---
title: 2020 KGHS i2trc2 Python
---

# Python Basic Syntax - Control Flow
###### tags:`2020KGHS` `i2trc2`

[Toc]

# Condition

## If else

if x < 0:
    x = 0
    print('Negative changed to zero')
elif x == 0:
    print('Zero')
elif x == 1:
    print('Single')
else:
    print('More')

## For Loop

### Iterating over a object
Iterating over a list object

words = ['cat', 'window', 'defenestrate']
for w in words:
    print(w, len(w))

Iterating over a dict object

people = {1: {'Name': 'John', 'Age': '27', 'Sex': 'Male'},
          2: {'Name': 'Marie', 'Age': '22', 'Sex': 'Female'}}

for p_id, p_info in people.items():
    print("\nPerson ID:", p_id, end=" ")
    print(", Info:", p_info)

:::warning
若在 iteration 過程中新增或刪除元素，會造成執行時期錯誤，因此若要刪除元素通常會對 list 或 dict 做 copy

# Strategy:  Iterate over a copy
for user, status in users.copy().items():
    if status == 'inactive':
        del users[user]

# Strategy:  Create a new collection
active_users = {}
for user, status in users.items():
    if status == 'active':
        active_users[user] = status

:::

### Iterating over a range of number

for i in range(5):
    print(i)
    
for i in range(0, 10, 3):
    print(i)
0, 3, 6, 9

a = ['Mary', 'had', 'a', 'little', 'lamb']
for i in range(len(a)):
    print(i, a[i])

### enumerate
當你同時需要 index 跟 value 的時候特別好用

for i,a in enumerate(a):
    print(i,a)

### zip
當你有兩個以上的 list 或 dict 需要同時取元素時

A = [0, 1, 2, 3, 4]
B = [4, 3, 2, 1, 0]
for a, b in zip(A, B):
    print(a, b)

### break / continue
中斷迴圈 or 略過這次(跳到下一次)，基本上和 C 的用法是一樣的

for num in range(2, 10):
    if num % 2 == 0:
        print("Found an even number", num)
        continue
    print("Found an odd number", num)

## While Loop
用法跟 C 幾乎都是一樣的

while True:
    pass

:::info
注意在 python 中，關係運算要用 `and` `or` `not`

while x <= 0 and y >= 1:
    x += 1
    y -= 1

:::

## Defining functions
和 C 的 function 最大的不同是 除了可以好幾個輸入，也可以好幾個輸出喔!!

def calculator(a, b):
    return a+b, a-b, a*b, a/b
    
x, y, z, w = calculator(1, 2)

### Default argument value

def calculator(a=1, b=2):
    return a+b, a-b, a*b, a/b
    
x, y, z, w = calculator()

當 L 不存在時，自動建立

def f(a, L=None):
    if L is None:
        L = []
    L.append(a)
    return L

# Reference
1. [Python document](https://docs.python.org/3/tutorial/controlflow.html)
2. Secret in Python
1. [Pointers in Python: What's the Point?](https://realpython.com/pointers-in-python/)
2. [Primer on Python Decorators](https://realpython.com/primer-on-python-decorators)
3. [Extending Python With C Libraries and the “ctypes” Module](https://dbader.org/blog/python-ctypes-tutorial)