Google python course basic exercise——string1

这是google python课程里的练习题一——string 1，通过这几个小程序的实现，足以窥一斑而知全豹，发现python的美。

题目及代码如下：

# A. donuts
# Given an int count of a number of donuts, return a string
# of the form 'Number of donuts: <count>', where <count> is  the number
# passed in. However, if the count is 10 or more, then use the word 'many'
# instead of the actual count.
# So donuts(5) returns 'Number of donuts: 5'
# and donuts(23) returns 'Number of donuts: many'
def donuts(count):
    if 0<count<10:      #这里注意哦，只有python等少数语言能这样写判断表达式，C/C++和java都是没有的。
        word = count
    elif count>=10:
        word = 'many'
    else:
        print 'please enter the positive number!'
    return 'Number of donuts: %s'% word


# B. both_ends
# Given a string s, return a string made of the first 2
# and the last 2 chars of the original string,
# so 'spring' yields 'spng'. However, if the string length
# is less than 2, return instead the empty string.
    def both_ends(s):
    if len(s)<2:
        word = ''
    else:
        word = '%s%s%s%s' % (s[0],s[1],s[-2],s[-1])  #这里[]内的"-"符号表示逆序
    return  word


# C. fix_start
# Given a string s, return a string
# where all occurences of its first char have
# been changed to '*', except do not change
# the first char itself.
# e.g. 'babble' yields 'ba**le'
# Assume that the string is length 1 or more.
# Hint: s.replace(stra, strb) returns a version of string s
# where all instances of stra have been replaced by strb.
def fix_start(s):
    c = s[0]
    a = s.replace(c,'*')
    b = c+a[1:]
    return b
    

# D. MixUp
# Given strings a and b, return a single string with a and b separated
# by a space '<a> <b>', except swap the first 2 chars of each string.
# e.g.
#   'mix', pod' -> 'pox mid'
#   'dog', 'dinner' -> 'dig donner'
# Assume a and b are length 2 or more.
def mix_up(a,b):
    if len(a)<2 or len(b)<2:
        print "please enter the string whose length is > 2"
    else:
        c = b[0:2]+a[2:]
        d = a[0:2]+b[2:]
        word = '%s %s' % (c,d)
    return word
    

# Provided simple test() function used in main() to print
# what each function returns vs. what it's supposed to return.
def test(got, expected):
  if got == expected:
    prefix = ' OK '
  else:
    prefix = '  X '
  print '%s got: %s expected: %s' % (prefix, repr(got), repr(expected))
    

# Provided main() calls the above functions with interesting inputs,
# using test() to check if each result is correct or not.
def main():
  print 'donuts'
  # Each line calls donuts, compares its result to the expected for that call.
  test(donuts(4), 'Number of donuts: 4')
  test(donuts(9), 'Number of donuts: 9')
  test(donuts(10), 'Number of donuts: many')
  test(donuts(99), 'Number of donuts: many')
  print
  print 'both_ends'
  test(both_ends('spring'), 'spng')
  test(both_ends('Hello'), 'Helo')
  test(both_ends('a'), '')
  test(both_ends('xyz'), 'xyyz')
  print
  print 'fix_start'
  test(fix_start('babble'), 'ba**le')
  test(fix_start('aardvark'), 'a*rdv*rk')
  test(fix_start('google'), 'goo*le')
  test(fix_start('donut'), 'donut')
  print
  print 'mix_up'
  test(mix_up('mix', 'pod'), 'pox mid')
  test(mix_up('dog', 'dinner'), 'dig donner')
  test(mix_up('gnash', 'sport'), 'spash gnort')
  test(mix_up('pezzy', 'firm'), 'fizzy perm')
# Standard boilerplate to call the main() function.
if __name__ == '__main__':
  main()

运行结果：

donuts
OK got: 'Number of donuts: 4' expected: 'Number of donuts: 4' 
OK got: 'Number of donuts: 9' expected: 'Number of donuts: 9' 
OK got: 'Number of donuts: many' expected: 'Number of donuts: many' 
OK got: 'Number of donuts: many' expected: 'Number of donuts: many' 

both_ends
OK got: 'spng' expected: 'spng' 
OK got: 'Helo' expected: 'Helo' 
OK got: '' expected: '' 
OK got: 'xyyz' expected: 'xyyz' 

fix_start
OK got: 'ba**le' expected: 'ba**le' 
OK got: 'a*rdv*rk' expected: 'a*rdv*rk' 
OK got: 'goo*le' expected: 'goo*le' 
OK got: 'donut' expected: 'donut' 

mix_up
OK got: 'pox mid' expected: 'pox mid' 
OK got: 'dig donner' expected: 'dig donner' 
OK got: 'spash gnort' expected: 'spash gnort' 
OK got: 'fizzy perm' expected: 'fizzy perm'

 

另外需说明的是c项fix_start()函数中：　

1. 调用s.replace()函数得到的是s修改的的副本，原串s保持不变，故用另一变量接收。
2. 得到变量a后很容易想到的下一步动作是 b=a.replace(a[0],c)，但运行后发现b=s了，可见此处a.replace(a[0],c)执行的效果并不是将串a中第一位a[0]替换成c字符，而是将串a中所有匹配a[0]的字符替换成了c，这就导致两次替换后，还原了。eg：s="google";c=s[0];a=s.replace(c,"*");b=a.replace(a[0],c)以为可以得到'goo*le',结果得到'google'。所以我采用了字符串的连接方法。