Python 编程入门/Python 编程 - 控制结构
一组动作定义了事件流，由流程图决定。在 Python 编程中，所有非“None”的值都返回 True，而具有“None”值的变量则返回 False。
最简单的控制 if 语句在条件满足时返回 True 值。有一个简单的单行 if 条件，接着是 if-else，然后是 if-elif 语句。
>>> var = 100 >>> if var==100:
print "yay! 变量确实是 100!!!"
yay! the variable is truly 100!!! >>> >>> >>> if (var==100): print "value is 100, its looks great!"
value is 100, its looks great!
>>>
var = 100
if var == 100:
print "yay! the variable is truly 100"
else:
print "Nope! Better luck next time!!!!"
>>>var=10
>>> if var == 100:
print "yay! the variable is truly 100"
else:
print "Nope! Better luck next time!!!!"
Nope! Better luck next time!!!!
Python 中的“for”循环会对一个代码块执行已知次数的迭代。Python 中“for”循环的特殊之处在于，可以对列表、字典、字符串变量中的项目数量进行迭代，以特定步骤数对特定范围的数字进行迭代，以及对元组中存在的项目数量进行迭代。
>>> a=(10,20,30,40,50)
>>> for b in a:
print "square of " + str(b) + " is " +str(b*b)
square of 10 is 100
square of 20 is 400
square of 30 is 900
square of 40 is 1600
square of 50 is 2500
>>> c=["new", "string", "in", "python"]
>>> for d in c:
print "the iteration of string is %s" %d
the iteration of string is new
the iteration of string is string
the iteration of string is in
the iteration of string is python
>>>
>>> for i in range(10):
print " "+str(i)+" 的值为 "+str(i)
the value of 0 is 0
the value of 1 is 1
the value of 2 is 2
the value of 3 is 3
the value of 4 is 4
the value of 5 is 5
the value of 6 is 6
the value of 7 is 7
the value of 8 is 8
the value of 9 is 9
>>> a={'a':10,'b':20,'c':30}
>>> for k in a:
print "the key is %s and the value is %d" %(k,a[k])
the key is a and the value is 10
the key is c and the value is 30
the key is b and the value is 20
while 循环在条件语句返回 true 时执行。在执行代码块之前，每次都会评估条件语句，并且在条件语句返回 false 的那一刻，执行就会停止。
>>> count = 0
>>> while (count<9):
print "the count is at iteration %d" %count
count+=1
the count is at iteration 0
the count is at iteration 1
the count is at iteration 2
the count is at iteration 3
the count is at iteration 4
the count is at iteration 5
the count is at iteration 6
the count is at iteration 7
the count is at iteration 8