the output is 1.why???

[indiver]

main() { int i=5; printf(“%d” ,i=++i ==6); }

[whodgson]

Because i=++i promotes undefined behavior and the output could be any value.

[donbock]

I don't believe there is any undefined behavior here. Your snippet is equivalent to

main()
  {
  int i = 5;
  i = (++i == 6);
  printf(“%d”, i);
  }

The output of the == operator is either 1 or 0. In this case, it is 1 because ++i is equal to 6.

I assume the odd format string is a cut-and-paste artifact. By the way, the proper return value of main is int.

[Oralloy]

Folks,

All constructs of the form

i = i++;

are in the realm of undefined behaviour according to the language specification.

Basically all expressions have undefined evaluation order, so an expression that has two side effects on a variable is ambiguous.