Correct Method

[christianwos]

Hi all,

New to C++. I am learning on my own using C++ Primer and I am doing one of the problems about vectors. My code runs and does what I want. All I am looking for here is some tips on whether I am doing this in an efficient way or not. Thanks in advance.

/*Read a set of integers into a vector. Print the sum of each
pair of adjacent elements. Change your program so that it prints the sum of
the first and last elements, followed by the sum of the second and second-tolast,
and so on.*/
#include <iostream>
#include <vector>

int main()
{
    std::cout << "Enter a set of integers: " << std::endl;
    int number;
    std::vector<int> vec;
    while(std::cin >> number)
        vec.push_back(number);
    // Perform the sum of adjacent elements
    std::cout << '\n';
    std::cout << "The sume of adjacent terms is: " << std::endl;
    int sum_adj = 0;
    for(decltype(vec.size()) index = 0; index < vec.size()-1; ++index){// the range has to be up to size -1 to avoid out-of-range sums
        sum_adj = vec[index]+vec[index+1];
        std::cout << sum_adj << std::endl;
    }
    std::cout << '\n';
    std::cout << "The sume of opposite-end terms is: " << std::endl;
    // Perform the sum of opposite  elements
    int sum_opp = 0;
    if(vec.size()%2==0){
        for(decltype(vec.size()) index = 0; index < vec.size()/2; ++index){
            sum_opp = vec[index]+vec[vec.size()-1-index];
            std::cout << sum_opp << std::endl;
        }
    }
    else{
        for(decltype(vec.size()) index = 0; index < vec.size()/2; ++index){
            sum_opp = vec[index]+vec[vec.size()-1-index];
            std::cout << sum_opp << std::endl;
        }
        std::cout << vec[vec.size()/2] << " central value" << std::endl;
    }
    return 0;
}

[AbstractionAnon]

A couple of comments:

Line 14: Keep in mind that <vector> will periodically reallocate the underlying memory as you add elements. With a simple program such as this, that really is not an issue, but it is something to keep in mind as your programs get larger and more complex.

Lines 21,30,36: Are you sure you want to output the sum each time through the loop?
Generally you would output the sum after you have iterated through the array.

[keskiverto]

The std::endl does both print a newline ('\n') and flush the stream's buffer. Mere newline is usually sufficient.


Lines 28-31 is identical to lines 34-37. Consider an alternative:

int sum_opp = 0;
for (decltype(vec.size()) index = 0; index < vec.size()/2; ++index) {
  sum_opp = vec[index]+vec[vec.size()-1-index];
  std::cout << sum_opp << std::endl;
}
if ( vec.size()%2 ) {
  std::cout << vec[vec.size()/2] << " central value" << std::endl;
}

Line 19. I gave no values. vec.size() == 0.
https://stackoverflow.com/questions/47964986/returns-true-for-unsigned-int0-10?rq=1
The vec.size()-1 is thus very large positive value.

On the other hand: index + 1 < vec.size() is safe.

[christianwos]

Thank you to both for the useful comments. I will implement them in my code.