Given an input array, the output must be the length of the longest arithmetic subarray of the given array.

I am getting a different output other than the desired one. I don't understand where I went wrong, I'm still a beginner so please ignore the rookie mistakes and kindly help me out wherever I'm wrong. Thanks in advance.

Here's the code:

#include <iostream>

using namespace std;

int main () {
    int n;
    cin>>n;
    int array[n];
    for (int i=0;i<n;i  )
  {
        cin>>array[i];
    }
    
    int length = 2;
    int cd = array[1] - array[0];
    for(int i=2; i<n; i  ){
        if(array[i] - array[i-1] == cd){
            length  ;
        }
        else {
            cd = array[i] - array[i-1];
            length=2;
        }
        cout<<length<<" ";
    }
    return 0;
}

CodePudding user response：

If you are looking for a subsequence then what you did would not accomplish that.

For example:

Input: nums = [9,4,7,2,10]
Output: 3
Explanation: 
The longest arithmetic subsequence is [4,7,10].

You would require a nested loop structure (a for loop within the for loop you currently have) to accomplish that as you want to check a certain cd with the entire array and not just the next element.

If you require to find a subsequence/subarray given that the elements must be adjacent to one another then your program would work correctly.

Also a big error in your code is that you are printing the length inside the for loop. Unsure of whether that was for debugging purposes.

CodePudding user response：

The problem here is you're resetting length after every update. You need a variable to store the maximum of every length.

#include <iostream>
using namespace std;

const int maxn = 1e6;
int arr[maxn];

int main ()
{
    int n; cin>>n;
    for (int i=0;i<n;i  ) { cin >> arr[i]; }

    int length = 2;
    int maxLength = 2; //max variable
    int cd = arr[1] - arr[0];
    for(int i=2; i<n; i  ){
        if(arr[i] - arr[i-1] == cd) {length  ;}
        else {
            cd = arr[i] - arr[i-1];
            length=2;
        }
        //cout<<length<<" "; //remove this
        maxLength = max(maxLength, length); //update maxLength
    }
    cout << maxLength;
}

A few more aesthetic notes:

• array is a keyword in C used to declare std::array. Although the program may still run, it could create unnecessary confusion.

• int array[n] is a VLAs (variable length array). It's not a C standard. It may or may not work depends on the compiler.

• Why is "using namespace std;" considered bad practice?