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.

Answer 1

std::string has a replace method, is that what you are looking for?

You could try:

s.replace(s.find("$name"), sizeof("Somename")-1, "Somename");

I haven't tried myself, just read the documentation on find() and replace().

Answer 2

If you want to do it quickly you can use a two scan approach. Pseudo code:

1. first parse. find how many matching chars.
2. expand the length of the string.
3. 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 <string>
#include <iostream>
#include <algorithm>
using namespace std;

void ReplaceString(string& subject, char search, const string& 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 = &subject[initSize - 1];
    while (end_ptr >= &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 << s << "\n";

}

Answer 3

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& str, const string& from, const string& 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 > 0 && replaceCount >= count) break;
        startpos = str.find(from, startpos);
    }

    return replaceCount;
}

Answer 4

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.

Answer 5

wstring myString = L"Hello $$ this is an example. By $$.";
wstring search = L"$$";
wstring replace = L"Tom";
for (int i = myString.find(search); i >= 0; i = myString.find(search))
    myString.replace(i, search.size(), replace);

Answer 6

With C++11 you can use std::regex like so:

    std::string string("hello $name");
    string = std::regex_replace(string, std::regex("\\$name"), "Somename");

The double backslash is required for escaping an escape character.