Output of C++ code

Question

Hello again,
Can someone explain this code. I understand n1, but not n2. How can sizeof(p) be 8 bytes, when it is an array of 9 ints?

#include <iostream>
using namespace std;

int main() {
int mas[9]={2,6,3,9,1,0,5,8,4};
int *p = new int[9];
for (int i=0; i<9; ++i)
{
p[i] = mas[i];
}
int n1 = sizeof(mas)/sizeof(*mas);
int n2 = sizeof(p)/sizeof(*p);
cout << n1 << n2;
return 0;
}

Answer 1

It is not an array!
It's a pointer, and pointers store memory addresses, they have size 32 or 64 bits (8bytes)

And please delete the memory you borrow when you don't need it anymore

Answer 2

Martin Taylor, Thanks, learned something new today

Answer 3

Martin Taylor It appears you are right:

https://code.sololearn.com/crxx92841Pm3/#

How did you know it? Why is memcpy more efficient?

Answer 4

Martin Taylor, Then it makes sense why memcpy takes a shorter time since there is only one transfer as opposed to 9, in this case? But I don't know what you mean by optimized, since this appears to be a standard function from string.h, I would assume it is optimized, or else no one would use it?

Yes, the clock_t type and clock() function are pretty nifty, I came across them when trying to compare recursive vs iterative solution for Fibonacci, if you input 30 below, you see there is a major difference.

https://code.sololearn.com/cr0bbz1x5t65/#
https://code.sololearn.com/cr0bbz1x5t65/#

Btw, you seem very knowledgeable, what is your background? I am an undergrad physics student, graduating this May.

update: it seems I flopped the cout statements, now it seems for-loop is faster?

Answer 5

Martin Taylor "When I ran your code on my local machine I couldn't get a reading higher than 0.00005 msecs for anything! The SoloLearn server is multitasking multiple users at the same time so will process the code slower than a local machine."

So I increased the array size to 2000 (https://code.sololearn.com/cRi071czkxxM/#) and now the difference is more apparent:

https://code.sololearn.com/cCQdSZCyH562/#

Answer 6

Nice very nice discussed and agreed that we can all learn from each other. This is very educational for a lot of SoloLearn enthousiasts. Thanks for your efforts to teach 👍🤗

Answer 7

sizeof(p) will give you size of that pointer only not for whole integers stored inside.

Answer 8

Angelo, thanks. This is from a code challenge, but I agree there needs to be a
delete [] p;
at the end.