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I dont have any idea why this code isnt letting me input. Sorry for being totally naive as a beginner but pls help.

Pranav1105 (3)
I have just made a simple code to taken in some inputs (as testcases and values), but this code is just running and completing execution on both codeblocks and the onlinegdb compiler. I am not able to understand why it isn't letting me input anything despite having cin included.

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  #include <iostream>

using namespace std;

int main() {
	int t,n,i,A[n];
	cin>>t;
	while(t>0)
	{
		cin>>n;
		for(i=0;i<n;i++)
		{
			cin>>A[i];
		}
                t--;
	}
	return 0;
}
Last edited on
Peter87 (11235)
The program has undefined behaviour (UB) because you use n (to specify the size of A) before being initialized.

Note that using a non-constant value to specify the size of a local array is not a standard feature, but some compilers allow it anyway so this shouldn't cause any problems as long as the compiler doesn't complain.
Last edited on
seeplus (6591)
What are you trying to achieve? Assuming that your compiler allows a non-const value to be used as the size of a local array, A is defined at L6 but n isn't obtained until L10. Remove A[n] from L6 and after L10 insert int A[n];

But even when this works as coded, it really does nothing apart from obtaining sets of values for A t times. Nothing is actually done with the entered values...
Pranav1105 (3)
Thanks Peter87 I got it. And @seeplus, I was actually trying to use it to sort a no of arrays (based on their testcases) in reverse order. I was able to rectify the mistake thanks to both of you. :)

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#include <iostream>
using namespace std;

int main() {
	int t,i,temp=0;
	cin>>t;
	while(t--)
	{
	    int n;
	    cin>>n;
	    int A[n];
	    for(i=0;i<n;i++)
	   {
		cin>>A[I];
	   }
	    for(i=0;i<n/2;i++)
	    {
		    temp = A[i];
		    A[i] = A[n-1-i];
		    A[n-1-i]=temp;
	    }
	    for(i=0;i<n;i++)
		    cout<<A[I]<<" ";
        }
	return 0;
}


Last edited on
seeplus (6591)
For info, C++ has an in-built swap function to replace L18-20. std::swap()

https://cplusplus.com/reference/utility/swap/

Also, C++ has an inbuilt sort function std::sort()

https://cplusplus.com/reference/algorithm/sort/
seeplus (6591)
To reverse order:

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#include <iostream>
#include <utility>

int main() {
	int A[1000] {};
	int t {};

	std::cin >> t;

	while (t--) {
		int n {};

		std::cin >> n;

		//int A[n];

		for (int i = 0; i < n; i++)
			std::cin >> A[i];

		for (int i = 0; i < n / 2; i++)
			std::swap(A[i], A[n - 1 - i]);

		for (int i = 0; i < n; i++)
			std::cout << A[i] << " ";

		std::cout << '\n';
	}
}


To sort by reverse order:

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#include <iostream>
#include <algorithm>
#include <functional>

int main() {
	int A[1000] {};

	int t {};

	std::cin >> t;

	while (t--) {
		int n {};

		std::cin >> n;

		//int A[n];

		for (int i = 0; i < n; i++)
			std::cin >> A[i];

		std::sort(A, A + n, std::greater<int>());

		for (int i = 0; i < n; i++)
			std::cout << A[i] << " ";

		std::cout << '\n';
	}
}

Last edited on
thmm (703)
Being a newbie is totally fine, but I would recommend to learn the basics of C++ first before trying to solve this challenge on codechef(?).

A good way to start is: https://www.learncpp.com
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