implementation counting sort - c++

Question

here is code for counting sorting
#include <iostream>
using namespace std;
int main(){
int a[]={2,3,1,2,3};
int n=sizeof(int)/sizeof(int);
int max=a[0];
for (int i=1;i<n;i++) {
if (a[i]>max) {
max=a[i];
}
}
int *output=new int[n];
for (int i=0;i<n;i++) {
output[i]=0;
}
int *temp=new int[max+1];
for (int i=0;i<max+1;i++) {
temp[i]=0;
}
for (int i=0;i<n;i++){
temp[a[i]]=temp[a[i]]+1;
}
for (int i=1;i<max+1;i++) {
temp[i]=temp[i]+temp[i-1];
}
for (int i=n-1;i>=0;i--) {
output[temp[a[i]]-1]=a[i];
temp[a[i]]=temp[a[i]]-1;
}
for (int i=0;i<n;i++) {
cout<<output[i]<<" ";
}
return 0;
}
but output is just 2,only one number. what is wrong i can't understand please guys help me

Answer 1

int n=sizeof(int)/sizeof(int);
is wrong. That just assigns 1 to n.
You mean
int n=sizeof(a)/sizeof(int);
I've not looked beyond this. No doubt there are more problems, but this is the most significant.
This is the kind of thing you can work out very easily with a debugger.

Answer 2

Look at this expression:
int n=sizeof(int)/sizeof(int);
What do you think the value of n is after this? (1)
Is that the appropriate value? (no, the value should be 5)
Does that explain the output you are seeing? (yes, that explains why only one number is shown)

Answer 3

My advice would be that if you're going to do this in C++, you actually try to use what's available in C++ to do it. I'd look up std::max_element to find the largest element in the input, and use an std::vector instead of messing with dynamic allocation directly.
When you want the number of elements in an array in C++, you might consider a function template something like this:
template <class T, size_t N>
size_t num_elements(T const (&x)[N]) {
return N;
}
Instead of dumping everything into main, I'd also write the counting sort as a separate function (or, better, a function template, but I'll leave that alone for now).
// warning: Untested code:
void counting_sort(int *input, size_t num_elements) {
size_t max = *std::max_element(input, input+num_elements);
// allocate space for the counts.
// automatically initializes contents to 0
std::vector<size_t> counts(max+1);
// do the counting sort itself.
for (int i=0; i<num_elements; i++)
++counts[input[i]];
// write out the result.
for (int i=0; i<counts.size(); i++)
// also consider `std::fill_n` here.
for (int j=0; j<counts[i]; j++)
std::cout << i << "\t";
}
int main() {
int a[]={2,3,1,2,3};
counting_sort(a, num_elements(a));
return 0;
}