Using ONLY

```
! ~ & ^ | +
```

How can I find out if a 32 bit number is TMax?

TMax is the maximum, two's complement number.

My thoughts so far have been:

```
int isTMax(int x)
{
int y = 0;
x = ~x;
y = x + x;
return !y;
}
```

That is just one of the many things I have unsuccessfully have tried but I just cant think of a property of TMax that would give me TMax back. Like adding tmax to itself would be unique compared to all the other integers.

Here is the actual problem:

```
/*
* isTMax - return 1 if x is the maximum, two's complement number,
* and 0 return otherwise.
* Legal ops: ! ~ & ^ | +
* Max ops: 10
* Rating: 1
*/
int isTMax(int x) {
int y = 0;
x = ~x;
y = x + x;
return !y;
}
```

int is 32 bits so the max signed would probably be 0x7FFFFFFF

As far as I know, there is no way to determine if a particular value is the max value of a *signed* type without already knowing the maximum value of that type and making a direct comparison. This is because signed expressions experience undefined behavior on overflow. If there were an answer to your question, it would imply the existence of an answer to a serious unsolved problem that's been floating around on SO for some time: how to programmatically determine the max value for a given signed type.

answered on Stack Overflow Sep 4, 2011 by R..

```
int isTmax(int x) {
//add one to x if this is Tmax. If this is Tmax, then this number will become Tmin
//uses Tmin = Tmax + 1
int plusOne = x + 1;
//add to x so desired input becomes 0xFFFFFFFF, which is Umax and also -1
//uses Umax = 2Tmax + 1
x = x + plusOne;
plusOne = !(plusOne);
//is x is 0xffffffff, then this becomes zero when ~ is used
x = ~x;
x = x | plusOne;
x = !x;
return x;
}
```

Spend 3 hours on this problem. I know this problem comes from csapp's data lab and its newest requirement is

```
1. Integer constants 0 through 255 (0xFF), inclusive. You are
not allowed to use big constants such as 0xffffffff
....
* isTmax - returns 1 if x is the maximum, two's complement number,
* and 0 otherwise
* Legal ops: ! ~ & ^ | +
* Max ops: 10
* Rating: 1
```

So, shift operator(`<<`

/`>>`

and `0x7FFFFFFF`

from accepted answer is forbidden now)

Below is my way:

TDD-style:

```
isTmax(2147483647) == isTmax(0b011111111111...1) == 1
isTmax(2147483646) == isTmax(0b011111111111...0) == 0
isTmax(-1) == isTmax(0b111111111...1) == 0
isTmax(-2147483648) == isTmax(0b100000000...0) == 0
```

the return should be either `0`

or `1`

. In, c, `!`

+ all nonzero will return `0`

. So `!`

is a must, otherwise we cannot guarantee getting `0`

for all numbers.

because `0b0111111...1`

(aka `2147483647`

) is the only argument which should make `isTmax`

return `1`

and `2147483647 + 1`

should be `10000000...0`

(aka `-2147483648`

)

`0b011111111...1 xor 0b1000000000...0`

is `0b11111111111...111`

. Because we must use `!`

, what we hope to see is `0`

(aka `0b0000000000000...0`

). Obviously, just apply *logic not*(aka `!`

) to `0b1111111...1`

), then we will get `0b000000000000`

):

```
!(~(x ^ (x + 1))
```

let's printf it

```
void print(int x)
{
printf("%d\n", !(~(x ^ (x + 1))));
}
int main() {
print (2147483647);
print(2147483646);
print(-1);
print(-2147483648);
}
```

1

0

1

0

Not bad, only `-1`

doesn't work as we expected.

Let's compare `-1`

and `2147483647`

11111111111111111111111111111111

01111111111111111111111111111111

We can find `-1 + 1 = 0`

while `2147483647 + 1 = -2147483648`

. Emphasize again, what we want is diff `-1`

and `2147483647`

, because both of them return `1`

as above shows. Look back to the protety of *logic not* in c: *all nonzero will return 0*, so `!-2147483648 == 0`

and `!(-1 + 1) != 0`

. Just modify left part of `x ^ (x + 1)`

(`x`

) into `x + !(x + 1)`

. If x is `2147483647`

, `x + !(x + 1)`

will equal to `x`

.

Run again:

```
void print(int x)
{
printf("%d\n", !(~( x + !(x + 1) ^ (x + 1))));
}
int main() {
print (2147483647);
print(2147483646);
print(-1);
print(-2147483648);
}
```

1

0

0

0

Done!

Something like this perhaps? 0x7FFFFFFF is the maximum positive signed 32 bit two's complement number.

```
int isTMax(int x){
return !(x ^ 0x7FFFFFFF);
}
```

I am not sure, you may need to cast it to unsigned for it to work.

answered on Stack Overflow Sep 4, 2011 by Ben Goodrich

```
#include <stdio.h>
#include <stdlib.h>
int test(int n) {
return !(n & 0x80000000) & !~(n | (n + 1));
}
// or just effectively do a comparison
int test2(int n) {
return !(n ^ 0x7fffffff);
}
int main(int ac, char **av) {
printf("is%s TMax\n", test(atoi(av[1])) ? "" : " not");
return 0;
}
```

answered on Stack Overflow Sep 4, 2011 by DigitalRoss

if it is Tmax : 011111.....

then we xor it with 10000....

we get 11111....

then we ~ to get all 0s = 0 , !0 we get 1:

```
int isTmax(int x) {
return !(~((1 << 31) ^ x ));
}
```

answered on Stack Overflow Sep 30, 2018 by XueYu

User contributions licensed under CC BY-SA 3.0