Python Find a substring in a string and returning the index of the substring

Question

I have     a function  def find str s  char  and a string   Happy Birthday     I essentially want to input  py  and return 3 but I keep getting 2 to return instead    Code    def find str s  char       index   0                if char in s          char   char 0          for ch in s              if ch in s                  index    1             if ch    char                  return index      else          return -1  print find str  Happy birthday    py      Not sure what s wrong

User · Answer

There is one other option in regular expression, the search method

import re

string = 'Happy Birthday'
pattern = 'py'
print(re.search(pattern, string).span()) ## this prints starting and end indices
print(re.search(pattern, string).span()[0]) ## this does what you wanted

By the way, if you would like to find all the occurrence of a pattern, instead of just the first one, you can use finditer method

import re

string = 'i think that that that that student wrote there is not that right'
pattern = 'that'

print([match.start() for match in re.finditer(pattern, string)])

which will print all the starting positions of the matches.

User · Answer

Not directly answering the question but I got a similar question recently where I was asked to count the number of times a sub-string is repeated in a given string  Here is the function I wrote   def count substring string  sub string       cnt   0     len ss   len sub string      for i in range len string  - len ss   1           if string i i len ss     sub string              cnt    1     return cnt   The find   function probably returns the index of the fist occurrence only  Storing the index in place of just counting  can give us the distinct set of indices the sub-string gets repeated within the string   Disclaimer  I am  extremly  new to Python programming

User · Answer

There s a builtin method find on string objects   s    Happy Birthday  s2    py   print s find s2     Python is a  batteries included language  there s code written to do most of what you want already  whatever you want    unless this is homework     find returns -1 if the string cannot be found

User · Answer

Here is a simple approach   my string    abcdefg  print text find  def      Output   3  I the substring is not there  you will get -1  For example   my string    abcdefg  print text find  xyz      Output   -1  Sometimes  you might want to throw exception if substring is not there   my string    abcdefg  print text index  xyz      It returns an index only if it s present   Output   Traceback  most recent call last    File  test py   line 6  in      print text index  xyz     ValueError  substring not found

User · Answer

Ideally you would use str find or str index like demented hedgehog said  But you said you can t      Your problem is your code searches only for the first character of your search string which the first one  is at index 2   You are basically saying if char 0  is in s  increment index until ch    char 0  which returned 3 when I tested it but it was still wrong  Here s a way to do it   def find str s  char       index   0      if char in s          c   char 0          for ch in s              if ch    c                  if s index index len char      char                      return index              index    1      return -1  print find str  Happy birthday    py    print find str  Happy birthday    rth    print find str  Happy birthday    rh      It produced the following output   3 8 -1

User · Answer

Adding onto  demented hedgehog answer on using find    In terms of efficiency  It may be worth first checking to see if s1 is in s2 before calling find     This can be more efficient if you know that most of the times s1 won t be a substring of s2  Since the in operator is very efficient   s1 in s2   It can be more efficient to convert   index   s2 find s1    to  index   -1 if s1 in s2     index   s2 find s1    This is useful for when find   is going to be returning -1 a lot   I found it substantially faster since find   was being called many times in my algorithm  so I thought it was worth mentioning

User · Answer

late to the party  was searching for same  as  in  is not valid  I had just created following   def find str full  sub       index   0     sub index   0     position   -1     for ch i ch f in enumerate full            if ch f lower      sub sub index  lower                position   -1             sub index   0         if ch f lower      sub sub index  lower                if sub index    0                   position   ch i              if  len sub  - 1   lt   sub index                   break             else                  sub index    1      return position  print find str  Happy birthday    py    print find str  Happy birthday    rth    print find str  Happy birthday    rh      which produces   3 8 -1   remove lower   in case case insensitive find not needed

[python] Python: Find a substring in a string and returning the index of the substring

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