Replace part of a string with another string

Question

Is it possible in C   to replace part of a string with another string   Basically  I would like to do this   QString string  hello  name    string replace   name    Somename      But I would like to use the Standard C   libraries

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If all strings are std  string  you ll find strange problems with the cutoff of characters if using sizeof   because it s meant for C strings  not C   strings  The fix is to use the  size   class method of std  string   sHaystack replace sHaystack find sNeedle   sNeedle size    sReplace     That replaces sHaystack inline -- no need to do an   assignment back on that   Example usage   std  string sHaystack    This is  XXX  test    std  string sNeedle     XXX    std  string sReplace    my special   sHaystack replace sHaystack find sNeedle  sNeedle size   sReplace   std  cout  lt  lt  sHaystack  lt  lt  std  endl

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This sounds like an option  string replace string find   s    string   s   size     Something     You could wrap this in a function but this one-line solution sounds acceptable  The problem is that this will change the first occurence only  you might want to loop over it  but it also allows you to insert several variables into this string with the same token   s

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Yes  you can do it  but you have to find the position of the first string with string s find   member  and then replace with it s replace   member   string s  hello  name    size type pos   s find    name     if   pos    string  npos        s replace  pos  5   somename          5   length   name       If you are planning on using the Standard Library  you should really get hold of a copy of the book  The C   Standard Library which covers all this stuff very well

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std  string replace std  string base  const std  string from  const std  string to        std  string SecureCopy   base       for  size t start pos   SecureCopy find from   start pos    std  string  npos  start pos   SecureCopy find from start pos                 SecureCopy replace start pos  from length    to              return SecureCopy

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To have the new string returned use this   std  string ReplaceString std  string subject  const std  string amp  search                            const std  string amp  replace        size t pos   0      while   pos   subject find search  pos      std  string  npos             subject replace pos  search length    replace            pos    replace length              return subject      If you need performance  here is an optimized function that modifies the input string  it does not create a copy of the string   void ReplaceStringInPlace std  string amp  subject  const std  string amp  search                            const std  string amp  replace        size t pos   0      while   pos   subject find search  pos      std  string  npos             subject replace pos  search length    replace            pos    replace length              Tests   std  string input    abc abc def   std  cout  lt  lt   Input string     lt  lt  input  lt  lt  std  endl   std  cout  lt  lt   ReplaceString   return value                lt  lt  ReplaceString input   bc          lt  lt  std  endl  std  cout  lt  lt   ReplaceString   input string not modified                lt  lt  input  lt  lt  std  endl   ReplaceStringInPlace input   bc          std  cout  lt  lt   ReplaceStringInPlace   input string modified                lt  lt  input  lt  lt  std  endl    Output   Input string  abc abc def ReplaceString   return value  a   a   def ReplaceString   input string not modified  abc abc def ReplaceStringInPlace   input string modified  a   a   def

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With C  11 you can use std  regex like so    include  lt regex gt      std  string string  hello  name    string   std  regex replace string  std  regex     name     Somename      The double backslash is required for escaping an escape character

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I m just now learning C    but editing some of the code previously posted  I d probably use something like this   This gives you the flexibility to replace 1 or multiple instances  and also lets you specify the start point   using namespace std      returns number of replacements made in string long strReplace string amp  str  const string amp  from  const string amp  to  size t start   0  long count   -1        if  from empty    return 0       size t startpos   str find from  start       long replaceCount   0       while  startpos    string  npos           str replace startpos  from length    to           startpos    to length            replaceCount             if  count  gt  0  amp  amp  replaceCount  gt   count  break          startpos   str find from  startpos              return replaceCount

