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# Learning python (general usage)
*For the beginners who have learned a little bit of programming. Even Excel will help.*
## Lesson 1
### Environment Setup
Install the 2 IDEs (Integrated Development Environment) below.
1. http://www.python.org
Download Python3.X.X. Do not download Python2.X.X.
Light weight.
Use "python IDLE" to test and run the code.
2. https://www.jetbrains.com/pycharm
A fancy IDE which has autocomplet and many useful features. There are 2 versions: one is Professional, and the other is Community. Please download the Community.
### Online environment
https://repl.it/languages/python3
### Some useful links if you want to learn online
https://www.codeschool.com/courses/try-python
https://www.learnpython.org/
### Solving problems in detail steps with variables, if-else statement, and looping.
Ex. 1: Calculate the average of several numbers.
>1. Let *sum* be the variable to accumate the numbers.
>1. Let *count* be the variable to store the count we process so far.
>1. Read each number one by one:
>a) increase the sum by the number
>b) increase count by 1
>1. Finally, the average is sum/count
Ex. 2: Is x a prime:
(assume the x is an integer)
>1. If x is less then 2, it is not a prime.
>1. If x is 2, it is a prime.
>1. If x is divided by any interger from 2 ~ x-1, it is not a prime.
>1. It is a prime.
Ex. 3: Count the number of each letter in an English article.
>1. Let a, b, c, ..., x, y, z be the variable which are used to stored the count of each letter.
>1. Read each charactor from the article one by one:
> If the charactor is not an English letter, just skip it
> else increase the value of the corresponding variable by 1.
Ex. 4: Make a histogram for the scores of the students in a class.
Ex. 5: Part of "master mind" - guess 1234 for answer 1357
``` python
for each digit d in the guessing number(1234 in this example):
compare the digit in the same position in the answer with d
if it is the same
get 1 A
else
compare the digit in the different 3 positions in the answer with d
```
### Python code for Ex.2
```python
# assume x is an integer
def is_prime(x):
if x < 2:
return False
if x == 2:
return True
for i in range(2,x): # range(a,b) means [a,b)
if x % i == 0:
return False
return True
```
### Small Test
A kid filled all the grids of a Sudoku, but does not know if it is correct.
How do you use your word to discribe the Sudoku rule?
### Summary
Think a problem with your own language first. Then try to transform the idea into pseudo code, which uses variables, if-else, and looping. Finally, transform the pseudo code into the target programming language. Don't worry about the last step. You will learn how to code in Python after this lesson.
## Lesson 2
### Basic input
```python
my_number = int(input())
my_string = input()
my_number = int(input("Please enter a number(1~100): "))
```
### Basic output
```python
print(123)
print("the string")
print("no new line", end="")
```
### Basic objects: integer, float, string, None
```python
a, b = 1, 2
print(a + b) # 3
c, d = "1",'2'
print(c + d)
print(1 / 3 == 0.3333333333333333) # True
print(1 == 1.00000000000000000001)
print(1 == 1.000000000001)
print(1 == "1")
print(None == 0)
```
```python
x = 20
print(x*10)
x = "20"
print(x*10)
```
```python
print(123 == 123)
print(123 == "123")
print(123 == int("123"))
print(str(123) == "123")
```
#### Advanced print (https://pyformat.info/)
```python
x, y = 10, 20
print("{}+{}={}".format(x,y,x+y))
print("{:<4}+{:<4}={:<4}".format(x,y,x+y))
```
### Conditional statement: if-else, if-elif-else
```python
if my_number == ans:
print("Correct!")
elif my_number > ans:
print("too large")
else:
print("too small")
```
### Basic logic operator: <, <=, ==, >=, >
```python
while True:
x = int(input("input a number 1~100"))
if 0 <= x <= 100:
break
```
### Loop statement: for, while, continue, break
```python
for i in range(1,20,2): # range(start, exclude end, each step)
print(i)
```
```python
for i in range(10):
if i == 5:
continue
print(i, end="")
```
```python
while True:
x = int(input("input a number 1~100"))
if 0 <= x <= 100:
break
```
### Small Tests
#### Q1: Print
```
*
**
***
****
*****
******
*******
********
*********
**********
```
#### Q2: Print
```
* *
** **
*** ***
**** ****
**********
```
#### Q3: Let user input an integer N. Output the avg of summation(1~N).
#### Q4: a, b = 10, 20, please swap the value of a and b
#### Q5: Print 99 multiplication table with some alignment.
```
1 * 1 = 1
1 * 2 = 2
1 * 3 = 3
...
4 * 4 = 16
4 * 5 = 20
4 * 6 = 24
4 * 7 = 28
4 * 8 = 32
...
9 * 5 = 45
9 * 6 = 54
9 * 7 = 63
9 * 8 = 72
9 * 9 = 81
```
#### Q6: List the primes <= the user input number N. For example, if user input is 19. List 2,3,5,7,11,13,17,19.
```
Input the integer as the upper bound: 19
2
3
5
7
11
13
17
19
```
#### Q7: Print the first 20 Fabonacci numbers from 0,1,...
```
0
1
1
2
3
5
8
13
21
34
55
89
144
233
377
610
987
1597
2584
4181
```
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