For loops

[muffins123]

Hi I have a question regarding for-loops. What value would be saved in x?

int x = 0;
for( int i = 0; i < 4; x += i++ );

As I understand, the loop starts from initial 0 with condition that i<4. It will also increment by 1. But I do not get how the value of x can be determined. Any help would be appreciated :) Thanks

p.s x=6

[keskiverto]

Lets "debug":

#include <iostream>

int main() {
  int x = 0;
  for ( int i = 0; i < 4; x += i++ ) {
    std::cout << "i=" << i << " x=" << x << '\n';
  }
  std::cout << "final x=" << x << '\n';

  return 0;
}

Does that give you any clues?

[muffins123]

It is unclear to me why x=6 is. I know this is a post increment which means we return the value and then increment it if I have understood it correctly. So that means x and i starts from 0 and will be incremented. First loop: x=0 i=1; second loop: x=1 i=2 and so on. Am I understanding this correctly? But how does x add up to 6? because i stops at 3.

I'm really sorry, I tried looking up tutorials but it doesn't provide any concrete explanation to this. I found a post with a similar topic : http://www.cplusplus.com/forum/beginner/125466/

but there are some things I didn't quite understand particularly this:

int x = 0;
int y = 5;

x = y++;   // x = 5, y = 6
x = ++y;   // x = 7, y = 7; 

[z05DSL3A]

But how does x add up to 6? because i stops at 3.

x += i++ is the same as

x = x + i;
i = i + 1;

Loop 1, i = 0 so x = x + 0 = 0
Loop 2, i = 1 so x = x + 1 = 1
Loop 3, i = 2 so x = x + 2 = 3
Loop 4, i = 3 so x = x + 3 = 6

-=-=-=-=-=-=-=-=-=-=-=-=-

but there are some things I didn't quite understand particularly this:

x = 0
y = 5

x = y++
// evaluate y then increment y,
//so y is evaluated to 5 and assigned to x and then y is incremented

x = ++y
// increment y then evaluate it,
// So y in incremented and then evaluated as 7 and 7 is assigned to x.

[MAhmed]

if you wanna increase the value you of "x" with loop
then bellow code is gonna help you

int x = 0;
for(int i = 0; i < 4; i++ );
{
x++;
}

your value of x will be changed to 4 because of that loop
Hope that helped :-)

[keskiverto]

But how does x add up to 6? because i stops at 3.

No. The loop stops, when i < 4 is not true.

The condition is not true only if 3 < i.
When 3==i, the condition is still true, the loop body is executed and the increment statement x += i++ is evaluated.

On the next time 3 < i (because i is already 4), the loop condition is false, and the looping ends.

for ( int i = 0; i < 4; x += i++ );

// is same as

for ( int i = 0; i < 4; x += i++ )
{
}

// is same as

for ( int i = 0; i < 4; )
{
  x += i++;
}

// is same as

for ( int i = 0; i < 4; )
{
  x += i;
  ++i;
}

// is same as

for ( int i = 0; i < 4; ++i )
{
  x += i;
}

[Handy Andy]

kevin84,

All these answers are good, but they have kind of missed the small point of x += . So after x becomes larger than zero you start with that number and then add to it. As Grey Wolf wrote Loop 4, i = 3 so x = x + 3 = 6 remember x starts out as 3 before you add three to get 6.

Hope that helps,

Andy