How to separate two different arrays in one loop?

Question

I want to separate in this way a -> num and b -> num2 But I can’t add another part of the variable to the for loop, for (auto& i, i2 : arr, arr2)

Are there other ways to do this?

#include <charconv>

int64_t a = 123567893,
        b = 85162,
        test = 0,
        test2 = 0;

    string arr = to_string(a),
           arr2 = to_string(b),
           num,
           num2 ;

    for (auto& i : arr, arr2)
    {
        num.push_back(i);
        num2.push_back(i);
        cout << i;
        // cout << i2;
    }

    from_chars(num.data(), num.data() + num.size(), test);
    from_chars(num2.data(), num2.data() + num2.size(), test2);


    cout << "\n" << test << endl;
    cout << "\n" << test2 << endl;

That looks like a very complicated way to copy two strings. What is the actual purpose of this code? — Albjenow, Oct 18 '19 at 08:44
Possible duplicate of [Sequence-zip function for c++11?](https://stackoverflow.com/questions/8511035/sequence-zip-function-for-c11) — Thomas Sablik, Oct 18 '19 at 08:55
What is your expectation? zip view (but `arr` and `arr2` have different size)? concatenation (but what would be `i2` in that case)? — Jarod42, Oct 18 '19 at 10:51

Thomas Sablik · Accepted Answer · 2019-10-18T10:50:03.530

2

You can use boost

#include <boost/range/combine.hpp>
#include <iostream>
#include <vector>

using namespace std;

int main() {
    std::string arr = "ABC";
    std::string arr2 = "XYZ";
    std::string num, num2;
    assert(arr.size() == arr2.size());
    for (const auto &i : boost::combine(arr, arr2)) {
        decltype(arr)::value_type a;
        decltype(arr2)::value_type b;
        boost::tie(a, b) = i;
        num.push_back(a);
        num2.push_back(b);
        cout << a;
        cout << b;
    }
}

Output is

AXBYCZ

I don't know if there is a solution for range-based for loops without boost . Of course, you could

#include <algorithm>
#include <iostream>
#include <vector>

using namespace std;

int main() {
    std::string arr = "ABC";
    std::string arr2 = "XYZ";
    std::string num, num2;
    for (unsigned int i{0}; i < std::max(arr.size(), arr2.size()); ++i) {
        if (i < arr.size()) num.push_back(arr[i]);
        if (i < arr2.size()) num2.push_back(arr2[i]);
        if (i < arr.size()) cout << arr[i];
        if (i < arr2.size()) cout << arr2[i];
    }
}

edited Oct 18 '19 at 10:50

answered Oct 18 '19 at 08:48

Thomas Sablik

16,127
7
34
62

@loli AFAIK there is no option that meets all your requirements. You could use a simple loop. You could use two loops. You could implement the zip function from boost. – Thomas Sablik Oct 18 '19 at 10:19
I need to somehow perform interactions between arrays, and if I make two loops, I will not be able to interact with them when I parse the array. – loli Oct 18 '19 at 10:26
@loli You can use a basic loop. I added a code example to my answer – Thomas Sablik Oct 18 '19 at 10:27
Thank you, I can somehow get around the size requirements of the array. eg: `arr = "123"; arr2 = "1234567"` array2 is truncated – loli Oct 18 '19 at 10:43
That depends on the use case. What should happen after the end of the shorter array? _"I need to somehow perform interactions between arrays"_ How can they interact after the end of one array? I added an idea to my answer but that makes no sense. Usually you have to increase the size of the smaller array and fill it with default values. – Thomas Sablik Oct 18 '19 at 10:48
Thanks, I got a little confused, but now I realized that the arrays should be the same. – loli Oct 18 '19 at 10:54
can I find out what means `i{ 0 }` ? – loli Oct 18 '19 at 11:08
this is the same as `i = 0;`? – loli Oct 18 '19 at 11:14
@loli Yes: https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Res-list – Thomas Sablik Oct 18 '19 at 11:47

score 1 · Answer 2 · answered Oct 18 '19 at 08:43

This here

for (auto& i : arr, arr2)

Won't work because that's not what the comma operator does. You can read about the comma operator here. In short, arr, arr2 returns arr2 so for (auto& i : arr, arr2) is equivalent to for (auto& i : arr2).

Instead, since arr and arr2 are std::string, you can just concatenate them with +:

for (auto& i : arr + arr2)
{
    num.push_back(i);
}

score -1 · Answer 3 · answered Oct 18 '19 at 08:49

-1

Why not this

for_each(arr.begin(),arr.end(), [&num](char c) {num.push_back(c);})
for_each(arr2.begin(),arr2.end(), [&num](char c) {num.push_back(c);})

answered Oct 18 '19 at 08:49

v78

2,803
21
44

How to separate two different arrays in one loop?

3 Answers3