Having trouble with histogram c function It is passed an arr

Having trouble with histogram c++ function. It is passed an array of values then I run it through my algorithm and then the function is supposed to return an array. I believe the program is not liking my algorithm but I do not know why. This is what I have so far

int* histogram(int* a)

{

static int histArray[100] = { 0 };

for (int i = 0; i < 100; i++)

histArray[a[i]]++;

return histArray;

}

Example of passed array: {1,2,2,1,3,4,5,6,7}

should return an array such: {0,2,2,1,1,1,1,1,0,0,} for a histogram with 10 slots.

Solution

#include <iostream>
#include <cstdlib>
using namespace std;

int* histogram(int *a,int len)
{
   int *histArray = new int[10];
   for (int i = 0; i < 10; i++)
   {
       histArray[i]=0;
   }
   //int len = sizeof(a)/sizeof(int);
   //cout<<len<<\"**\"<<endl;
   for (int i = 0; i < len; i++)
   {
       histArray[a[i]]++;
   }
   /*for (int i = 0; i < 10; i++)
   {
       cout<<histArray[i]<<\' \';
   }*/
   return histArray;
}
int main()
{
   int b[] = {1,2,2,1,3,4,5,6,7};
   int *a= new int[9];
   for(int i=0;i<9;i++)
   {
       a[i]=b[i];
   }
   //int len1 = sizeof(a)/sizeof(int);
   //cout<<len1<<endl;
   int *hist = histogram(a,9);
   //int len = sizeof(hist)/sizeof(int);
   //cout<<len<<\'(\'<<endl;
   for (int i = 0; i < 10; i++)
   {
       cout<<hist[i]<<\' \';
   }
   cout<<endl;
   return 0;
}