#include "stdafx.h"
int _tmain(int argc, _TCHAR* argv[])
{
int x=0;
int y=0;
while (x<15)y++,x+=++y;
printf ("%i %i",x, y);
getchar ();
getchar ();
return 0;
}
I don't know why x is 20 and y is 8 at the end. Please explain it step by step.
while (x<15)y++,x+=++y;
=>
while (x<15) {
y++;
x += ++y;
}
=>
while (x < 15) {
y += 2;
x += y;
}
So:
Before 1st iteration: x = 0, y = 0;
After 1st iteration: x = 2, y = 2;
After 2nd iteration: x = 6, y = 4;
After 3rd iteration: x = 12, y = 6;
After 4th iteration: x = 20, y = 8;
Note that there is a simple closed formula for these values as well: x = n*n - n and y = 2*n.
Remember that :
Which gives the following values for x and y :
iterations x y
0 0 0
1 2 2
2 6 4
3 12 6
4 20 8
x<15 -> y=1, x=0+(y=2)=2
2<15 -> y=3, x=2+(y=4)=6
6<15 -> y=5, x=6+(y=6)=12
12<15 -> y=7, x=12+(y=8)=20
Done x=20, y=8
The comma operator enforces the order of execution. x,y means that x is executed first, and then y.
If you follow what is going to happen to your variables :
First Loop:
x=0, y=0
y++ => y=1
x+=++y => x=2, y=2
Second Loop:
x=1, y=2
y++ => y=3
x+=++y => x=6, y=4
Third Loop:
y++ => y=5
x+=++y => x=12, y=6
Fourth Loop:
y++ => y=7
x+=++y => x=20, y=8
And while loop will exit.
I changed it to
#include <stdio.h>
int main()
{
int x=0;
int y=0;
while (x<15) {
y++,x+=++y;
printf ("%i %i\n",x, y);
}
return 0;
}
and it yields
H:\Temp>a.exe
2 2
6 4
12 6
20 8
now.
Why? Because y gets incremented twice in each step, and x gets added the new value. So you essentially get 2+4+6+8 = 20.
But I'm not sure it is defined behaviour. It is if the , operator defines a sequence point.
Let's go through the loops (conditions after the loop):
In each loop, what effectively is done is:
y+=2
x+=y
which results in the states written above.
Alright, this is what is happening every loop in your while clause:
Now in numbers, right before the end of the loop:
in every iteration
y increased by 2 ( y +=2 ) ,
after that x increased by x + y ( x= x+y)
it skips when x = 20 ( as x is now > 15 )
y++,x+=++y;is pretty horrible code; is this homework?