"First time" test

Question

What is the shortest C expression that is defined to evaluated to false the first time and true for every time after that:

Reference implementation:

bool b = false;  // you get this for free.

(b ? true : b = true, false)  // only this line is measured.

Answer 1

3 chars:

As Keith Randall suggested in a comment, if we can use floats or doubles, this should do the trick:

float b = 0;  // free

b++

Eventually, as b is incremented repeatedly, two things may happen: either the addition overflows (and thus evaluates to HUGE_VAL, which may be infinite or a large positive value) or, more likely, the roundoff step size simply grows larger than one, turning the increment into a no-op. In either case, the expression should continue to evaluate as true.

Answer 2

6 characters

int i = 0; //free
i||i++

OTOH: I like Peter Taylor's solution as it has no branch so it may be faster (depending on compiler details).

p.s. I came up with that after posting the question (honest!).

Answer 3

With:

long long b = 0;

then three characters are enough for many applications:

b++

Evaluates false first and true for a long time thereafter ;-)

Answer 4

7 characters

As Peter Taylor suggested, still int a = 2;

!(a/=2)

8 characters (old)

Start with int a = 2;

!(a>>=1)

Another version of this: int b = 0x40000000;

!(b<<=1)

Answer 5

9 chars:

b=0;//free
(b&(b=1))

at least if it's valid c (not undefined behavior)

12 chars

(b?b:!(b=1))

this is valid c

Answer 6

11 10 chars

I can't beat the three char approach, but make it different:

int b = 0;  // free

b?b:0*(b=1) // 11 characters
b||0*(b=1)  // one character less thanks to breadbox

Answer 7

int b=1; // for free
!b|(b=0)

bit-wise or works as well since in first trial 0|0 yields 0 and in other trials !0|0 yields !0 which is guaranteed to be true. It is also ANSI-compliant, I've tried the following code with GCC 4.6.1 with -ansi flag:

#include <stdio.h>
main () {
  int b=1, i=0;
  for (;i<5;++i)
    printf ("%d",!b|(b=0));
}

Answer 8

APL (5 or 6)

If I'm only allowed to initialize the variable to 0, it's 6 characters:

A←0    ⍝ free
⊃A←⍴⍴A

How it works: ⍴A is the size of A (which is the empty list the first time around, because 0 is a scalar), so ⍴⍴A is the size of the size of A (which is [0] the first time, because a one-dimensional empty list has zero values in one dimension). This is then assigned to A (A←) and the first element is returned (⊃).

1. A is 0, ⍴A is [], then A is set to ⍴⍴A which is [0], and the first element is returned (0).
2. A is [0], ⍴A is [1], then A is set to ⍴⍴A which is [1], and the first element is returned (1).
3. A is [1], so ⍴A remains [1], so A is set to ⍴⍴A which remains [1] and it returns 1.

If I'm allowed to initialise the variable to anything I want, I can set it to the empty list and drop one of the ⍴s to make it 5 characters:

A←⍬    ⍝ free, set to empty list
⊃A←⍴A

Answer 9

9 chars

b=1; //free
!b||(b=0)  

edited: no longer undefined behavior

Answer 10

bool b = false; // for free?
b = b?b:!b;

Answer 11

What about this:

bool b = false;

!b+b++

Answer 12

1 Character

Since you didn't specify which compiler and/or which command line options are to be used, here's my 1 character solution:

bool b = false;

X

Compile with

gcc -DX='(b ? true : b = true, false)'