Transfer to Python
Now we will get our hands dirty to play with Python in the Command-Line. Python can be run in REPL(Read–eval–print loop). It is a simple way to code in Python. Type
python without any parameters in the shell.python
You will get something like this:
Python 3.9.7 | packaged by conda-forge | (default, Sep 29 2021, 19:24:02)
[Clang 11.1.0 ] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>>
Try to type a simple calculation formula here.
>>> 26374 + 88140
114514
Then you have a simple CLI calculator! To use it as a calculator,
_ refers to the last output, just like the ANS in calculators.>>> _ + 1805296
1919810
We can also set a variable and let it remember.
>>> a = -1
And we can just type
a and we can get its value>>> a
-1
Now using
Ctrl + D to quit REPL. And let's try to write some Python code by Vim. First, cd to anywhere you like. Then:vim hello.py
And press
i to Enter INSERT mode to type the code.print("Hello, World")
Python allows you not to type
; at the end of a line. Press the Esc key and type :x to exit Vim. Then run the code by:python hello.py
In Python, we have List, Set, Dictionary, Tuple.
>>> a = {1, 2, 3}
>>> type(a)
<class 'set'>
>>> b = [1, 2, 3]
>>> type(b)
<class 'list'>
>>> c = {1: 'a', 2: 'b'}
>>> type(c)
<class 'dict'>
>>> d = (1, 2, 3)
>>> type(d)
<class 'tuple'>
List | Tuple | Set | Dictionary |
List is a non-homogeneous data structure which stores the elements in single row and multiple rows and columns | Tuple is also a non-homogeneous data structure which stores single row and multiple rows and columns | Set data structure is also non-homogeneous data structure but stores in single row | Dictionary is also a non-homogeneous data structure which stores key value pairs |
List can be represented by [ ] | Tuple can be represented by ( ) | Set can be represented by { } | Dictionary can be represented by { } |
List allows duplicate elements | Tuple allows duplicate elements | Set will not allow duplicate elements | Set will not allow duplicate elements and dictionary doesn’t allow duplicate keys. |
List can use nested among all | Tuple can use nested among all | Set can use nested among all | Dictionary can use nested among all |
Example: [1, 2, 3, 4, 5] | Example: (1, 2, 3, 4, 5) | Example: {1, 2, 3, 4, 5} | Example: {1, 2, 3, 4, 5} |
List can be created using list() function | Tuple can be created using tuple() function. | Set can be created using set() function | Dictionary can be created using dict() function. |
List is mutable i.e we can make any changes in list. | Tuple is immutable i.e we can not make any changes in tuple | Set is mutable i.e we can make any changes in set. But elements are not duplicated. | Dictionary is mutable. But Keys are not duplicated. |
List is ordered | Tuple is ordered | Set is unordered | Dictionary is ordered |
Creating an empty list l=[] | Creating an empty Tuple t=() | Creating a set a=set() b=set(a) | Creating an empty dictionary d={} |
To access a list item by index, the syntax is the same as Java.
arr = [1, 2, 3, 4, 5]
print(arr[3]) # This will output 4
To work with dictionaries, we can use the key to get the value.
dict = {"Alice": 12, "Bob": 13, "Charlie": 14}
print(dict["Alice"]) # This will output 12
We want to print out all of the items in a list, we can simply use a for-each loop:
arr = [1, 2, 3, 4, 5]
for a in arr:
print(a)
Or something similar as Java:
arr = [1, 2, 3, 4, 5]
for i in range(len(arr)):
print(arr[i])
int[] arr = {1, 2, 3, 4, 5};
for (int i = 0; i < arr.length; i++) {
System.out.println(arr[i]);
}
Or If we want to loop a dictionary:
dict = {"Alice": 12, "Bob": 13, "Charlie": 14}
for key in dict:
print(key, dict[key])
if and else are very similar to that in Java:num = 10
if num >= 10:
print("num >= 10")
else:
print("num < 10")
int num = 10;
if (num >= 10) {
System.out.println("num >= 10");
} else {
System.out.println("num < 10");
}
And Python can make checking easier:
primes = [2, 3, 5, 7, 11, 13, 17, 19]
if 10 in primes:
print("10 is prime")
else:
print("10 is composite")
Defining a function in Python is also similar to that in Java.
def say_hello():
print("Hello")
public static void sayHello() {
System.out.println("Hello");
}
Adding return value. And
if __name__ == '__main__': means the main method.import math
def quadratic(a, b, c):
delta = b * b - 4 * a * c
if delta >=0:
root_A = ( -b - math.sqrt(delta) ) / 2 * a
root_B = ( -b + math.sqrt(delta) ) / 2 * a
return root_A, root_B
else:
return None
if __name__ == '__main__':
roots = quadratic(2, 3, 1)
print(roots)
👍 Goal: Rewrite this Java code into Python code(script)
class HelloWorld {
static boolean isPrime(int num) {
if (num == 0) return false;
if (num == 1) return false;
boolean flag = true;
for (int i = 2; i <= num / 2; i++) {
if (num % i == 0) {
flag = false;
break;
}
}
return flag;
}
public static void main(String[] args) {
for (int i = 0; i < 100; i++) {
if (isPrime(i)) System.out.println(i);
}
}
}
Note that:
TrueorFalse
