Python | Check order of character in string using OrderedDict( )

Given an input string and a pattern, check if characters in the input string follows the same order as determined by characters present in the pattern. Assume there won’t be any duplicate characters in the pattern.


string = "engineers rock"
pattern = "er";
Output: true
All 'e' in the input string are before all 'r'.

string = "engineers rock"
pattern = "egr";
Output: false
There are two 'e' after 'g' in the input string.

string = "engineers rock"
pattern = "gsr";
Output: false
There are one 'r' before 's' in the input string.

We have existing solution for this problem, please refer Check if string follows order of characters defined by a pattern or not | Set 1. Here we solve this problem quickly in python using OrderedDict(). Approach is very simple,

  • Create an OrderedDict of input string which contains characters of input strings as Key only.
  • Now set a pointer at the start of pattern string.
  • Now traverse generated OrderedDict and match keys with individual character of pattern string, if key and character matches with each other then increment pointer by 1.
  • If pointer of pattern reaches it’s end that means string follows order of characters defined by a pattern otherwise not.




# Function to check if string follows order of 
# characters defined by a pattern
from collections import OrderedDict
def checkOrder(input, pattern):
    # create empty OrderedDict
    # output will be like {'a': None,'b': None, 'c': None}
    dict = OrderedDict.fromkeys(input)
    # traverse generated OrderedDict parallel with
    # pattern string to check if order of characters 
    # are same or not
    ptrlen = 0 
    for key,value in dict.iteritems():
        if (key == pattern[ptrlen]):
            ptrlen = ptrlen + 1
        # check if we have traverse complete 
        # pattern string
        if (ptrlen == (len(pattern))):
            return 'true'
    # if we come out from for loop that means 
    # order was mismatched
    return 'false'
# Driver program
if __name__ == "__main__":
    input = 'engineers rock' 
    pattern = 'egr'
    print checkOrder(input,pattern)




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