Replacing elements in vector using erase and insert

Question

void replace(vector<string> my_vector_2, string old, string replacement){

    vector<string>::iterator it;
    for (it = my_vector_2.begin(); it != my_vector_2.end(); ++it){

        if (*it==old){
            my_vector_2.erase(it);
            my_vector_2.insert(it,replacement);

        }
    }

}

So, I'd like this function to replace all occurrences of the string old in the vector with the string replacement. But when calling this function, it simply doesn't change the vector at all. I'm not sure if I am using the erase and insert functions properly. Any ideas?

Solution 1

At first you need to pass vector by reference, not by value.

void replace(vector<string>& my_vector_2, string old, string replacement){

Second erase and insert invalidates it, you need to update it with new iterator returned by erase

it = my_vector_2.erase(it);  
it = my_vector_2.insert(it,replacement);

Solution 2

There's an ready-made algorithm for your problem:

#include <algorithm>
#include <string>
#include <vector>

std::vector<std::string> v;  // populate

std::replace(v.begin(), v.end(), "old", "new");

Solution 3

You are passing your std::vector as a value. In order to Change the std::vector you pass to the function, declare it as a reference

void replace(vector<string>& my_vector_2, string old, string replacement){ }

The & denotes that you pass your std::vector by reference and so you can access the object you passed.

And don't erase the element, simply replace it.

Comments

[Shimon Rachlenko]

Shouldn't you pass the vector as a reference?

[Pete Becker]

You don't need to erase and insert. Just assign: *it = replacement;. That eliminates any issues about iterator invalidation, and removes a bunch of churning to remove an element then open up a hole for insertion where that element used to be.

[Enn Michael]

Isn't this UB? Mentioning it after the call to erase is UB, and you do mention it in your call to insert. I think you have to use a workaround with std::distance.

[Toby Speight]

@Enn - did you not notice that it is assigned with the result of erase()? The invalidated iterator cannot be used, because it now holds a valid iterator; the same is true for the following insert() line.