 import get_int

    # Prompt user for x
    x = get_int("x: ")

    # Prompt user for y
    y = get_int("y: ")

    # Perform addition
    print(x + y)

    Notice how the CS50 library is imported. Then, x and y are gathered from the user. Finally, the result is printed. Notice that the main function that would have been seen in a C program is gone entirely! While one could utilize a main function, it is not required.

  • It’s possible for one to remove the training wheels of the CS50 library. Modify your code as follows:

    # Addition with int [using input]

    # Prompt user for x
    x = input("x: ")

    # Prompt user for y
    y = input("y: ")

    # Perform addition
    print(x + y)

    Notice how executing the above code results in strange program behavior. Why might this be so?

  • You may have guessed that the interpreter understood x and y to be strings. You can fix your code by employing the int function as follows:

    # Addition with int [using input]

    # Prompt user for x
    x = int(input("x: "))

    # Prompt user for y
    y = int(input("y: "))

    # Perform addition
    print(x + y)

    Notice how the input for x and y is passed to the int function which converts it to an integer.

  • We can expand the abilities of our calculator. Modify your code as follows:

    # Division with integers, demonstration lack of truncation

    # Prompt user for x
    x = int(input("x: "))

    # Prompt user for y
    y = int(input("y: "))

    # Divide x by y
    z = x / y
    print(z)

    Notice that executing this code results in a value, but that if you were to see more digits after .333333 you’d see that we are faced with floating-point imprecision.

  • We can reveal this imprecision by modifying our codes slightly:

    # Floating-point imprecision

    # Prompt user for x
    x = int(input("x: "))

    # Prompt user for y
    y = int(input("y: "))

    # Divide x by y
    z = x / y
    print(f"{z:.50f}")

    Notice that this code reveals the imprecision. Python still faces this issue, just as C does.

Compare​

  • In C, we faced challenges when we wanted to compare two values. Consider the following code:

    // Conditionals, Boolean expressions, relational operators

    #include <cs50.h>
    #include <stdio.h>

    int main(void)
    {
        // Prompt user for integers
        int x = get_int("What's x? ");
        int y = get_int("What's y? ");

        // Compare integers
        if (x < y)
        {
            printf("x is less than y\n");
        }
        else if (x > y)
        {
            printf("x is greater than y\n");
        }
        else
        {
            printf("x is equal to y\n");
        }
    }

  • In Python, we can execute the above as follows:

    # Conditionals, Boolean expressions, relational operators

    from cs50 import get_int

    # Prompt user for integers
    x = get_int("What's x? ")
    y = get_int("What's y? ")

    # Compare integers
    if x < y:
        print("x is less than y")
    elif x > y:
        print("x is greater than y")
    else:
        print("x is equal to y")

    Notice that the CS50 library is imported. Further, minor changes exist in the if statement.

  • Further looking at comparisons, consider the following code in C:

    // Logical operators

    #include <cs50.h>
    #include <stdio.h>

    int main(void)
    {
        // Prompt user to agree
        char c = get_char("Do you agree? ");

        // Check whether agreed
        if (c == 'Y' || c == 'y')
        {
            printf("Agreed.\n");
        }
        else if (c == 'N' || c == 'n')
        {
            printf("Not agreed.\n");
        }
    }

  • The above can be implemented as follows:

    # Logical operators

    from cs50 import get_string

    # Prompt user to agree
    s = get_string("Do you agree? ")

    # Check whether agreed
    if s == "Y" or s == "y":
        print("Agreed.")
    elif s == "N" or s == "n":
        print("Not agreed.")

    Notice that the two vertical bars utilized in C is replaced with or. Indeed, people often enjoy Python because it is more readable by humans. Also, notice that char does not exist in Python. Instead, strs are utilized.

  • Another approach to this same code could be as follows:

    # Logical operators, using lists

    from cs50 import get_string

    # Prompt user to agree
    s = get_string("Do you agree? ")

    # Check whether agreed
    if s in ["y", "yes"]:
        print("Agreed.")
    elif s in ["n", "no"]:
        print("Not agreed.")

    Notice how we are able to express multiple keywords like y and yes.

