Python 教學 - 控制流程

控制流程是程式設計的重要組成部分。控制流有兩種類型，包括分支和循環。

比較和邏輯運算符

比較和邏輯運算子常用於控制流機制。這是可在 Python 中使用的比較運算子的清單。比較運算子傳回一個布林值。

Operator	Description
==	Equals to
!=	Not equals to
>	Greater than
>=	Greater than or equals
<	Less than
<=	Less than or equals

這是 Python 中比較運算子用法的範例。

first = 5
second = 6

print(first > second)
print(first < second)
print(first != second)

輸出

False
True
True

and 運算子是如果兩個條件都傳回 true 則傳回 true 的運算子。

Condition	Condition	Result
False	False	False
False	True	False
True	False	False
True	True	True

or 運算符是一個運算符，如果一個或兩個條件回傳 true，則傳回 true。

Condition	Condition	Result
False	False	False
False	True	True
True	False	True
True	True	True

This is an example of logical operator usage.

first = 6
second = 7

result_1 = first != 0 and first > second
result_2 = second > 5 or second == first

print(result_1)
print(result_2)




<p>Output<br>
</p>

<pre class="brush:php;toolbar:false">False
True

Based on the code above, the and operator returns False because there is a condition that returns False which is first > second.
The or operator returns True because there is a condition that returns True which is second > 5.

and returns true if all conditions are true.

or returns true if one of the other conditions is true.

Branching in Python

Branching is a mechanism to execute a code based on a specific condition. There are many approaches to performing branching in Python.

Branching with if

This is the basic structure of branching with if.

if condition:
    code

The code inside the if block is executed if the condition inside the if block is true.

This is an example of branching using if.

num = 24

if num % 2 == 0:
    print('even number')

Output

even number

Based on the code above, the code inside the if block is executed because the condition inside the if block is true.

Branching with if-else

This is the basic structure of branching with if-else.

if condition:
    code
else:
    other code

The code inside the if block is executed if the condition inside the if block is true. Otherwise, the other code is executed inside the else block.

This is the example of branching with if-else.

num = 35

if num % 2 == 0:
    print('even number')
else:
    print('odd number')

Output

odd number

Based on the code above, the condition inside the if block is false then the code inside the else block is executed.

Branching with if-elif-else

This is the basic structure of branching with if-elif-else.

if condition:
    code
elif condition:
    code
else:
    code

The if-elif-else is useful to perform branching with many conditions. This is an example of if-elif-else usage.

role = "Back-end"

if role == "Front-end":
    print("learn HTML, CSS and JS")
elif role == "Back-end":
    print("learn Java, Go and PHP")
else:
    print("learn any related topics")

Output

learn Java, Go and PHP

Based on the code above, the code is executed based on the role value.

Branching with match-case

The match-case branching works like the switch-case in other programming languages. This is the basic structure.

match value:
    case condition:
        code
    case condition:
        code
    case condition:
        code
    case _:
        default code

This is the example of branching with match-case.

role = "Back-end"

match role:
    case "Front-end":
        print("learn HTML, CSS and JS")
    case "Back-end":
        print("learn Java, Go and PHP")
    case _: # default condition
        print("learn any related topics")

Output

learn Java, Go and PHP

Looping in Python

Looping is a mechanism to execute a code repeatedly. Looping is an essential tool for completing repetitive tasks like reading a record in a file, basic calculations, and so on. There are many approaches to performing looping in Python.

Looping with for

The for loop is suitable for executing repetitive tasks within an exact amount of time. For example, a certain task has to be completed 10 times. This is the basic structure of the for loop.

for value in iterables:
    code

This is the sample code without using the for loop.

print(1)
print(2)
print(3)
print(4)
print(5)

This is the modified code using the for loop.

for i in range(1,6):
    print(i)

Output

1
2
3
4
5

Based on the code above, the for loop code is executed with the range() function. The range() function returns a sequence of numbers based on the given start and end of the range. This is the basic structure of the range() function.

range(start,end,step)

• start defines the start of the sequence.
• end defines the end of the sequence. The end value is not included in the sequence.
• step defines the step value for generating the next sequences. By default, the step value is 1 which means each sequence value is incremented by 1.

