How to figure out a loop problem in C++ ?

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I can't figure out why the code while(i<y) outputs an included y.

Here is the code:

C++

#include <iostream>
using namespace std;
                                      
int main() {
	int x, y, i = 0, s = 0, max;
	cout << "enter x y " << endl;
	cin >> x >> y;
	if (x < y) 
		while (i < y) {
			i = x;
			s += x;
			x += 1;

			cout << i << endl;
		}
	else 
		while (i < x) {
			i = y;
			s += y;
			y += 1;

			cout << i << endl;
		}
cout << "sum = " << s << endl;

}

What I have tried:

When I input x=3 and y=6, the output I get is 3 4 5 6.
Why is 6 included while i.

Posted 22-Oct-23 3:39am

kozlok

Updated 24-Oct-23 1:25am

Deeksha Shenoy

v5

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2 solutions

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Mike Hankey · Accepted Answer · 2023-10-22T04:06:00

Solution 1

Think about this, you are testing for i < y, then in the while loop you set i = x and then later you increment x.

So when i = 5 you increment x then you check for i so x is set to 6 but i is still 5.

C++

while (i < y) {
i = x;
s += x;
x += 1;

cout << i << endl;
}

Posted 22-Oct-23 4:06am

Mike Hankey

Comments

Mike Hankey 22-Oct-23 11:07am

Because the last time through i which is 5 is less than 6 and then you set i to 6.

kozlok 22-Oct-23 11:21am

after setting i to 6 , i wouldn't then check the while condition wich is less then 6 so i=6 wouldn't be executed ?

Mike Hankey 22-Oct-23 12:09pm

For x=3 and y=6

Loop: i x x+1 prints
-----------------------------
3 3 4 3
4 4 5 4
5 5 6 5
passes i
6 6 7 6
i=6 is not less than 6 so it fails

CPallini · Accepted Answer · 2023-10-22T20:24:00

If you just need to sum number from x to (y-1), then why don't you simplify the approach?
The following code would do the trick:

C++

#include <iostream>
using namespace std;

int main()
{
  int x, y, sum = 0;
  cout << "enter x y " << endl;
  cin >> x >> y;

  if (y < x)
    swap(x,y);

  for (int i=x; i<y; ++i)
    sum += i;

  cout << "sum = " << sum << endl;
}

Also, knowing that

1 + 2 + 3 + ... + n = n * (n + 1) / 2

you could write

C++

#include <iostream>
using namespace std;

int main()
{
  int x, y;
  cout << "enter x y " << endl;

  if ( x > y ) swap(x,y);

  int sum = y*(y+1)/2 - x*(x+1)/2;

  cout << sum;
}