Replace specific character in a string in C++

Question

I have a code like following -

Value = "Current &HT"; //this is value
void StringSet(const char * Value)
{
    const char *Chk = NULL; 
    Chk = strpbrk(Value,"&");
  if(Chk != NULL)
  {    
    strncpy(const_cast<char *> (Chk),"&amp",4)
  }
}

In above code I would like to replace "&" from Value with "&amp.It works fine if I have "&" single character but in current case strpbrk() return "&HT"and in below strncpy whole "&HT"is replaced.

Now I would like to know methods by which I can only replace a single character from a string.

Answer 1

You cannot replace one character in a C style string with several because you cannot know in a C style string how much room you have available to add new characters. You can only do this by allocating a new string and copying the old string to the new. Something like this

char* StringSet(const char* value)
{
    // calculate how many bytes we need
    size_t bytes = strlen(value) + 1;
    for (const char* p = value; *p; ++p)
        if (*p == '&')
             bytes += 3;
    // allocate the new string
    char* new_value = new char[bytes];
    // copy the old to the new and replace any & with &amp
    char* q = new_value;
    for (const char* p = value; *p; ++p)
    {
        *q = *p;
        ++q;
        if (*p == '&')
        {
             memcpy(q, "amp", 3);
             q += 3;
        }
    }
    *q = '\0';
    return new_value;
}

But this is terrible code. You really should use std::string.

Answer 2

I think you need some temp array to hold string past & and then replace & in original string and append temp array to original. Here is the above code modified, I believe you can use strstr instead of strchr it accepts char* as second argument.

void StringSet(char * Value)
{
    char *Chk = NULL,*ptr = NULL;
    Chk = strchr(Value,'&');
  if(Chk != NULL)
  {
    ptr = Chk + 1;
    char* p = (char*)malloc(sizeof(char) * strlen(ptr));
    strcpy(p,ptr);
    Value[Chk-Value] = '\0';
    strcat(Value,"&amp");
    strcat(Value,p);
    free(p);
  }
}

Thanks Niraj Rathi

Answer 3

You should not modify a constant string, and certainly can't modify a string literal. Although it is much much better to use a std::string instead of dealing with resource management yourself, one way is to allocate a new c-style string and return a pointer to it:

char *StringSet(const char *Value) {
  char buffer[256];
  for (char *p = (char*)Value, *t = buffer; p[0] != 0; p++, t++) {
    t[0] = p[0];
    if (p[0] == '&') {
      t[1] = 'a'; t[2] = 'm'; t[3] = 'p';
      t += 3;
    }   
    t[1] = 0;
  }
  char *t = new char[strlen(buffer)+1];
  strcpy(t, buffer);
  return t;
}

Answer 4

string str="Current &HT";
str.replace(str.find('&'),1,"&amp");