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I need to generate a unique random number everytime a user press y

Anomaly007 (1)
#include <iostream>
#include <time.h>
#include<windows.h>
using namespace std;

int main ()
{
srand(time(NULL));
int arr[]={1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20,21,22,23,24,25,26,27,28,29,30,31,32,33,34,35,36,37,38,39,40,41,42,43,44,45,46,47,48,49,50,51,52,53,54,55,56,57,58,59,60,61,62,63,64,65,66,67,68,69,70,71,72,73,74,75};
char ans;
int i,j;
int elem;
int Maxsize = 75;
cout<<"Press y to generate number"<<"\n";
cin >> ans;

while(ans=='y'||ans=='Y')
{

elem = (rand() % 75) + 1;
cout<<elem << " " << "will be delete"<<"\n";
for(i=0; i<=Maxsize; i++)
{

if(arr[i]==elem)
{
for(j=i; j<(Maxsize-1); j++)
arr[j] = arr[j+1];
i--;
Maxsize--;

}
}
cout<<"\nElement Deleted Successfully!";
cout<<"\n\nThe New Array is:\n";
for(i=0; i<Maxsize; i++){
cout<<arr[i]<<" ";

}
cout<<"\n";
cout<<"Press y to generate number"<<"\n";
cin >> ans;
Sleep(2);
system("CLS");
}
cout<<endl;
cout << "\n\n";
return 0;

}

//I have try to delete the position of the number generated but the same number get generated
jonnin (11437)
one way to do it is to just std::swap() the value you have consumed with the end of the array and decrease the array's logical (kept track of) size by 1 each time.

say you got 42.
your array becomes
1,2,...40,41,75,43,... 74,42
and its size goes from 75 to 74.
next iteration, you pick rand()%74 and do the same thing.

another way to do it, more efficient, is to shuffle (<algorithm> https://www.cplusplus.com/reference/algorithm/shuffle/) the array and just iterate it.
so 1,2,3,4,5 could become
3,2,5,1,4 or whatever.
then first iteration, take [0], next, take[1], ...
both approaches work. The second again is much more efficient, getting rid of all the rand calls and extra nonsense and just tapping a value from a container. If you use a vector you can pop off the back until its empty, same idea.
consider: <random> instead of rand (from C), and <ctime> instead of time.h.

does this help/ make sense?
Last edited on
deleted account xyzzy (5768)
PLEASE learn to use code tags, they make reading and commenting on source code MUCH easier.

http://www.cplusplus.com/articles/jEywvCM9/
http://www.cplusplus.com/articles/z13hAqkS/

HINT: you can edit your post and add code tags.

Some formatting & indentation would not hurt either

Using C++ constructs to generate random numbers from a sequence.
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#include <iostream>
#include <random>
#include <numeric> // std::iota
#include <vector>
#include <cctype>  // ::tolower

int main()
{
   // set size, no magic numbers 
   const int SIZE { 75 };

   // create a C++ pseudo-random engine, seeded with
   // C++ true non-deterministic random device
   std::default_random_engine prng(std::random_device {}());

   // create a vector of SIZE elements
   std::vector<int> vec(SIZE);

   // fill the vector 1 - SIZE
   std::iota(vec.begin(), vec.end(), 1);

   // shuffle the vector to have truly random element placement
   // using the random engine instance
   std::shuffle(vec.begin(), vec.end(), prng);

   char yes_no { };

   std::cout << "\tThe vector has " << vec.size() << " elements.\n\n";

   while (std::cout << "Generate a random number? (y/n) ", // display message
          std::cin >> yes_no,                              // get input
          'n' != ::tolower(yes_no))                        // compare lower cased input
   {
      // take the first element from the shuffled vector
      std::cout << "The generated number is " << vec[0] << "\n\n";

      // erase the first element
      vec.erase(vec.begin());
   }

   std::cout << "\n\tThe vector has " << vec.size() << " elements.\n";
}
        The vector has 75 elements.

Generate a random number? (y/n) y
The generated number is 15

Generate a random number? (y/n) Y
The generated number is 70

Generate a random number? (y/n) y
The generated number is 14

Generate a random number? (y/n) y
The generated number is 19

Generate a random number? (y/n) N

        The vector has 71 elements.
deleted account xyzzy (5768)
Repeatedly erasing individual elements from the front of a vector can cause performance issues as the vector updates the memory usage. Erasing from the back of a vector will still have potential performance issues, they should be less severe than from front erasures.

