Lecture 6
- Welcome!
- Python
- Hello
- Types
- Speller
- Image Recognition
- CS50 Library
- Conditionals
- Variables
- Loops
- Calculator
- Compare
- Object-Oriented Programming
- Meow
- Mario
- Scores
- Uppercase
- Greet
- Exit Status
- Search
- Phonebook
- Comparison
- Swap
- CSV
- Speech
- Summing Up
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 runningpython hello.py
. Notice how the+
sign concatenates"hello, "
andanswer
. -
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 theanswer
such thatanswer
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:
bool float int str
Notice that
long
anddouble
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 aword
is inwords
, it returnsTrue
. So much easier than an implementation in C! Similarly, in theload
function the dictionary file is opened. For each line in that file, we add that line towords
. Usingrstrip
, the trailing new line is removed from the added word.size
simply returns thelen
or length ofwords
.unload
only needs to returnTrue
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
replaceselse if
. Parentheses are also no longer required in theif
andelif
statements.
Variables
- Variable declaration is simplified too. In C, you might have
int counter = 1;
. In Python, this same line would readcounter = 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 typecounter++
.
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
andy
are gathered from the user. Finally, the result is printed. Notice that themain
function that would have been seen in a C program is gone entirely! While one could utilize amain
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
andy
to be strings. You can fix your code by employing theint
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
andy
is passed to theint
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 thatchar
does not exist in Python. Instead,str
s 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
andyes
.
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
andyes
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 ofs.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 theprint
statement. Further, notice that themain
function appears at the top of the file. At the bottom of the file, themain
function is called. By convention, it’s expected that you create amain
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 variablen
. In themain
function, you can callmeow
and pass a value like3
to it. Then,meow
utilizes the value ofn
in thefor
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.
-
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 awhile
loop, as Python does not have ado while
loop. You can write code as follows in a file calledmario.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 theget_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 convertn
to an integer. If it cannot do so, an error is outputted. -
Consider the following image:
-
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 repeat4
times. -
What about a large block of bricks?
-
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. Theprint
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
list
s are a data structure within Python.list
s 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 canappend
the score to the list. Also notice that we use thesum
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 aroundscore
because only alist
can be added to anotherlist
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
str
s. 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 thef
present in theprint
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 wordpython
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 usesys.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
.
Search
-
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 thename
is in thepeople
dictionary. Further, notice how, in theprint
statement, we can index into the people dictionary using the value ofname
. 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 thewriter
and its work happening below it indented, prevents us from needing toclose
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
andnumber
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!