在 C++ 中，如何将变量放入数组中？

Question

#include <iostream>
#include <cmath>
using namespace std;
bool narcissistic(int value)
{
    cout << "value is:" << value << endl;
    int digitNumber = (log10(value) + 1);
    cout << "Digit Number:" << digitNumber << endl;
    int sum = 0;
    int arr[5];
    for (int i = 0; i <= digitNumber - 1; i++)
    {
        cout << "i digit:" << i << endl;
        int exponential = pow(10, i);
        arr[i] = (value % (exponential *10)) / pow(10, i);
        cout << arr[i] << endl;
        sum += pow(arr[i], digitNumber);
        cout << pow(arr[i], digitNumber) << endl;
        cout << i << "'th sum value:" << sum << endl;

    }
    
    return true;

}

int main() {
    int value = 153;
    narcissistic(value);

    return 0;
}

在此代码中； 我不得不写：

int arr[5];

但我希望这个数组的大小是一个变量，以便可以定义它的大小，并且可以通过 for 循环放置它的值，因此它的大小可以等于循环的数量。 我想这样写：

for (int i = 0; i <= digitNumber - 1; i++)
    {
        int arr[i];
        cout << "i digit:" << i << endl;
        int exponential = pow(10, i);
        arr[i] = (value % (exponential *10)) / pow(10, i);
        cout << arr[i] << endl;
        sum += pow(arr[i], digitNumber);
        cout << pow(arr[i], digitNumber) << endl;
        cout << i << "'th sum value:" << sum << endl;

    }

Visual Studio 说数组作为长度的值必须是常数，这就是为什么你不能通过循环等来确定它。

有没有办法将变量作为大小放入数组中，并通过 C++ 中的循环分配数组的大小？

Answer 1

你可能想要这样的东西：

#include <iostream>
#include <cmath>
#include <vector>

using namespace std;

bool narcissistic(int value)
{
  cout << "value is:" << value << endl;
  int digitNumber = (log10(value) + 1);
  cout << "Digit Number:" << digitNumber << endl;
  int sum = 0;
  std::vector<int> arr(digitNumber);  // initialize a vector containing 
                                      // digitNumber elements

  for (int i = 0; i <= digitNumber - 1; i++)
  {
    cout << "i digit:" << i << endl;
    int exponential = pow(10, i);
    arr[i] = value % (exponential * 10) / pow(10, i);
    cout << arr[i] << endl;
    sum += pow(arr[i], digitNumber);
    cout << pow(arr[i], digitNumber) << endl;
    cout << i << "'th sum value:" << sum << endl;
  }

  // printf values in arr
  cout << "arr: ";
  for (auto& value : arr)
    cout << value << ", ";

  return true;
}

int main() {
  int value = 153;
  narcissistic(value);

  return 0;
}

但这是将数字分解为数字的一种相当复杂的方法。你根本不需要浮点运算来做到这一点：

#include <iostream>
#include <cmath>
#include <vector>

using namespace std;

bool narcissistic(int value)
{
  cout << "value is:" << value << endl;
  std::vector<int> arr;  // start off with an empty vector

  for (int i = 0; value != 0; i++)
  {
    arr.push_back(value % 10);  // add to the vector
    value /= 10;
  }

  // printf values in arr
  cout << "arr: ";
  for (auto& value : arr)
    cout << value << ", ";

  return true;
}

int main() {
  int value = 153;
  narcissistic(value);

  return 0;
}

Answer 2

如果你想避免使用vector，你可以使用这个：

#include <iostream>
#include <cmath>
#include <vector>

using namespace std;

bool narcissistic(int value)
{
  cout << "value is:" << value << endl;
  int digitNumber = (log10(value) + 1);
  cout << "Digit Number:" << digitNumber << endl;
  int sum = 0;
  
  vector<int> v(digitNumber);
  int *arr = &v[0];

  for (int i = 0; i <= digitNumber - 1; i++)
  {
    cout << "i digit:" << i << endl;
    int exponential = pow(10, i);
    arr[i] = value % (exponential * 10) / pow(10, i);
    cout << arr[i] << endl;
    sum += pow(arr[i], digitNumber);
    cout << pow(arr[i], digitNumber) << endl;
    cout << i << "'th sum value:" << sum << endl;
  }

  // print values in arr
  cout << "arr: ";
  for (int i = 0; i <= digitNumber - 1; i++)
    cout << arr[i] << ", ";

  return true;
}

int main() {
  int value = 153;
  narcissistic(value);

  return 0;
}