How do I find highest value using pointer? - c++

Question

I want to find the highest value from an array using two given pointer int *p,*max;, but the code doesn't work.
#include <iostream>
#include <string>
using namespace std;
int main() {
int a[10], i, index;
int *p, *max;
for (i = 0; i < 10; i++) cin >> a[i];
max = 0;
p = &a[10];
for (index = 0; index < 10; index++) {
if ((p[index]) > *max) {
*max = (p[index]);
}
}
cout << "Highest value=" << *max << endl << "is at index=" << index << endl;
return 0;
}

Answer 1

The code is buggy. First of all, you assign
p=&a[10];
This assigns p to a memory address past a. Furthermore, you then index as p[index], which essentially is the same as a[10 + index].
Also, max is a wild pointer. It does not point to anything. You are assigning values to an undefined memory location.
I would strongly suggest to read up on pointers and to properly understand them before using them. Also, in modern C++, it is not very often than you need pointers.
Also, in idiomatic C++, we would probably write
auto p = std::max_element(a, a + 10);

Answer 2

There are several problems.
First, p should point to the array's first element, so you should have p = &a[0].
You can also rely on implicit conversion and just write p = a;, which is exactly the same.
&a[10] is the pointer "one-past-the-end" of the array, and dereferencing it is undefined.
Next, you want max to point to the maximum element.
It should also start at the beginning of the array, like p.
Then, when you find a new maximum, you should make max point to that element, not change the value max points to.
Lastly, index will always be 10 after the search loop.
(Take a few moments to think about why.)
You don't need it – the index is the difference between the location of the maximum element and the beginning of the array.
int main()
{
int a[10];
for(int i = 0; i < 10; i++)
cin >> a[i];
int* max = &a[0];
int* p = &a[0];
for (int index = 0; index < 10; index++){
if (p[index] > *max){
max = &p[index];
}
}
cout << "Highest value= " << *max << endl << "is at index= "<< max - a << endl;
}

Answer 3

I'd remove p and use a range-based for-loop where possible and iterators when it'll improve performance.
Comments in the code:
#include <iostream>
#include <iterator>
// using namespace std; // don't do this
int main() {
using std::cin, std::cout;
int a[10];
// use a range-based for-loop:
for(int& aref : a) { // aref is a reference to the current element in a
// check that extraction from std::cin actually works
if(!(cin >> aref)) {
std::cerr << "error reading int\n";
return 1;
}
}
// initialize max to point at the first element
auto max = std::begin(a);
// Start at the second element since max is already set to point at the first element.
// Don't use magic numbers. Define a constant or use std::size(<array>)
// ...or use iterators like in this example:
for(auto curr = std::next(std::begin(a)); curr != std::end(a); ++curr) {
if(*curr > *max) {
max = curr;
}
}
// you can use std::distance ot calculate the index for max:
cout << "Highest value=" << *max << '\n'
<< "is at index=" << std::distance(std::begin(a), max) << '\n';
}

Answer 4

The solution to this problem is recognizing that max should always point to the maximum item seen in the array a so far so instead of initializing max to 0 you start by initializing it to point to the first item in a which is &a[0] or just a.
I tried to make the least amount of changes to the original code:
#include <iostream>
#include<string>
using namespace std;
int main()
{
int a[10],i,index;
int *p,*max;
for(i=0;i<10;i++)
cin>>a[i];
max=a; // Initialize max to point to the first item in a
p=a;
for(index=0;index<10;index++){
if((p[index])>*max){
max=(&p[index]); // Now make max point to the new maximum item
}
}
cout<<"Highest value="<<*max<<endl<<"is at index="<<max - p<<endl;
return 0;
}
Here is the code in ideone:
https://ideone.com/BwE45C
As mentioned in the comments below p probably is not being used as the question expects so I have rewritten the code to iterate using p
#include <iostream>
#include<string>
using namespace std;
int main()
{
int a[10],i,index;
int *max;
for(i=0;i<10;i++)
cin>>a[i];
max=a;
for(int* p=a;p<a+10;p++){ // p is now a pointer that is used to iterate through the array
if(*p>*max){
max=p; // max points to the new maximum
}
}
cout<<"Highest value="<<*max<<endl<<"is at index="<<max - a<<endl;
return 0;
}
The new ideone link for this is here: https://ideone.com/bk3zoS