Get index of current element in C++ range-based for-loop

My code is as follows:

std::cin >> str;
for ( char c : str )
    if ( c == 'b' ) vector.push_back(i) //while i is the index of c in str

Is this doable? Or I will have to go with the old-school for loop?

Maybe it's enough to have a variable i?

unsigned i = 0;
for ( char c : str ) {
  if ( c == 'b' ) vector.push_back(i);
  ++i;
}

That way you don't have to change the range-based loop.

Assuming str is a std::string or other object with contiguous storage:

std::cin >> str;
for (char& c : str)
    if (c == 'b') v.push_back(&c - &str[0]);

The range loop will not give you the index. It is meant to abstract away such concepts, and just let you iterate through the collection.

What you are describing is known as an 'each with index' operation in other languages. Doing some quick googling, it seems that other than the 'old-school for loop', you have some rather complicated solutions involving C++0x lambas or possibly some Boost provided gems.

EDIT: As an example, see this question

You can use lambdas in c++11:

#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
#include <iterator>

using namespace std;


int main() {
    std::string str;
    std::vector<char> v;
    auto inserter = std::back_insert_iterator<decltype(v)>(v);
    
    std::cin >> str;
    //If you don't want to read from input
    //str = "aaaaabcdecccccddddbb";
    
    std::copy_if(str.begin(), str.end(), inserter, [](const char c){return c == 'b';});
    
    std::copy(v.begin(),v.end(),std::ostream_iterator<char>(std::cout,","));
    
    std::cout << "Done" << std::endl;
    
}

In C++ 20, I use initializer like this:

for(unsigned short i = 0; string item : nilai){
   cout << i << "." << "address " << &item << " -> " << item << endl;
   i++;
}

So, your case will be like:

for (unsigned short i = 0; char c : str ) {
  if ( c == 'b' ) vector.push_back(i);
  ++i;
}

I don't know what 'vector' mean in your case, and what is push_back(). Don't forget to add -std=c++20 (I just use g++ for compiling, so i don't know much about other compiler). You can also start the 'i' value from 1 if you want to. I think it's elegant enough