The range(1,6) means generating a sequence of numbers from 1 up to but not including 6.

This is another example of a for loop with a custom range (range(2,11,2)) to display even numbers.

for i in range(2,11,2):
    print(i)

Output

2
4
6
8
10

Based on the code above, the even numbers from 2 up to but not including 11 are displayed. The range(2,11,2) generates sequence from 2 up to but not including 11 with the step value equals 2 which means each sequence value is incremented by 2.

break and continue keyword

The break keyword stops the code execution. This keyword is usually used inside the loop. This is an example of break usage.

num = 10

while num != 0:
    if num % 4 == 0:
        print("done!")
        break # stops the execution
    print("stil running...")
    num -= 1 # decrement the value by 1

Output

stil running...
stil running...
done!

Based on the code above, the break keyword stops the execution if the condition is met.

The continue keyword continues the code execution into the next phase or skips to the next iteration. This is an example of continue keyword usage.

for i in range (1,7):
    if i == 5:
        print("skipped!")
        continue
    print(f"value: {i}")

Output

value: 1
value: 2
value: 3
value: 4
skipped!
value: 6

Based on the code above, the continue keyword skips to the next iteration if the condition is met.

Looping with while

The while loop is suitable for executing repetitive tasks within an uncertain amount of time. For example, a certain task has to be completed until a specific condition is met. This is the basic structure of the while loop.

while expression:
    code

This is an example of while loop usage.

num = 1

while num <= 5:
    print(num)
    num += 1

Output

1
2
3
4
5

Based on the code above, the code inside the while loop is executed while the condition (in this case num <= 5) is true. If the condition is not met, then the loop is stopped. The += operator increments the num value by 1.

The num += 1 is equals to num = num + 1. The ++ and -- operator is not supported in Python.

When working with a loop, ensure the implementation and applied condition are correct to avoid an infinite loop.

Example 1 - Grade Checking

Let's create a Python program to check the grade based on the given score. For example, this table is used to check the grade based on the given score.

Score	Grade
81-100	A
65-80	B
50-64	C
30-49	D
0-29	E

The solution is using branching. This is the complete example:

score = int(input("enter a score: "))

if score >= 81 and score <= 100:
    print("A")
elif score >= 65 and score <= 80:
    print("B")
elif score >= 50 and score <= 64:
    print("C")
elif score >= 30 and score <= 49:
    print("D")
elif score >= 0 and score <= 29:
    print("E")
else:
    print("invalid score")

Output

Example 2 - Palindrome Checking

Let's create a program to check if the given word is a palindrome. Palindrome is a word that can be read equally forward and backward.

The naive solution is to compare the original word and the reversed word. If both of them are equal, then the given word is a palindrome. The naive solution walkthrough is illustrated in the picture below.

This is the naive solution implementation using a while loop.

# get the word input from the user
word = input("insert a word: ")

# sanitize the input
word = word.lower()

# create a variable to store reversed word
reversed = ""
# create a variable to store the index
idx = len(word) - 1

# generate reversed word
while idx >= 0:
    reversed += word[idx]
    idx = idx - 1

# compare the original and reversed word
if word == reversed:
    print("palindrome")
else:
    print("not palindrome")

Output

Another approach is to create two indices for iterating through each character from forward and backward. If two characters from forward and backward are not equal, then the given word is not palindrome. The walkthrough of this approach is illustrated in the picture below.

This is the implementation of using two indices.

# get the word input from the user
word = input("insert a word: ")

# sanitize the input
word = word.lower()

# create two indices
front = 0 # forward tracking
back = len(word) - 1 # backward tracking

# store result
result = "palindrome"

# track each characters forward and backward
while front < len(word):
    if word[front] != word[back]:
        result = "not palindrome"
        break
    front += 1
    back -= 1

# display the check result
print(result)

Output

Sources

• Python Official Page.
• Python Tutorial.
• Control Flow in Python.

I hope this article helps you learn Python. If you have any feedback, please let me know in the comment section.

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