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

Question

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?

Answer 1

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.

Answer 2

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]);

Answer 3

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

Answer 4

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.

Answer 5

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

Answer 6

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;

}