if语句（If Statement）: used to implement a decision

1个if语句的“单独模式”

**floor=int(input(“Floor: ”))
if floor>13:** # if condition:
	**actualFloor=floor-1**
# statement what you would like to do
#如果楼层数高于13层的话，实际层数是楼层数减1（传说中的13楼标14层）
**else:**                    # else:
	**actualFloor=floor**        # 12楼就是12楼~
**print(“The elevator will travel to the actual floor ”,actualFloor)**

# 特别提示！！！格式一定要写成这样！一般写完if条件回车之后，statement那行会自动缩进表示这句statement是在if条件下执行。如果编辑器没有缩进，自己一定要缩进！！！如果下一句不是if条件下执行的语句，取消缩进就可以了。

# 注意执行顺序和跳过语句！！！使用前可比较if…else…和if…if…if…和if…elif…else…等模式的结果决定用哪一个

# 一般程序执行条件是，如果符合if，会跳过else，和matlab下switch运行模式相同

# 所以不用找了，python没有switch语句

Nested Branches # 多个if的“嵌套模式”

constant variables, usually in uppercase

**RATE1=0.10
RATE2=0.25
RATE1_SINGLE_LIMIT=32000.0
RATE2_MARRIED_LIMIT=64000.0
income=float(input("Please enter you income: "))
maritalStatus=input("Please enter s for single, m for married: ")
tax1=0.0
tax2=0.0

if maritalStatus == "s" :
	if income <= RATE1_SINGLE_LIMIT :
		tax1 = RATE1 * income
	else :
		tax1 = RATE1 * RATE1_SINGLE_LIMIT
	tax2 = RATE2 * (income - RATE1_SINGLE_LIMIT)
else :
	if income <= RATE2_MARRIED_LIMIT :
		tax1 = RATE1 * income
	else :
		tax1 = RATE1 * RATE2_MARRIED_LIMIT
	tax2 = RATE2 * (income - RATE2_MARRIED_LIMIT)
totalTax = tax1 + tax2
print("The tax is $%.2f" % totalTax)

# **Output:** 
# Please enter your income: 80000
# Please enter s for single, m for married: m
# The tax is $10400.00**

Multiple Alternatives # 多个if的平行模式

**richter = float(input("Enter a magnitude on the Richter scale: "))
if richter >= 8.0 :
	print("Most structures fall")
elif richter >= 7.0 :**# elif == else if
	**print("Many buildings destroyed")
elif richter >= 6.0 :
	print("Many buildings considerably damaged, some collapse")
elif richter >= 4.5 :
	print("Damage to poorly constructed buildings")
else :
	print("No destruction of buildings")**

关系运算符（Relational Operators）

> ， > = ， < ， < = ， = = (equal)， ! = (not equal)

完全遵照三次元的逻辑。但是！需要注意！Python中 **=** 的意思是赋值（等价于R里面的**->** 或者 <- ）, 两个等号，也就是 **= =** 才是等于，用于逻辑判断.

一定要区分赋值和判断！！！

# 栗子 1

**c = 6 #** 给变量c赋值6；
**c == 6 #** 是判断c等于6.

# 栗子 2