This function is included in the "boost/algorithm/string" library. This library contains some brilliant methods which help in accomplishing string manipulations that are lacking in STL library.
This function is used to replace the n'th instance of the specified pattern in the input string with a new pattern. It first performs a search for the n'th occurence of the specified pattern to be replaced and then it replaces it with a new one.
input_string : an input string
seacrh_string : the pattern that has to be replaced in the input_string;
N : An integer which specifies the occurence of the pattern to be replaced. i.e., An index of the match to be replaced.
format_string : the pattern that replaces the search_string.
The index is 0-based index i.e., the first occurence is considered 0. For negative values of N, matches are counted from the end of the string i.e., the last occurence is -1 and so on.
replace_nth() performs an inplace replacement. There is another variation present replace_nth_copy() which instead of doing an inplace replacement, returns a copy of the modified string.
replace_nth() performs a case-sensistive search. There is another variation present ireplace_nth() which performs a case-insensitive search. Similarly ireplace_nth_copy() returns a copy of modified string instead of doing an inplace replacement.
using namespace std;
using namespace boost::algorithm;
string str1 = "Hello and Welcome Hello and Welcome";
replace_nth(str1,"Hello ",0,"Hi ");
string str2 = "Apple Banana Mango Apple Banana Apple";
string str4 = "Hi Hi Hi Hi Hi Hi";
string str3 = replace_nth_copy(str4,"Hi ",3,"Guys ");
As we can observe from the output we are first performing an inplace replacement where we are replacing the 0th occurence of "Hello " with "Hi" . Next we are performing replacement on the original string while performing the search case-insensitively. In str3, we are storing the string obtained after performing replacement while searching case-sensitively.