nested if statements

1. 語法

   if expression1:
   	statement(s)
   	if expression2:
   		statement(s)
   	elif expression3:
   		statement(s)
   	else:
   		statement(s)
   else:
   	statement(s)
   

2. 範例：輸入一個數字，如果小於 100，再判斷是否等於 50？等於 10？小於 10？。

   [dywang@deyu zzz]$ cat if3.py 
   #!/usr/bin/env python3
   # coding: utf-8
   
   var = input("Enter a number: ")
   num = int(var)
   if num == 100:
   	print("Expression value is equal to 100")
   elif num < 100:
   	print("Expression value is less than 100")
   	if num == 50:
   		print("Which is 50")
   	elif num < 50:
   		print("Expression value is less than 50")
   else:
   	print("Expression value is larger than 100")
   

3. 執行程式，輸入 100 , 50, 10, 2, 150 等數字進行測試。

   [dywang@deyu zzz]$ chmod +x if3.py
   [dywang@deyu zzz]$ ./if3.py 
   Enter a number: 100
   Expression value is equal to 100
   [dywang@deyu zzz]$ ./if3.py 
   Enter a number: 50
   Expression value is less than 100
   Which is 50
   [dywang@deyu zzz]$ ./if3.py 
   Enter a number: 10
   Expression value is less than 100
   Expression value is less than 50
   [dywang@deyu zzz]$ ./if3.py 
   Enter a number: 2
   Expression value is less than 100
   Expression value is less than 50
   [dywang@deyu zzz]$ ./if3.py 
   Enter a number: 150
   Expression value is larger than 100