rather a stupid question

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06-25-2009
BEN10

rather a stupid question

Halo all,
I know it might look a simple and stupid question to you all but please help me out of this.
I wonder how the output of the code below can be known in advance

Code:

#include <stdio.h> int main(void) { char ch=-74; printf("%d %c",ch,ch); }

The output is coming out to be

Quote:

-74 ||

I know that ch ranges from -128 to 127 and it's printing -74 correctly but the %c is making me confuse. I found the 2's complement of -74 and matched it with ASCII value but that also didn't match. Isn't my PC storing -74 in its 2's complement form?
Thanks
06-25-2009
MK27

Quote:

Originally Posted by BEN10

I found the 2's complement of -74 and matched it with ASCII value but that also didn't match. Isn't my PC storing -74 in its 2's complement form?
Thanks

Negative signed datatypes are stored in two's complement, yes. It does not really make sense, however, to be be looking at the two's complement form of -74 (which is a binary number) and expecting that to add any new meaning. It is still -74 for all intents and purposes.

You may have noticed -74 is not an ascii value. Char values less than zero are used in multibyte characters, which are not part of ascii. Since a multibyte character is several sequential values (not just a single char), the %c represented by -74 is not very meaningful either...
06-25-2009
brewbuck

Quote:

Originally Posted by BEN10

I found the 2's complement of -74 and matched it with ASCII value but that also didn't match.

You couldn't have possibly done that, the unsigned char with the same bit pattern as -74 is equal to 182, and 182 is not an ASCII code.

The character set your computer uses is not ASCII. It simply happens to be the same as ASCII in the range 0-127.

Quote:

Isn't my PC storing -74 in its 2's complement form?

Yes.
06-25-2009
BEN10

Now I saw that char corresponding to ASCII of 182 is similar to the one which got printed. But how did you get 182?
06-25-2009
tabstop

Quote:

Originally Posted by BEN10

Now I saw that char corresponding to ASCII of 182 is similar to the one which got printed. But how did you get 182?

182 is the two's complement of 74.

Code:

74 = 01001010 flip 10110101 add 1 10110110 = 182

Note though that there is no ASCII code for 182.
06-25-2009
BEN10

Quote:

Originally Posted by tabstop

182 is the two's complement of 74.

Code:

74 = 01001010 flip 10110101 add 1 10110110 = 182

Note though that there is no ASCII code for 182.

Thanks. Actually I was taking only 7 bits for 74(stupid me).
The character corresponding to 182 will be printed in my code, which is getting printed.
06-25-2009
cpjust

Quote:

Originally Posted by tabstop

182 is the two's complement of 74.

Code:

74 = 01001010 flip 10110101 add 1 10110110 = 182

Note though that there is no ASCII code for 182.

what about just doing: 256 - 74 = 182?
06-25-2009
BEN10

Quote:

Originally Posted by cpjust

what about just doing: 256 - 74 = 182?

By which logic are you doing this? Is this logic applicable for any negative number? Or it's just a fluke for -74?
06-25-2009
tabstop

Quote:

Originally Posted by BEN10

By which logic are you doing this? Is this logic applicable for any negative number? Or it's just a fluke for -74?

If you take any (binary) number, and then flip all the 0's and 1's around, and then add those two numbers, you will always get all 1's. (For a char, that's 255, for a 2-byte int 32767, for a 4-byte int 4billion and change.)
06-26-2009
cpjust

Quote:

Originally Posted by BEN10

By which logic are you doing this? Is this logic applicable for any negative number? Or it's just a fluke for -74?

Try writing a program to do this for all possible char numbers and see if it works.