#include "stdafx.h"
#include <iostream>
using namespace std;
int _tmain(int argc, _TCHAR* argv[])
{
int j = 5;
while ( ++j < 9)
cout << j++ << endl;
cin.get();
return 0;
}
So I am working on a book, and I understand how these things work, but I am seeking a more concrete explanation of why this prints 6 and 8. Please help me.
Thanks
To understand what it outputs you need to step through the code:
j = 5
j = j + 1 // 6
j < 9 ? yes
output j // 6
j = j + 1 // 7
go to start of loop
j = j + 1 // 8
j < 9 ? yes
output j // 8
j = j + 1 // 9
go to start of loop
j = j + 1 // 10
j < 10 ? no
j
starts as 5. It then gets incremented to 6, then compared with 9 in in ++j < 9
. This passes the condition, so we go to the next line, where it outputs 6 and then increments j
to 7. We go back to the conditional, j
is incremented to 8, which is still less than 9, then output, then j
is incremented to 9
, where the condition fails and the program ends.
Given:
int j = 5;
while ( ++j < 9)
cout << j++ << endl;
Why are only 6
and 8
printed?
Upon entering the loop, j
is 5. Then the while (++j ...
increments that to 6. Then the cout
executes, printing out 6. Then the side effect of the j++
takes place, incrementing it to 7.
On the next iteration of the loop, the ++j
increments it again, giving 8
, which is then printed out. Then the side effect of the j++
takes place, incrementing j
to 9.
Then when while (++j < 9)
is executed again, j
has been incremented to 10, so the while
loop exits, and no more is printed.
++j means the value of j is incremented by 1 and then used in the expression where it appears
j++ means the ( current ) value of j is used in the expression, after that j is incremented
++j means increase j by one and then evaluate j. j++ on the other hand means evaluate j first and then increase it by 1.
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