Object-Oriented Programming​

  • Up until this point, our programs in this course have been linear: sequential.

  • It’s possible to have certain types of values not only have properties or attributes inside of them but have functions as well. In Python, these values are known as objects

  • In C, we could create a struct where you could associate multiple variables inside a single self-created data type. In Python, we can do this and also include functions in a self-created data type. When a function belongs to a specific object, it is known as a method.

  • For example, strs in Python have a built-in methods. Therefore, you could modify your code as follows:

    # Logical operators, using lists

    from cs50 import get_string

    # Prompt user to agree
    s = get_string("Do you agree? ")

    # Check whether agreed
    if s.lower() in ["y", "yes"]:
        print("Agreed.")
    elif s.lower() in ["n", "no"]:
        print("Not agreed.")

    Notice how we are able to express multiple keywords like y and yes and convert any user input to lowercase.

  • This could be further simplified as:

    # Logical operators, using lists

    from cs50 import get_string

    # Prompt user to agree
    s = get_string("Do you agree? ")

    s = s.lower()

    # Check whether agreed
    if s in ["y", "yes"]:
        print("Agreed.")
    elif s in ["n", "no"]:
        print("Not agreed.")

    Notice how the old value of s is overwritten with the result of s.lower().

  • In this class, we will only scratch the surface of Python. Therefore, the Python documentation will be of particular importance as you continue.

  • You can learn more about string methods in the Python documentation

Meow​

  • Returning to meow.c from weeks earlier, recall the following code:

    // Demonstrates while loop

    #include <stdio.h>

    int main(void)
    {
        int i = 0;
        while (i < 3)
        {
            printf("meow\n");
            i++;
        }
    }

  • The above can be implemented within Python as:

    # Demonstrates while loop

    i = 0
    while i < 3:
        print("meow")
        i += 1

  • Similarly, using a for loop, we can write code as follows:

    # Better design

    for i in range(3):
        print("meow")

  • As we hinted at earlier today, you can further improve upon this code using functions. Modify your code as follows:

    # Abstraction

    def main():
        for i in range(3):
            meow()

    # Meow once
    def meow():
        print("meow")
main()

```

Notice that the `meow` function abstracts away the `print` statement. Further, notice that the `main` function appears at the top of the file. At the bottom of the file, the `main` function is called. By convention, it’s expected that you create a `main` function in Python.

  • Indeed, we can pass variables between our functions as follows:

    # Abstraction with parameterization

    def main():
        meow(3)
# Meow some number of times
def meow(n):
    for i in range(n):
        print("meow")


main()

```

Notice how `meow` now takes a variable `n`. In the `main` function, you can call `meow` and pass a value like `3` to it. Then, `meow` utilizes the value of `n` in the `for` loop.
  • Reading the above code, notice how you, as a C programmer, are able to quite easily make sense of the above code. While some conventions are different, the building blocks you previously learned are very apparent in this new programming language.

Mario​

  • Recall a few weeks ago our challenge of building three blocks on top of one another, like in Mario.

    three vertical blocks

  • In Python, we can implement something akin to this as follows:

    # Prints a column of 3 bricks with a loop

    for i in range(3):
        print("#")

  • In C, we had the advantage of a do-while loop. However, in Python it is convention to utilize a while loop, as Python does not have a do while loop. You can write code as follows in a file called mario.py:

    # Prints a column of bricks, using a helper function to get input

    from cs50 import get_int
def main():
    height = get_height()
    for i in range(height):
        print("#")


def get_height():
    while True:
        n = get_int("Height: ")
        if n > 0:
            return n


main()

```

Notice how the scope of `n` is everywhere in the `get_height` function once it is assigned a value. Further notice that by convention, there are double spaces between functions.

  • We can take away the training wheels of the CS50 library as follows:

    # Prints a column of bricks, catching exceptions

    def main():
        height = get_height()
        for i in range(height):
            print("#")
def get_height():
    while True:
        try:
            n = int(input("Height: "))
            if n > 0:
                return n
        except ValueError:
            print("Not an integer")


main()

```

Notice that `try` is utilized to attempt to convert `n` to an integer. If it cannot do so, an error is outputted.

