Debugging a histogram in C++ - c++

Question

I want to build a histogram with ell intervals. The size of each interval is computed as k = ceil(m/ell) where m is the maximum number in the data set. That is, the interval i should be [(i - 1) * k, i * k). If a data set is given by the numbers 16 33 55 57 8 47 1 21 14 73 6 59 29 57 20 95 77 5 62 48 and the number of intervals is ell = 10, the histogram has to be textually represented as
0: 4
10: 2
20: 3
30: 1
40: 2
50: 4
60: 1
70: 2
80: 0
90: 1
I need to write a program, that reads in the following values (from the standard input cin) :
the number ell of intervals
the size n of the data set
n non-negative integers
Here is my code so far:
#include<iostream>
#include<math.h>
#include<algorithm>
using namespace std;
int main() {
double ell;
int n; // size of the data set
double m = 0;
int * a;
int * x;
int * y;
cin >> ell;
cin >> n;
a = new int[n];
// finding max element in array a
for (int i = 0; i < n; i++) {
cin >> a[i];
if (a[i] > m) {
m = a[i];
}
}
int k = ceil(m/ell);
x = new int[(int)(ell)];
y = new int[(int)(ell)];
for (int i = 1; i <= ell; i++) {
x[i] = (i - 1) * k;
for (int j = 0; j < n; j++) {
if (a[j] >= (i - 1) * k && a[j] < i * k && x[i] != (ell - 1) * k) {
y[i] += 1;
} else if (a[j] >= (ell - 1) * k && x[i] == (ell - 1) * k) {
y[i] += 1;
} else {
y[i] += 0;
}
}
cout << x[i] << ": " << y[i] << endl;
}
delete [] a;
delete [] x;
delete [] y;
return 0;
}
If I input this:
10 20 16 33 55 57 8 47 1 21 14 73 6 59 29 57 20 95 77 5 62 48
I get
0: 4
10: 2
20: 3
30: 1
40: 2
50: 4
60: 1
70: 2
80: 0
90: 1
But if I input the same many times, I will get a weird output like
0: 4
10: 268501820
20: 1073741827
30: 112
40: 2
50: 4
60: 1
70: 2
80: 0
90: 6
Why is this happening and how can I fix my problem?

Answer 1

x = new int[(int)(ell)];
y = new int[(int)(ell)];
Creates objects on the heap, but unless the objects (i.e. int) have a constructor, then the memory is left uninitialized.
The array x is correctly initialized, but y is left (expected to be 0).
So the fix is ...
x = new int[(int)(ell)];
y = new int[(int)(ell)];
for( int i = 0; i < ell ; i++ ){
x[i] = 0; // tidier to do it obviously.
y[i] = 0; // essential to get y[i] += 1; to be meaningful.
}

Answer 2

You never initialise your memory. You create dynamic arrays with both x and y but you are not guaranteed that the memory is zeroed. You get lucky for the most part that but when there are values left in the memory your program uses you get the outliers that you see. Put something like this in and it should fix your issues.
for(int m = 0;m < (int)ell; m++) {
x[m] = 0;
y[m] = 0;
}