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My own implementation  taking into account that string needs to be resized only once  then replace can happen   template  lt typename T gt  std  basic string lt T gt  replaceAll const std  basic string lt T gt  amp  s  const T  from  const T  to        auto length   std  char traits lt T gt   length      size t toLen   length to   fromLen   length from   delta   toLen - fromLen      bool pass   false      std  string ns   s       size t newLen   ns length         for  bool estimate     true  false                  size t pos   0           for     pos   ns find from  pos      std  string  npos  pos                          if  estimate                                newLen    delta                  pos    fromLen                            else                               ns replace pos  fromLen  to                   pos    delta                                   if  estimate              ns resize newLen              return ns      Usage could be for example like this   std  string dirSuite   replaceAll replaceAll relPath parent path   u8string

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This could be even better to use   void replace string amp  input  const string amp  from  const string amp  to        while true                size t startPosition   input find from           if startPosition    string  npos              break          input replace startPosition  from length    to

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If you want to do it quickly you can use a two scan approach   Pseudo code    first parse  find how many matching chars   expand the length of the string   second parse  Start from the end of the string when we get a match we replace  else we just copy the chars from the first string    I am not sure if this can be optimized to an in-place algo   And a C  11 code example but I only search for one char    include  lt string gt   include  lt iostream gt   include  lt algorithm gt  using namespace std   void ReplaceString string amp  subject  char search  const string amp  replace           size t initSize   subject size        int count   0      for  auto c   subject             if  c    search    count             size t idx   subject size  -1   count   replace size  -1      subject resize idx   1    0         string reverseReplace  replace        reverse reverseReplace begin    reverseReplace end            char  end ptr    amp subject initSize - 1       while  end ptr  gt    amp subject 0                 if   end ptr    search                for  auto c   reverseReplace                    subject idx - 1    c                  --idx                                                           else               subject idx - 1     end ptr              --idx                    --end ptr           int main         string s   Mr John Smith         ReplaceString s         20        cout  lt  lt  s  lt  lt    n

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There s a function to find a substring within a string  find   and a function to replace a particular range in a string with another string  replace   so you can combine those to get the effect you want   bool replace std  string amp  str  const std  string amp  from  const std  string amp  to        size t start pos   str find from       if start pos    std  string  npos          return false      str replace start pos  from length    to       return true     std  string string  hello  name    replace string    name    Somename        In response to a comment  I think replaceAll would probably look something like this   void replaceAll std  string amp  str  const std  string amp  from  const std  string amp  to        if from empty            return      size t start pos   0      while  start pos   str find from  start pos      std  string  npos            str replace start pos  from length    to           start pos    to length       In case  to  contains  from   like replacing  x  with  yx

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I use generally this   std  string amp  replace std  string amp  s  const std  string amp  from  const std  string amp  to        if  from empty            for size t pos   0   pos   s find from  pos      std  string  npos  pos    to size                s replace pos  from size    to       return s      It repeatedly calls std  string  find   to locate other occurrences of the searched for string until std  string  find   doesn t find anything  Because std  string  find   returns the position of the match we don t have the problem of invalidating iterators

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std  string has a replace method  is that what you are looking for   You could try   s replace s find   name    sizeof   name   - 1   Somename      I haven t tried myself  just read the documentation on find   and replace

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You can use this code for remove subtring and also replace   and also remove extra white space   code    include lt bits stdc   h gt  using namespace std   void removeSpaces string  amp str            int n   str length         int i   0  j   -1       bool spaceFound   false       while    j  lt   n  amp  amp  str j                while  j  lt   n                if  str j                                           if   str j            str j                             str j           amp  amp  i - 1  gt   0  amp  amp                   str i - 1                          str i - 1    str j                  else                                  str i      str j                               spaceFound   false                    else if  str j                                               if   spaceFound                                str i                           spaceFound   true                                     if  i  lt   1          str erase str begin     i  str end         else         str erase str begin     i - 1  str end       int main         string s      cin gt  gt s      for int i s find  quot WUB quot   i gt  0 i s find  quot WUB quot                  s replace i 3  quot   quot              removeSpaces s       cout lt  lt s lt  lt endl       return 0

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wstring myString   L Hello    this is an example  By       wstring search   L      wstring replace   L Tom   for  int i   myString find search   i  gt   0  i   myString find search       myString replace i  search size    replace

[c++] Replace part of a string with another string

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