  • Consider the following image:

    four horizontal question blocks

  • In Python, we could implement by modifying your code as follows:

    # Prints a row of 4 question marks with a loop

    for i in range(4):
        print("?", end="")
    print()

    Notice that you can override the behavior of the print function to stay on the same line as the previous print.

  • Similar in spirit to previous iterations, we can further simplify this program:

    # Prints a row of 4 question marks without a loop

    print("?" * 4)

    Notice that we can utilize * to multiply the print statement to repeat 4 times.

  • What about a large block of bricks?

    three by three block of mario blocks

  • To implement the above, you can modify your code as follows:

    # Prints a 3-by-3 grid of bricks with loops

    for i in range(3):
        for j in range(3):
            print("#", end="")
        print()

    Notice how one for loop exists inside another. The print statement adds a new line at the end of each row of bricks.

  • You can learn more about the print function in the Python documentation

Scores​

  • lists are a data structure within Python.

  • lists have built in methods or functions within them.

  • For example, consider the following code:

    # Averages three numbers using a list and a loop

    from cs50 import get_int

    # Get scores
    scores = []
    for i in range(3):
        score = get_int("Score: ")
        scores.append(score)

    # Print average
    average = sum(scores) / len(scores)
    print(f"Average: {average}")

    Notice that you can use the built-in append method, whereby you can append the score to the list. Also notice that we use the sum function to add all elements in the list.

  • You can even ultilize the following syntax:

    # Averages three numbers using a list and a loop with + operator

    from cs50 import get_int

    # Get scores
    scores = []
    for i in range(3):
        score = get_int("Score: ")
        scores += [score]

    # Print average
    average = sum(scores) / len(scores)
    print(f"Average: {average}")

    Notice that += is utilized to append the score to the list. In this case we place square brackets around score because only a list can be added to another list using + or +=.

  • You can learn more about lists in the Python documentation

  • You can also learn more about len in the Python documentation

Uppercase​

  • Similarly, consider the following code:

    # Uppercases string one character at a time

    before = input("Before: ")
    print("After:  ", end="")
    for c in before:
        print(c.upper(), end="")
    print()

    Notice that each character is uppercased one at a time.

  • Python has a built-in method for strs. You could modify your code as follows:

    # Uppercases string all at once

    before = input("Before: ")
    after = before.upper()
    print(f"After:  {after}")

    Notice the upper method is utilized to uppercase the entire string at once.

Greet​

  • As with C, you can also utilize command-line arguments. Consider the following code:

    # Prints a command-line argument

    from sys import argv

    if len(argv) == 2:
        print(f"hello, {argv[1]}")
    else:
        print("hello, world")

    Notice that argv[1] is printed using a formatted string, noted by the f present in the print statement.

  • You can print all the arguments in argv as follows:

    # Printing command-line arguments, indexing into argv

    from sys import argv

    for i in range(len(argv)):
        print(argv[i])

    Notice that the above will not present the word python if executed, and the first argument will be the name of the file you are running. You can think of the word python as being analogous to ./ when we were running programs in C.

  • You can slice pieces of lists away. Consider the following code:

    # Printing command-line arguments using a slice

    from sys import argv

    for arg in argv[1:]:
        print(arg)

    Notice that executing this code will result in the name of the file you are running being sliced away.

  • You can learn more about the sys library in the Python documentation

Exit Status​

  • The sys library also has built-in methods. We can use sys.exit(i) to exit the program with a specific exit code:

    # Exits with explicit value, importing sys

    import sys

    if len(sys.argv) != 2:
        print("Missing command-line argument")
        sys.exit(1)

    print(f"hello, {sys.argv[1]}")
    sys.exit(0)

    Notice that dot-notation is used to utilize the built-in functions of sys.

  • Python can also be utilized to search. In your terminal window, type code names.py and write code as follows:

    # Implements linear search for names

    import sys

    # A list of names
    names = ["Bill", "Charlie", "Fred", "George", "Ginny", "Percy", "Ron"]

    # Ask for name
    name = input("Name: ")

    # Search for name
    for n in names:
        if n == name:
            print("Found")
            sys.exit(0)

    print("Not found")
    sys.exit(1)

    Notice that this code functions. Indeed, it implements a linear search.