C++ has iterators (think pointers) that can point to the last element of the container:
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#include <iostream>
#include <random>
#include <numeric> // std::iota
#include <vector>
#include <cctype>  // ::tolower

int main()
{
   // set size, no magic numbers 
   const int SIZE { 75 };

   // create a C++ pseudo-random engine, seeded with
   // C++ true non-deterministic random device
   std::default_random_engine prng(std::random_device {}());

   // create a vector of SIZE elements
   std::vector<int> vec(SIZE);

   // fill the vector 1 - SIZE
   std::iota(vec.begin(), vec.end(), 1);

   // shuffle the vector to have truly random element placement
   // using the random engine instance
   std::shuffle(vec.begin(), vec.end(), prng);

   char yes_no { };

   std::cout << "\tThe vector has " << vec.size() << " elements.\n\n";

   while (std::cout << "Generate a random number? (y/n) ", // display message
          std::cin >> yes_no,                              // get input
          'n' != ::tolower(yes_no))                        // compare lower cased input
   {
      // take the last element from the shuffled vector
      std::cout << "The generated number is " << *(vec.crbegin()) << "\n\n";

      // erase the last element
      vec.pop_back();
   }

   std::cout << "\n\tThe vector has " << vec.size() << " elements.\n";
}
seeplus (6591)
Is there a need to remove the last element? Why not just inc the start itr while less than the end itr?

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#include <iostream>
#include <random>
#include <numeric>
#include <vector>
#include <cctype>

int main() {
	constexpr size_t SIZE {75};
	std::vector<int> vec(SIZE);

	std::iota(vec.begin(), vec.end(), 1);
	std::shuffle(vec.begin(), vec.end(), std::random_device {});

	char yes_no {};

	std::cout << "\tThe vector has " << vec.size() << " elements.\n\n";

	auto ritr {vec.crbegin()};

	while (ritr != vec.crend() && (std::cout << "Generate a random number? (y/n) ") &&
			(std::cin >> yes_no) && 'n' != ::tolower(yes_no))
		std::cout << "The generated number is " << *ritr++ << "\n\n";
}

deleted account xyzzy (5768)
No, there is no need to remove any of the elements, from the front or back. The OP was trying to delete elements from the array so I did it with a vector using the container's deletion methods.

In actuality a pointer to the current element that gets updated is a better way to go.

Or use an indexing variable, such as size_t index { };.

Either method can be used in some code logic the container has reached the last element, either from the front or back.

Using an indexing variable would work with the OP's original design of using a regular array, PLUS being able to bail if the user wants more than SIZE random numbers.
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#include <iostream>
#include <random>
#include <numeric> // std::iota
#include <array>   // std::begin, std::end, std::size
#include <cctype>  // ::tolower

int main()
{
   // set size, no magic numbers 
   constexpr size_t SIZE { 75 };

   // create a C++ pseudo-random engine, seeded with
   // C++ true non-deterministic random device
   std::default_random_engine prng(std::random_device {}());

   // create a array of SIZE elements
   int arr[SIZE];

   // fill the array 1 - SIZE
   std::iota(std::begin(arr), std::end(arr), 1);

   // shuffle the array to have truly random element placement
   // using the random engine instance
   std::shuffle(std::begin(arr), std::end(arr), prng);

   char yes_no { };

   std::cout << "\tThe array has " << std::size(arr) << " elements.\n\n";

   size_t index { };

   while (std::cout << "Generate a random number? (y/n) ", // display message
          std::cin >> yes_no,                              // get input
          'n' != ::tolower(yes_no))                        // compare lower cased input
   {
      // has the user already picked all SIZE random numbers?
      if (SIZE == index)
      {
         std::cout << "\nThere are no more available random numbers!\n";
         return 0;
      }

      // take the last element from the shuffled array
      std::cout << "The generated number is " << arr[index] << "\n\n";

      // increment the index to the next available element
      index++;
   }

   std::cout << "\n\tThe array has " << std::size(arr) - index << " elements.\n";
}
Last edited on
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