## if控制流语句执行条件原理

if语句也叫做控制流语句，给出条件来决定下一步应该怎么操作。其实原理就是如果条件为真，则执行语句块内容被执行；如果条件为假，则语句块就不会被执行。 Python条件语句是通过一条或多条语句的执行结果（True或者False）来决定执行的代码块。

## if控制流语句的语法

```if condition_1:
statement_block_1
elif condition_2:
statement_block_2
else:
statement_block_3
```

### 4）如果 "condition_2" 为False，将执行"statement_block_3"块语句；

Python 中用 elif 代替了 else if，所以if语句的关键字为：if – elif – else。

## 实例

```age = int(input("请输入你家狗狗的年龄: "))
print("")
if age < 0:
print("你是在逗我吧!")
elif age == 1:
print("相当于 14 岁的人。")
elif age == 2:
print("相当于 22 岁的人。")
elif age > 2:
human = 22 + (age -2)*5
print("对应人类年龄: ", human)
```

```people = 30
cars = 40
buses = 15

if cars > people:
print("We should take the cars.")
elif cars < people:
print("We should not take the cars.")
else:
print("We can't dicide.")

if buses > cars:
print "That's too many buses."
elif buses < cars:
print("Maybe we could take the buses.")
else:
print("We still can't decide.")

if people > buses:
print("Alright, let's just take the buses.")
else:
print("Fine, let's stay home then.")
```

```num = int(input("enter number"))
if num%2 == 0:
if num%3 == 0:
print ("Divisible by 3 and 2")
else:
print ("divisible by 2 not divisible by 3")
else:
if num%3 == 0:
print ("divisible by 3 not divisible by 2")
else:
print  ("not Divisible by 2 not divisible by 3")
```
right