  • You can utilize the built-in abilities of Python as follows:

    # Implements linear search for names using `in`

    import sys

    # A list of names
    names = ["Bill", "Charlie", "Fred", "George", "Ginny", "Percy", "Ron"]

    # Ask for name
    name = input("Name: ")

    # Search for name
    if name in names:
        print("Found")
        sys.exit(0)

    print("Not found")
    sys.exit(1)

    Notice that the in preposition is utilized. Python understands how to implement the lower-level code to do a linear search.

Phonebook​

  • Recall that a dictionary or dict is a collection of key and value pairs.

  • You can implement a dictionary in Python as follows:

    # Implements a phone book

    from cs50 import get_string

    people = {
        "Carter": "+1-617-495-1000",
        "David": "+1-949-468-2750"
    }

    # Search for name
    name = get_string("Name: ")
    if name in people:
        print(f"Number: {people[name]}")

    Notice that the dictionary is implemented using curly braces. Then, the statement if name in people searches to see if the name is in the people dictionary. Further, notice how, in the print statement, we can index into the people dictionary using the value of name. Very useful!

  • Python has done their best to get to constant time using their built-in searches.

Comparison​

  • We can implement comparisons as follows in Python:

    # Compares two strings

    # Get two strings
    s = input("s: ")
    t = input("t: ")

    # Compare strings
    if s == t:
        print("Same")
    else:
        print("Different")

    Notice how Python utilizes the == to be able to compare two variables. Further, notice that Python allows you to compare two strings without examining strings character by character using pointers as in C.

Swap​

  • Further, we can implement a program that swaps values as we did in C. Consider the following code in Python:

    # Swaps two integers

    x = 1
    y = 2

    print(f"x is {x}, y is {y}")
    x, y = y, x
    print(f"x is {x}, y is {y}")

    Notice that each value is swapped, using some very Pythonic syntax x, y = y, x.

CSV​

  • You can also utilize Python to engage with CSV files. Consider the following program called phonebook.py:

    # Saves names and numbers to a CSV file

    import csv

    # Get name and number
    name = input("Name: ")
    number = input("Number: ")

    # Open CSV file
    with open("phonebook.csv", "a") as file:

        # Print to file
        writer = csv.writer(file)
        writer.writerow([name, number])

    Notice that utilizing the with block of code, with the writer and its work happening below it indented, prevents us from needing to close our file once finished.

  • Commonly, CSV files have columns that carry specific names. A DictWriter can used to create the CSV file and assign specific names to each column. Consider the following modification to our code:

    # Saves names and numbers to a CSV file using a DictWriter

    import csv

    # Get name and number
    name = input("Name: ")
    number = input("Number: ")

    # Open CSV file
    with open("phonebook.csv", "a") as file:

        # Print to file
        writer = csv.DictWriter(file, fieldnames=["name", "number"])
        writer.writerow({"name": name, "number": number})

    Notice the name and number columns are defined in the penultimate row of code, and values are added in the final line.

  • You can learn more about the CSV files in Python in the Python documentation

Speech​

  • Using a third-party library, Python can do text-to-speech.

    # Says hello to someone

    import pyttsx3

    engine = pyttsx3.init()
    name = input("What's your name? ")
    engine.say(f"hello, {name}")
    engine.runAndWait()

  • Further, you can run the following code:

    # Says "This was CS50"

    import pyttsx3

    engine = pyttsx3.init()
    engine.say("This was CS50")
    engine.runAndWait()

Summing Up​

In this lesson, you learned how the building blocks of programming from prior lessons can be implemented within Python. Further, you learned about how Python allowed for more simplified code. Also, you learned how to utilize various Python libraries. In the end, you learned that your skills as a programmer are not limited to a single programming language. Already, you are seeing how you are discovering a new way of learning through this course that could serve you in any programming language – and, perhaps, in nearly any avenue of learning! Specifically, we discussed…

  • Python
  • Variables
  • Conditionals
  • Loops
  • Types
  • Libraries
  • Dictionaries
  • Command-line arguments
  • Regular expressions

See you next time!