C++ - my loop keeps on adding up to 0 - c++

Question

so far here's my code
#include <iostream>
using namespace std;
int main ()
{
int num1 = 0;
int num2 = 0;
int sum = 0;
for(num2 = num1; num1 <= num2; num1 +=2) sum += num1;
num1 = num1 / 2 == 0? num1 : num1 + 1;
num2 = num2 / 2 == 0? num2 : num2 - 1;
cout << "Enter the First Number:" << endl;
cin >> num1;
cout << "Enter the Second Number:" << endl;
cin >> num2;
cout << "Total Sum: " << sum << endl;
} //end for
but the sum keeps on adding up to 0 :/
here's the problem.
Create a program that displays the sum of the even numbers between and including two numbers entered by the user. In other words, if the user enters an even number, that number should be included in the sum. For example, if the user enters the integers 2 and 7, the sum is 12 (2 + 4 + 6). If the user enters the integers 2 and 8, the sum is 20 (2 + 4 + 6 + 8 ). Display an error message if the first integer entered by the user is greater than the second integer.

Answer 1

The code is executed sequentially and the for loop initialization will make you lose the boundaries of the loop consider this code instead.
#include <iostream>
using namespace std;
int main ()
{
int num1 = 0;
int num2 = 0;
int sum = 0;
cout << "Enter the First Number:" << endl;
cin >> num1;
cout << "Enter the Second Number:" << endl;
cin >> num2;
if (num1 > num2) // swap the numbers and do not print error message
{
int temp = num1;
num1 = num2;
num2 = temp;
}
//make sure to start from even number
num1 = num1 % 2 ? num1+1 : num1;
for(; num1 <= num2; num1 +=2)
sum += num1;
cout << "Total Sum: " << sum << endl;
} //en

Answer 2

You need to calculate the sum after getting the input!
But your whole calculation and loop usage is wrong. Here it's fixed:
#include <iostream>
using namespace std;
int main ()
{
int num1 = 0;
int num2 = 0;
int sum = 0;
cout << "Enter the First Number:" << endl;
cin >> num1;
cout << "Enter the Second Number:" << endl;
cin >> num2;
if (num1 % 2 == 1) num1 += 1;
if (num2 % 2 == 1) num2 -= 1;
while (num1 <= num2) {
sum += num1;
num1 += 2;
}
cout << "Total Sum: " << sum << endl;
}
Note the following:
% returns modulus - num1 % 2 ==1 implies that num1 is odd. I took out your ternary ?: operators not because they're bad, but just because if is easier to read and in this case you're not doing anything if num1 is even.
You were setting num2 at the start of your for loop. A while loop makes more sense in this situation, or a for loop without initialization for (;num1<=num2; num1+=2) {.

Answer 3

While the homework (I assume) should be solved by you, here are a few hints to help you out:
1) Your for loop needs curly brackets around the code that is supposed to loop:
for(num2 = num1; num1 <= num2; num1 +=2)
{
sum += num1;
num1 = num1 / 2 == 0? num1 : num1 + 1;
num2 = num2 / 2 == 0? num2 : num2 - 1;
}
2) Your loop is above your cout and cin statements, so it runs before the user ever enters any numbers. You need to move the loop to after (below) the numbers have been given to the program by the user.
3)
The logic of the loop is probably not what you want. Once curly brackets are added, this is what it is doing (in "pseudo-code"):
Let num2 equal num1 // Both are set to zero so this doesn't do anything
While num1 is less than or equal to num2:
{
Add the current value of num1 to sum.
if num1 /2 (ignoring remainder) is 0, then set num1 equal to itself. Otherwise, add 1 to it.
// num1 already equals itself, so this doesn't do anything when num1 / 2 is zero.
//
if num2 /2 (ignoring remainder) is 0, then set num1 equal to itself. Otherwise, subtract 1 from it.
Add 2 to num1.
}
Unless the assignment says otherwise, it's probably best not to use the ternary (? and :) syntax because it's pretty confusing when you're just starting to program (at least, I thought so).
C++ is a challenging language to learn, but hang in there!

Answer 4

1.)Get numbers
2.)Determine max and min
3.)Sum evens between
#include <iostream>
using namespace std;
void main()
{
int num1 = 0;
int num2 = 0;
int sum = 0;
int temp = 0;
int i;
//Get your input values
cout << "Enter the First Number:" << endl;
cin >> num1;
cout << "Enter the Second Number:" << endl;
cin >> num2;
cout << endl;
//just to reorganize and make num1 the smallest of the two
if ( num2 << num1 )
{
temp = num1;
num1 = num2;
num2 = temp;
}
//loop through and add even values
for(i = num1; i < num2; i++)
{
if(i%2 == 0)
{
sum = sum + i;
}
}
cout << "Sum: " << sum << endl;
}