std::copy 不会在 C++ 中复制向量

Question

To find all sequences of fixed length which contain only 0 and 1 I use this code:

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

using namespace std;

void print_array(vector<string> arr) {
  cout << '[';
  int n = arr.size();
  for (size_t i = 0; i < n; i++) {
    cout << arr[i];
    if (i < (n - 1)) {
      cout << ", ";
    }
  }
  cout << ']' << endl;
}

vector<string> get_variants(int n) {
  vector<string> result = {"0", "1"};
  vector<string> temp;
  temp.reserve(2);
  result.reserve(2);
  for (int i=0; i < (n - 1); ++i) {
    copy(result.begin(), result.end(), temp.end()); // [1]
    for (int j=0; j < result.size(); ++j) {
      temp[j] += "0";
      result[j] += "1";
    }
    copy(temp.begin(),temp.end(), result.end());
    temp.clear();
  }
  return result;
}

int main(int argc, char const *argv[]) {
  int n;
  cin >> n;
  vector<string> maybe = get_variants(n);
  print_array(maybe);
  return 0;
}

But vector temp is empty, before copying in line which I marked [1] and after. So, my program's output was [0111, 1111] . What I'm doing wrong?

Answer 1

A more straightforward way than using std::copy is the use of .insert() :

temp.insert(temp.end(), result.begin(), result.end()); //1
...
result.insert(result.end(), temp.begin(), temp.end()); // 2nd copy

Answer 2

You are writing to temp.end() and result.end() . These iterators represent "one past the end", and therefore writing to these iterators is Undefined Behavior.

You seem to be looking for std::back_inserter . This will create an iterator that will insert a new element to your container when it is written through.

std::copy(result.begin(), result.end(), std::back_inserter(temp));

While this answers the posted question, there remain other errors in your code leading to Undefined Behavior.

Answer 3

Trying to compile your program with a C++ compiler will not work, because you include #include <bits/stdc++.h> which is a non tC++ standard compliant header.

You should never include this file.

You are using typical competitive programming stuff, but including all C++ headers and not use them will eat up Compile time for no good reason.

Then, you typedef the typical competitive programming abbreviations. 2 of them, you do not use. Then there is no reason to define them.

I recommend to not do this any longer. And in C++, please use the using statement.

Then, although you want to be fast, you pass arr by value to your print function. This will copy the whole vector.

You assign/compare a lot of int with unsigned int values. This you should not do.

Additionally: Please use meaningful variable names and write comments. The more the better.

---

Regarding your specific bug. Both std::copy statements use end iterator as target. End is end. It is past the end of the vector. Please use std::back_inserter instead.

---

Regarding the algorithm. I took a while for me to realize that you basically want to create binary numbers. Nothing else. Unfortunately you translated that in a very complicated way.

Normally, you just would count from 0 to 2^n-1 and then show the data. Thats all. Becuase the numbers may be of arbitraty length, we will use manual addition of digits like in scholl on a peice of paper. Very simple.

Everthing then biols down to some lines of code.

Please see:

#include <iostream>
#include <vector>

int main() {
    // Read length of binary number to create and validate input
    if (int numberOfDigits{}; (std::cin >> numberOfDigits and numberOfDigits > 0)) {

        // Here we will store the binary digits, so 0s or 1s
        std::vector<int> digits(numberOfDigits,0);

        // Som printing helper
        std::cout << '[';
        bool printComma{};

        // We need to print 2^n possible combinations
        for (int i = 0; i < (1 << numberOfDigits); ++i) {

            // Print comma, if need
            if (printComma) std::cout << ','; printComma = true;

            // Print all digits of the binary number
            for (const int d : digits) std::cout << d;
           
            // Calculate next binary number
           int carry = 0;
           for (int index=numberOfDigits -1; index >=0; --index)  {
                
                const int sum = digits[index] + ((index == (numberOfDigits - 1)?1:0)) + carry;
                carry = sum / 2;
                digits[index] = sum % 2;
            } 
        }
        std::cout << ']';
    }
}

If there should be questions, then I am happy to answer.

Answer 4

reserve does't increse vector size.

Answer 5

Trying to compile your program with a C++ compiler will not work, because you include #include <bits/stdc++.h> which is a non tC++ standard compliant header.

You should never include this file. Never.

So, you are using typical competitive programming stuff, but why include all C++ headers and not use them. This is a waste of time.

Then, you typedef the typical nonsens competitive programming abbreviations. 2 of them, you do not use, so why define them?

Do not do this any longer. And in C++, please use the using statement.

Then, although you want to be fast, you pass arr by value to your print function. This will copy the whole vector.

You assign/compare a lot of int with unsigned int values. This you should not do.

Additionally: Use meaningful variable names and write comments. The more the better.

---

Regarding the algorithm. I took a while for me to realize that you basically want to create binary numbers. Nothing else. Unfortunately you translated that in a very complicated way.

Normally, you just would count from 0 to n-1 and then show the data. Thats all. Becuase the numbers may be of arbitraty length, we will use manual addition of digits like in scholl on a peice of paper. Very simple.

Everthing then biols down to some lines of code.

Please see:

#include <iostream>
#include <vector>

int main() {
    // Read length of binary number to create and validate input
    if (int numberOfDigits{}; (std::cin >> numberOfDigits and numberOfDigits > 0)) {

        // Here we will store the binary digits, so 0s or 1s
        std::vector<int> digits(numberOfDigits,0);

        // Som printing helper
        std::cout << '[';
        bool printComma{};

        // We need to print 2^n possible combinations
        for (int i = 0; i < (1 << numberOfDigits); ++i) {

            // Print comma, if need
            if (printComma) std::cout << ','; printComma = true;

            // Print all digits of the binary number
            for (const int d : digits) std::cout << d;
           
            // Calculate next binary number
           int carry = 0;
           for (int index=numberOfDigits -1; index >=0; --index)  {
                
                const int sum = digits[index] + ((index == (numberOfDigits - 1)?1:0)) + carry;
                carry = sum / 2;
                digits[index] = sum % 2;
            } 
        }
        std::cout << ']';
    }
}