if --- Sort Three Numbers

今天做练习遇到如下题目，只用if 对3个数排序，又把很久没用的排序给忘了，只记得sort了。用if写了下从大到小排列，自测没问题，int条件下。

This is an exercise in constructingif-statements. Using only simple variables and if statements, youshould be able to get this to work; a loop is not needed.

Given 3 numbers (X, Y,Z),assign variablesx,y,z so that andx , y,and z are fromX,Y, andZ. Use only a series of if-statements and assignment statements.

Hint. You must define the conditions under which you choose between x← X, x ← Y or x ←Z. You will do a similar analysis for assigningvalues toy andz. Note that your analysisfor settingy will depend on the value set forx;similarly, your analysis for settingz will depend onvalues set forx andy.

#!/usr/bin/python
# Filename: sort_three.py

a = input("a = ")
b = input("b = ")
c = input("c = ")

if a < b :
    max = b
    b = a
    a = max
    print a,b
if b < c :
    max = c
    c = b
    b = max
    print b,c
if a < b :
    max = b
    b = a
    a = max
    print a,b
print a,b,c

python 还有另一种交换数值的方法  a,b = b,a   ，这样就不用再设置中间变量了，如下：

>>> a
6
>>> b
3
>>> a,b = b,a
>>> a
3
>>> b
6
>>> 

上面的3行用一行就可以解决。

if 的格式书写如下：

if expression :
    suite

或者：

if expression :
    suite
elif expression :
    suite

expression 可以是：
条件一 or 条件二：

条件一 and 条件二：

if expression :
    suite
elif expression :
    suite
else：
    suite

if expression :
    suite
else：
    suite


以上if也可以缩减为一句话：suite  表达式1 or/and 表达式2 等

eg:

average = sum/count if count != 0 else None

average = count != 0 and float(sum)/count