3 C++ For Loop Implementations

Using an index, pointer, and iterator to cycle through a vector.

TL;DR —The goal is to quickly review C++ syntax needed to iterate through a vector using a for loop. We will use an index, pointer, and iterator. You will find the full code listing at the bottom of this post.

Let’s begin by instantiating an integer vector named ageVector.

vector<int> ageVector = { 10,20,30,40,50 };

1. C++ for loop INDEX implementation.

for (int i = 0; i < ageVector.size(); i++) {
        cout << ageVector[i] << " ";
}

Initialize an integer i = 0, this will be used as an index. Then set the loop condition i < ageVector.size() where the index is incremented by one, i++. In the code block, access the element contents using an access operator, here we use ageVector[i].

2. C++ for loop POINTER implementation.

for (int* ptr = &ageVector[0]; 
     ptr <= &ageVector[ageVector.size()-1]; 
     ptr++) {
        cout << *ptr << " ";
    }

First we declare an integer pointer and set it to the address of the first element in the vector, ptr = &ageVector[0]. Then set the loop condition to compare the pointer against the address of the last element, ptr <= &ageVector[ageVector.size()-1]. Finally, use pointer arithmetic to increment to the next address, ptr++.

To access the contents, remember to dereference the pointer. In the code block, we do this with *ptr.

3. C++ for loop ITERATOR implementation.

for (vector<int>::iterator itr = ageVector.begin(); 
     itr < ageVector.end(); 
     itr++) {
        cout << *itr << " ";
    }

Declare an integer vector iterator and set it to the beginning of the vector, vector<int>::iterator itr = ageVector.begin(). Then set the loop condition to compare the iterator against the past-the-end element, itr < ageVector.end(). Finally, use itr++ to increment to the next vector element.

To access the contents, remember to dereference the iterator. In the code block, we do this with *itr.

That’s our quick review. Here is the full code listing.

#include <iostream>
#include <vector>
using namespace std;
int main(void){
vector<int> ageVector = { 10,20,30,40,50 };
cout << "\n ***for loop with index" << endl;
for (int i = 0; i < ageVector.size(); i++) {
        cout << ageVector[i] << " ";
}
cout << "\n\n ***for loop with pointer" << endl;
for (int* ptr = &ageVector[0]; ptr <= &ageVector[ageVector.size()-1]; ptr++) {
        cout << *ptr << " ";
}
    
cout << "\n\n ***for loop with iterator" << endl;
for (vector<int>::iterator itr = ageVector.begin(); itr < ageVector.end(); itr++) {
        cout << *itr << " ";
}
cout << "\n\n ***Enjoy" << endl;
return 0;
}