Creating a new array in C++ using parameters not determined before the code is run

Question

I need to create an array with the size that a random number generator creates. Any help would be appreciated!

#include <iostream>
#include <iomanip>
#include <cstdlib>
#include <ctime>

using namespace std;

int main()
{
    srand(time(NULL));
    int parameter;
    char newArray[] = {};
    parameter = rand() % 10;
    newArray[parameter] = {};

    for (int i = 0; i < parameter; i++)
    {
        cout << "Enter a character: ";
        cin >> newArray[i];
    }
    for (int i = 0; i < parameter; i++)
    {
        cout << newArray[i];
        return (0);
    }
}

In C++, an array of dynamically determined size is spelled std::vector — Igor Tandetnik
– Igor Tandetnik, Commented Apr 26, 2020 at 0:44

Thomas Sablik · Accepted Answer · 2020-04-26 01:03:54Z

1

You can use a std::string

#include <iostream>
#include <iomanip>
#include <string>
#include <cstdlib>
#include <ctime>

int main() {
    std::srand(std::time(nullptr));
    int parameter = std::rand() % 10;
    std::string newArray(parameter, '\0');

    for (auto &c : newArray) {
        std::cout << "Enter a character: "; std::cin >> c;
    }
    std::cout << newArray;

    return 0;
}

or a std::vector

#include <iostream>
#include <iomanip>
#include <vector>
#include <cstdlib>
#include <ctime>

int main() {
    std::srand(std::time(nullptr));
    int parameter = std::rand() % 10;
    std::vector<char> newArray(parameter);

    for (auto &c : newArray) {
        std::cout << "Enter a character: "; std::cin >> c;
    }
    for (auto c : newArray) {
        std::cout << c;
    }
    return 0;
}

edited Apr 26, 2020 at 1:03

answered Apr 26, 2020 at 0:58

Thomas Sablik

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2 Comments

Omid CompSCI Over a year ago

I think he can also just declare newArray after the random number and keep it as a char newArray[parameter] = new char[parameter] and keep the code the same.

Thomas Sablik Over a year ago

@OmidCompSCI Do you mean a variable-length array char newArray[parameter]; or dynamic memory allocation char *newArray = new char[parameter]? Your example char newArray[parameter] = new char[parameter] causes a compile error. Many compilers don't support VLAs and dynamic memory allocation is a complex topic in C++. You have to remember to clean up. Strings and vectors are the most simple ways to achieve this.

anastaciu · Accepted Answer · 2020-04-26 01:27:15Z

1

The intuitive thing to do is to use std::string, after all, we want an array of characters:

#include <iostream>
#include <ctime>

int main()
{
    srand(time(NULL));
    int parameter;
    char c;

    std::string newArray; 

    parameter = rand() % 10 + 1; // you'd want to avoid 0 

    while(parameter > 0)
    {
        std::cout << "Enter a character: ";
        std::cin >> c;
        //ignore extra characters
        std::cin.ignore(std::numeric_limits<std::streamsize>::max(), '\n');
        newArray.push_back(c);
        parameter--;
    }
    std::cout << newArray;
}

edited Apr 26, 2020 at 1:27

answered Apr 26, 2020 at 1:12

anastaciu

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Comments

Scott Deagan · Accepted Answer · 2020-04-26 03:16:52Z

1

#include <iostream>
#include <memory>

int main()
{
  std::srand(time(nullptr));
  int count = (rand() % 10) + 1;
  std::unique_ptr<char[]> newArray(new char[count + 1]{});

  std::cout << "Count: " << count << std::endl;

  for (int i=0; i<count; i++) {
    std::cout << "Enter character " << i << ": "; 
    std::cin >> newArray[i];
  }

  std::cout << newArray.get() << std::endl;

  return 0;
}

edited Apr 26, 2020 at 3:16

answered Apr 26, 2020 at 3:06

Scott Deagan

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