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?
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1Almost the same: stackoverflow.com/questions/10962290/…jogojapan– jogojapan2013-03-01 02:50:32 +00:00Commented Mar 1, 2013 at 2:50
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1Also related: stackoverflow.com/questions/9005835/range-based-for-in-c11jogojapan– jogojapan2013-03-01 02:54:19 +00:00Commented Mar 1, 2013 at 2:54
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3Just want to add, for a 9 yo question, this does not age well. There are many modern improvements. Despite views::enumerate delayed until C++26, we still now have ranges and views that can emulates in C++23, so when C++23 and the libraries got implemented, we can do this: godbolt.org/z/dx8xs5G15 (or you know, use libs)Shane Hsu– Shane Hsu2022-04-17 06:35:15 +00:00Commented Apr 17, 2022 at 6:35
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7 Answers
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.
7 Comments
Shane Hsu
It's a solution, but it's not elegant. Also, I think it will be pointless as it's implementing traditional for loop in a range-based one.
Daniel Frey
Maybe it's not elegant, but it's the baseline wrt complexity, readability, overhead, etc. for all other upcoming solutions/work-arounds.
Shane Hsu
Sure. But I really think there should be a constant
index in C++ Range-Based for loop.Daniel Frey
You asked about the current language status, where something like
index does not exist. Whether and how the language could be extended is a different question and does not belong here.
Tomasz Gandor
One problem with this - the i variable is scoped outside the loop. In case of the traditional for, there is a place where you can declare extra variables, when they are all of the same type (as your main 'loop variable').
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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]);
7 Comments
Shane Hsu
Nice! I was thinking that vectors can use something like
it - vector.begin(), and you solved my problem! Thanks!
BadAtLaTeX
Thanks! I got
&it - &vector[0] to compile. But is there a nice way such as @ShaneHsu 's comment? I figured out, I had to use &it - vector.begin() using iterators such as const initializer_list<double>& it. However, in a loop such as for(const pair<double, int>& it : vector_of_pairs) this yields an error: no match for ‘operator-’ (operand types are ‘const pair<double, int>*’ and vector<pair<double, int> >::const_iterator . How can I get the address of the iterator ? (Note that the former is in a ctor, whereas the latter is in a const method)
Steven Lu
Note if the underlying storage changes into no longer a vector type, it will break. Likely your integer based semantics would also then break... so this isnt a strong point against doing this. But it does feel like you'd be strengthening your commitment to contiguous storage by doing this.
Steven Lu
@gr4nt3d your question appears to be about how you cannot obtain an ordering out of an iterator for an associative type, which (say it's a hashtable) pretty much has no ordering, you can only do stuff like check its equality to
container.end(), or fetch the next one, etc. See stackoverflow.com/a/43680932/340947BadAtLaTeX
@StevenLu, I don't get your point; I was referring to plain
std::vector (at least telling from the comment, as it's been quite some time since then). As such it would have the ordering that I need, but apparently the expression &it - vector_of_pairs.begin()did not work even though I'd expect the pointer arithmetic to work, as it is contiguously stored. However, I cannot recall the exact case anymore. |
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
Comments
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.
Comments
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
Comments
Since C++23 there is a way combining iota_view and zip_view:
std::cin >> str;
for (const auto [i, value] : std::views::zip(std::ranges::iota_view(0), str)) {
if (value == 'b') {
vector.push_back(i);
}
}
1 Comment
Alexey Tikhvinsky
Agreed with Jade's edition, just prefer to inline iota_view(0) since it better answer to initial request
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;
}
