C++ Vector Array / WhileLoop - c++

Question

I am reading C++ Programming In Easy Steps 4th and have read in to something I am having trouble understanding. I've only just started reading it the other day.
I had a particularly hard time getting my head around vector arrays and elements when I done that section of the book, I still don't feel confident about it despite taking a bunch of time to try and understand them, so I plan to re-visit that again. I'm now on a later part of the book however which is demonstrating loops and here is the code that I am having trouble with (Just one line):
#include <iostream>
#include <vector> // Include vector support (Vector function library)
using namespace std;
int main()
{
//Declaring an integer vector and an integer variable for loop counter
vector <int> vec ( 10 );
int i = 0;
//Now inserting a while loop to assign a counter value to an element of the vector on each iteration
while ( i < vec.size() )
{
i++; //Increment the counter
vec[ i -1 ] = i; // Assign count to element
cout << " | " << vec.at ( i -1 );
}
return 0;
}
I understand everything, but this line:
vec[ i -1 ] = i; // Assign count to element
I'm not sure what this is doing exactly particularly the i -1 part? Can someone break this down for me in an easy to understand way? I'm going to quickly re-visit the section on vector arrays and see if I can get an understanding.

Answer 1

Vector indicies start at 0. A vector of 10 elements is indexed from 0 to 9. Assuming your goal is to load the array consecutively with the numbers 1 to 10, and since you start i at 0 and increment it before using it in the index expression, you need to subtract 1 from it to get the proper index.
This piece of code does the same thing.
vector<int> vec(10);
int i = 0;
while (i < vec.size())
{
vec[i] = i + 1;
i++;
}
i = 0;
while (i < vec.size())
{
cout << i << ": " << vec.at(i) << endl;
i++;
}

Answer 2

It may help you if you break the code down to what the while loop is actually accomplishing by individual statements (you would see similar if you were running through the while loop with a debugger and also adding watch statements such as i-1):
#include <iostream>
#include <vector> // Include vector support (Vector function library)
using namespace std;
int main()
{
//Declaring an integer vector and an integer variable for loop counter
vector <int> vec ( 10 ); // vector of 10 elements: 0...9
int i = 0; // i starts with the value of 0
// replace while loop with the individual statements the loop accomplishes
// note also that vec.size() = 10
// while (i < vec.size())
// increment i to 1 (started as 0, from above)
i++; //Increment the counter
vec[ 0 ] = i; // i - 1 = 0 here - Assign count to element
cout << " | " << vec.at ( 0 );
// increment i to 2
i++; //Increment the counter
vec[ 1 ] = i; // i - 1 = 1 here - Assign count to element
cout << " | " << vec.at ( 1 );
// continue to i = 9 ...
// increment i to 10
i++; //Increment the counter
vec[ 9 ] = i; // i - 1 = 9 here - Assign count to element
cout << " | " << vec.at ( 9 );
return 0;
}

Answer 3

you're assigning the numbers 1 to 1o to your vector.
vec[i-1] = i
follows directly after i++ which means,for your first index you're assigning a value a count/step more than index.
Put simply,let's follow the first 4 values of i and the elements inserted into the vector:
i = 0;
i = 1;here vec[ 1 - 1] = vec[0] = 1
i = 2;here vec [2 - 1] = vec[1] = 2
i = 3;and finally vec[ 2 ] = 3

Answer 4

Vectors are wrapper classes for arrays (Source:Link to C++ reference ) that also do some really cool things like use templates for allocators, provide dynamic space, and use stl iterators.
So, vectors can be accessed like arrays using the [ ] operator. The two code snippets are identical:
First, the array form:
//Allocate and assign array
int* arr[5]={1,2,3,4,5};
//Print the 3rd element, note that the first index corresponds to 0.
std::cout<< arr[2] <<std::endl;
//Reassign the 2nd element:
arr[1]=54;
Now the vector version:
//Allocate and assign vector
std::vector<int> arr( {1,2,3,4,5} );
//Print the 3rd element, note that the first index corresponds to 0.
std::cout<< arr[2] <<std::endl;
//Reassign the 2nd element:
arr[1]=54;
A tip for the wise: Accessing vectors like this can be dangerous since the size of an array can be changed elsewhere in code. Instead consider the following for assignment:
std::vector<int> arr( {1,2,3,4,5} );
int index=0;
for(std::vector<int>::iterator it = arr.begin(); it!= arr.end(); ++it){
if(index++ == 3){
(*it) = 54;
}
}