Counting elements in an array greater than next element C-CodePudding

Why is it giving wrong count In this question we have to find out how many elements are greater than the next element of an array
#include using namespace std;

    int main() {
    //number of testcases
        int t;
        cin>>t;
        while(t--){
//taking number of elements in an array
            int n,count;
            cin>>n;
//taking array
            int a[n];
            count=0;
            for(int i=1;i<=n;i  ){
                cin>>a[n];
            }
//checking array 
        for(int j=1;j<=n-1;j  ){
     
            if(a[j]>a[j 1])
            {
                count  ;
            }
               
        }
    cout<<count<<endl;
    }
        return 0;
    }

Input 5 1 2 3 4 1 Expected Output 1 Output through above code 2

CodePudding user response：

There are several problems with your code:

Problem 1

You're going out of bounds of the array a which leads to undefined behavior, when you wrote

cin >> a[n];  //undefined behavior

Note that indexing of arrays starts from 0 instead of 1 in C .

Problem 2

In standard C , the size of an array must be a compile-time constant. This mean the following is incorrect in your code:

int n;
cin>>n;

int a[n];   //NOT STANDARD C   BECAUSE n IS NOT A CONSTANT EXPRESSION

Better would be to use std::vector instead of built in array.

int n = 0;
std::cin >> n;
std::vector<int> a(n);   //this creates a vector of size n

//take input from user
for(int& element: a)
{
    std::cin >> element;
} 
int count = 0;
for(int i = 0; i< n - 1;   i)
{
    if(a[i] > a[i 1])
    {
          count;
    }
}