PYTHON CRASH COURSE -
A COMPARISON WITH JAVA
CS 334: Machine Learning
COURSE REMINDERS
• In-Class exercise #1 due 1/31
• Read the syllabus
• Python workshops running for the
first 3 weeks with 2 offerings
• You need a functional Python 3
environment for today’s class
2
PYTHON USES
• Web Development (e.g., Django, Flask)
• Data Analysis (e.g., NumPy, Pandas, Matplotlib, Seaborn)
• Internet of Things (e.g., Raspberry Pi + Python)
• Web Scraping
• Machine Learning + Computer Vision (e.g., scikit-learn, nltk,
tensorflow, OpenCV)
3
PYTHON VS JAVA
https://data-flair.training/blogs/python-vs-java/
4
PYTHON VS JAVA KEY DIFFERENCES
• Designed to be used interpretively (i.e., no need for explicit
compilation)
• Statement can be entered at the interpreter prompt (>>>>)
• Execution is immediate
• Line-oriented: End of line is completion of a statement (unless
explicitly escaped with \)
5
VARIABLES / BASIC DATA TYPES
• Python is dynamic typed
which means you can Basic Data Java Python Example
introduce a variable by Types
assigning some value to it Integer int/Integer int a=1
without explicitly declaring Float float/Float float a=1.5
String String str a=“Hello”
type
Boolean boolean bool a=True
• Variable type can change later
6
COMMENTS
• Single line comments begin with #
(// in Java)
• Multi-line comments can begin with """ and end with """
(/* and */ in Java)
• Example:
# initialize foo to 1
foo = 1
7
CODING EXAMPLE
• Declare a variable foo with your
favorite number
• Print the variable (i.e., print(foo))
• Assign foo with a string (Use “ or ‘)
• Print the variable again
COMMON OPERATORS
Operator Java Python Python Example
Addition + + 1+2
Minus - - 2-1
Division / / 4/2
Multiplication * * 2*4
Modulo % % 4%3
Equal == == 2 == 3
Less Than / Less Than Equal < / <= < / <= 2<3
Greater Than / Greater than Equal > / >= > / >= 2>3
And && and x < 5 and x > 2
Or || or x < 5 or x > 2
Not ! not not (x < 5)
9
COMMON DATA STRUCTURES
Type Java Python Python Declaration
Tuple --- tuple x=(1,2,3,3)
(immutable ordered list)
List List<V> list x = []
x = [1, 2, 3, 3]
Dictionary HashMap<K, V> dict x={}
x = {‘a’:1, ‘b’:2, ‘c’:3, ‘d’: 3}
Set Set<V> set x=set()
x = {1,2,3,3}
10
CODING EXAMPLE
• Create a list foo with 4 floats
• Print the 3rd element of the list (i.e.,
print(foo[2]))
• Create a dict food with 4 keys (‘a’, ‘b’,
‘c’, ‘d’) and 4 values (of your choice)
IF/ELSE
Java Python
if (x < 5) { if x < 5:
// call do1 # call do1
do1();
} else if (x < 10) {
do1()
elif x < 10:
Indentation matters
// call do2 # call do2 - it determines the
do2(); do2()
} else { else: scope!
// call do3 # call do3
do3(); do3()
}
12
FOR
Java Python
for (foo in bar) { for foo in bar:
// call do1 # call do1
do1(foo); do1(foo)
}
for (int i=0; i < 10; i++) { for i in range(10):
// call do1 # call do1
do1(foo[i]); do1(foo[i])
}
range creates a sequence between 0
to 9 with increments of 1
13
CODING EXAMPLE
• Loop through each element of the list
and only print out the elements that
are greater than a value of your
choosing. This value should be chosen
such that you print out at least 1
element of your list
• Print the key and values associated with
food by looping through dictionary
items using food.items()
WHILE
Java Python
while (foo < 10) { while foo < 10:
// call do1 # call do1
do1(); do1()
}
15
METHODS / FUNCTIONS
Java Python
double square(double x) { def square(x):
return x*x; return x*x
}
Major differences:
• Function DOES NOT need to occur in class
• Return type and input types are not declared
• Indentation defines body of function
16
CLASSES
Java (Foo.java) Python (Foo.py)
public class Foo { class Foo:
double a; // class attribute a = None # class attribute
// Constructor # Constructor
public Foo(double a) { def __init__(self, a):
this.a = a; self.a = a
}
# Method called bar
// Method called bar def bar(self, b):
double bar(double b) { return self.a * b
return a * b;
} def main():
} foo = new Foo(5.0)
print(foo.bar(2.4))
public static void main(String[] args) {
Foo foo = new Foo(5.0); if __name__ == “__main__”:
System.out.println(foo.bar(2.4)); main()
}
17
IN-CLASS EXERCISE #2
• Download exercise2.py from Canvas ->
Files -> Exercises -> Ex2
• Fill in the 5 functions by following the
specifications in the multi-line comments
(""" à """)
• Submit to Gradescope (Exercise #2)
• Closes 15 minutes after class ends today
18
GRADESCOPE AUTOGRADER TIPS
• Do not import any package that is not specified in requirements.txt
or already imported for you (ignore the gradescopeutils package)
• Public tests will give you an idea of whether your output format is
correct, logic correctness is generally not exposed until after grading
• Submit early and often to get feedback!
19
EXERCISE #2: ADD_TWO
EXERCISE #2: CONVERT_C_TO_F
EXERCISE #2: SUM_LIST
EXERCISE #2: SUM_DICT
EXERCISE #2: IS_PRIME
HOMEWORK #1 ANNOUNCEMENT
• Out today on Canvas and due 2/6 @ 11:59 PM ET on Gradescope
• 4 questions
• Q1: Get familiar with Python
• Q2: Numerical programming (Numpy)
• Q3-Q4: Dataset loading and visualization (Pandas)
25
