Explain

[ishwariamca]

#include<stdio.h>
void main()
{
printf("&#37;x",-1<<4);
}

please Explain this program... The output displayed as fff0

[matsp]

And what do you want us to explain? That is exactly the output I'd expect.

--
Mats

[ishwariamca]

I cannot understand the concept clearly. please let me know in detail.

[matsp]

Yes, but I still don't know what you are actually asking for:
- how a negative number is represented,
- what a shift operator does,
- how printf display hexadecimal numbers
- something else that I haven't been able to guess

I could spend quite a bit of time explaining any of the first three, and still not hit what you are actually asking about.

--
Mats

[QuantumPete]

I cannot understand the concept clearly. please let me know in detail.

Does this explanation need to be limited to a hundred words, so it fits into the answer box on your assignment sheet?

QuantumPete

[matsp]

Does this explanation need to be limited to a hundred words, so it fits into the answer box on your assignment sheet?

QuantumPete

I'm still confused as to what we should explain - how #include <stdio.h> works, why the output looks like it does, or something else.

I too am sure that the answer should fit in some box on an assignment sheet, but I also would like to have a distinct question that can be answered, rather than "explain this" and a bit of code - 'tis a bit vague!

--
Mats

[Dogmasur]

I'm not in school, so don't be wary of any assignment sheet answer. I read the post and wanted to know what is the purpose of a shift operator. This may be beyond where I'm at in learning C anyway, but I figured I would ask.

[QuantumPete]

rather than "explain this" and a bit of code - 'tis a bit vague!

I reckon it's something along the lines of: Please explain what is happening, given this program and this output. So the answer is along the lines of:
"-1 is represented in binary as [blah], the bitshift then moves [blah], finally, the format specifier [blah]. Thence you get the above output."
I've blah'ed out all the interesting bits of the answer, ishwariamca, if there is a part of the program you don't understand, ask a specific question.

QuantumPete

[zacs7]

>> I read the post and wanted to know what is the purpose of a shift operator. This may be beyond where I'm at in learning C anyway, but I figured I would ask.
The applications are countless, this site has a pretty good tutorial on them: http://www.cprogramming.com/tutorial...operators.html

It uses a car park example.

[Dogmasur]

Thank you.

[matsp]

I'm not in school, so don't be wary of any assignment sheet answer. I read the post and wanted to know what is the purpose of a shift operator. This may be beyond where I'm at in learning C anyway, but I figured I would ask.

The shift operator corresponds to a set of instructions that most processors have, which multiplies/divides by 2 to the power of N. (simple, old processors would only do shifts one bit at at time, so only multiply or divide by 2). Compared to regular divide/multiply operations on the processor, these are MUCH faster [and sometimes the only type of multiply/divide, so any other form of multiplication would have to be built on a basis of multiplying/dividing by 2 - a bit like we do long multiplication or division on paper, except we use multiply/divide by 10 to figure out the result step by step].

This is useful when working with the binary representation of numbers, or when for example extracting a number that occupies bits in the middle of a larger number.

--
Mats

[Dogmasur]

Okay, thank you.

[master5001]

I know someone probably answered, but this one calls for some visual aid:

(32-bit) Example:

-1 // our signed int version of a binary value
11111111111111111111111111111111 //in binary
-1 < 4 // means that 4 bits are shifted left
11111111111111111111111111110000 // in binary

Make sense?

[zacs7]

You should mention it's in 2s-complement

It makes more sense in signed magnitude, although that's not what happens...

-1 // our signed int version of a binary value
10000000000000000000000000000001 //in binary (signed magnitude)
-1 << 4 // means that 4 bits are shifted left
00000000000000000000000000010000 // in binary, note the value is NOT sign extended

[ishwariamca]

Thank You for your explanation.