FAQ: Vectors - Review

This community-built FAQ covers the “Review” exercise from the lesson “Vectors”.

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Why do you declare the int product_odd as being 1 at the beginning of the code? I thought generally you set it to 0…?? Is it something to do with the fact you will be multiplying it??


multiplication can’t be done by 0.
03 would be 0. Then again 01 would be 0 and then 0*9 =0.

If it was set to 2, the total would be 54


Is anyone having a problem with this exercise in which, you complete the task but it is forever attempting to verify if I did the problem correctly? I’ve been waiting about 5 minutes now and normally it only takes a matter of seconds

jokes i refreshed the page and now it works

Why do you declare the int total_even = 0 ?

1 Like
#include <iostream>

int main() {
	std::vector<int>numbers = {2, 4, 3, 6, 1, 9};
	int even_total = 0;
	int odd_total = 0;
  for (int i = 0; i < numbers.size(); i++)
    if (even_total % 2 == 0)
     even_total = even_total + numbers[i]; 
    if (odd_total % 2 == 1)
      odd_total = odd_total + numbers[i];
 std::cout << "Sum of even numbers is " << even_total;
 std::cout << "Sum of odd numbers is " << odd_total;

Is my code but it errors

review.cpp: In function 'int main()':
review.cpp:4:7: error: 'vector' is not a member of 'std'
  std::vector<int>numbers = {2, 4, 3, 6, 1, 9};
review.cpp:4:14: error: expected primary-expression before 'int'
  std::vector<int>numbers = {2, 4, 3, 6, 1, 9};
review.cpp:7:23: error: 'numbers' was not declared in this scope
   for (int i = 0; i < numbers.size(); i++)

It seems to say that std::vector isn’t an expression, even though I was taught that it is.

Hey Speed_reader,

You just forgot to add the library for vector in your code. Below the #include iostream you need #include vector for your program to recognize std::vector. Hopefully you figured this out.


I don’t understand how “if (vector[i] % 2 == 0)” is able to find the even numbers in the vector.

To my understanding, this line is saying “okay, use that value you just pulled from the vector and divide it by two. If you get zero, then yep, that’s an even number, now do the following.” But none of the values in the vector would yield zero if you cut them in half.

Does that make sense? What am I missing here?

1 Like

% doesn’t mean divide. It’s the modulo operator. It returns the remainder of dividing the dividend by the divisor. If any element in the vector divided by 2 has a remainder of 0 then it’s an even number. Hope this helps!


Ahhhh! Yes, that makes perfect sense now, thank you so, so much. That’s been bugging me all day.

1 Like

For some reason if I #include it just loads forever when I press run. If i leave that part out then it errors out which makes sense but then I can’t check if my program actually works.

Hello, @microblaster90973.

Welcome to the forum.

Sounds like you may have an infinite loop in your code. If you’ll include your code in a post using these guidelines, we can take a look.

Why do you declare the int total_even = 0 ?

If I wanted to print out under each for and else loop the value on the vector that was chosen, how would I do that so I can see what is happening behind the scenes. Thank you.


int main() {

int total_even = 0;
int product_odd = 1;

std::vector vector = {2, 4, 3, 6, 1, 9};

for (int i = 0; i < vector.size(); i++) {

if (vector[i] % 2 == 0) {

  total_even = total_even + vector[i];

} else {

  product_odd = product_odd * vector[i];



std::cout << "Sum of even: " << total_even << “\n”;
std::cout << "Product of odd: " << product_odd;


I don’t understand the whole problem can someone please help me