Replace specific character in a string in C++ - 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");