Learn Python

2020-02-11

Compare With Java

Based on the knowledge of Java, it should be easier to learn Python. I don’t wanna learn from the beginning such as: “this is a variable, and now let’s see what we can do with a variable.”
So I mark this down here, to help me learn well when comparing with Java.

number

Java:

int a = 1;
double b = 0.5;
boolean c = true;

int d = 3 / 4;				// 0

double e = Math.pow(2, 3);		// 8.0

int f = (int)2.5;

int g = Math.max(3, 4);

Scanner reader = new Scanner(System.in);
int h = reader.nextInt();

Python:

a = 1
b = 0.5
c = True

d = 3 / 4					# 0.75
d = 3 // 4					# 0

e = 2 ** 3					# 8

f = int(2.5)

g = max(3, 4)

h = input()
h = input("Enter a number: ")

String

Java:

String s = "hello";

String a = s.charAt(0);// "h"

String b = s.substring(0, 2)// "he"

String c = s(0, 5)// "hello"

String d = s.substring(s.length() - 3)// "llo"

boolean e = s.contains("he")// ture

String f = "hello"
String g = "hello"
boolean h = f.equals(g)
boolean i = f == g

Python:

s = 'hello'

a = s[0]								# 'h'
a = s[0:1]								# 'h'

b = s[0:2]								# 'he'

c = s[0:5]								# 'hello'
c = s[:5]								# 'hello'

d = s[-3:]								# 'llo'

e = "he" in s 							# True

f = "hello"
g = "hello"
h = f == g
i = f is g

list

Java:

List list = new ArrayList();
list.add("Ronaldo");
list.add("Messi")
list.add("Reus")

System.out.println(list);

int size = list.size();

System.out.println(list.get(1));

list.add(1, "Lewandowski");

list.remove(1);

/**********************/

/**********************/

Python:

list = []
list.append("Ronaldo")
list.append("Messi")
list.append("Reus")

print(list)

size = len(list)

print(list[1])

list.insert(1, "Lewandowski")

del(list[1])
del(list[1:3])

list2 = []
list2.append("Ronaldo")
list2.append("Messi")
list2.append("Reus")
list.extend(list2)
print(list[3])
list3 = list + list2
print(list3[1][0])
list3[0] = 4

# tuple
x = 1, 2, 3, 4
y = x, 5, "hello"
z = 5,		# (5,)

if, for, while

Java:

if(4 < 5){
	System.out.println(1);
}else if(4 > 3){
	System.out.println(2);
}else{
	System.out.println(3);
}

int x = 0;
while(x < 5){
	System.out.print(x + " ");
	x++;
}

for(int i = 0; i < 10; i++){
	System.out.print(i + " ");
}

int[] array = {1, 2, 3, 4};
for(int x : array){
	System.out.print(x + "'");
}

/*****************/

Python:

if 4 < 5:
	print(1)
elif 4 > 3:
	print(2)
else:
	print(3)

x = 0
while x < 5:
	print(x, end = " ")
	x += 1

for i in range(10):
	print(i, end = " ")
for i in range(2, 10):
	print(i, end = " ")		# 2, 3, 4, 5, 6, 7, 8, 9
for i in range(2, 10, 3):
	print(i, end = " ")		# 2, 5, 8

list = [1, 2, 3, 4]
for x in list:
	print(x, end = "")

for x in range(5):
	if x == 6:
		break	# if hit, skip else:
else:			# for loop end naturally
	print('entered else')

method/function

Java:

public static void main(String[] args){
	System.out.println(sum(1, 2, 3));
}
private static int sum(int a, int b, int c){
	return a + b + c;
}

/*****************/

/*****************/

Python:

def sum(a, b, c):
	return a + b + c
print(sum(1, 2, 3))

mystery = sum
print(mystery(1, 2, 3))

def sum(a, b, c = 0):
	return a + b + c
print(sum(1, 2))

OOD

Java:

public class Dog{
	private String name;
	private int age;

	public Dog(String name, int age){
		this.name = name;
		this.age = age;
	}

	public String getName(){
		return name;
	}
	public int getAge(){
		return age;
	}
	public void setName(String name){
		this.name = name;
	}
	public void setAge(int age){
		this.age = age;
	}

	public String toString(){
		return "Dog:\nname: " + name + "\nage: " + age;
	}

	public static void main(String[] args){
		Dog dog1 = new Dog("Oxygen", 2);
		System.out.println(dog1);
		System.out.println(dog1.getAge());
	}

}

Python:

class Dog:

	def __init__(self, name, age):
		self._name = name
		self._age = age

	def get_name(self):
		return self._name
	def get_age(self):
		return self._age
	def set_name(self, name):
		self._name = name;
	def set_age(self, age):
		self._age = age;
	def __str__(self):
		return "Dog:\nname: " + self._name + "\nAge: " + str(self._age)

	def random():
		return 7

dog2 = Dog("Coddy", 3)
print(dog2)
print(dog2.get_age())

print(Dog.random())

others

System.out.println(!(4 > 5));

System.out.println(4 < 5 && 6 < 7);
// !, &&, ||

// byebye

/*
good
bye
*/

System.out.println("hello");

System.out.print("hello");

System.out.println("hello" + "world");

int x = 1;
System.out.println("x = " + x);

print(not (4 > 5))

print(4 < 5 and 6 < 7)
# not and or

# byebye

'''
good
bye
'''

print('hello')

print('hello', end = "")

print("hello" + "world")

x = 1
print("x =", x)
print("x = ", x, sep = "")

Other Tricks

2d array

There is a methon like this:

1	arr = [[0] * 4] * 3

However when we change a number, such as:

1	arr[0][1] = 1

It will become:

1
2
3

[[0, 1, 0, 0],
 [0, 1, 0, 0],
 [0, 1, 0, 0]]

So we have to do like this:

1	arr = [([0] * 4 ) for i in range(4)]

Or another way using numpy:

import numpy as np

X = np.array([[1, 2], [4, 5], [7, 8]])

Z = np.array([[0] * 3] * 4)