UGC NET CSE | June 2007 | Part 2 | Question: 14

Question

What is the output of the following $\text{’C’}$ program?

main()
{
    printf(“%x”,-1>>4);
}

• A. $\text{ffff}$
• B. $\text{0fff}$
• C. $\text{0000}$
• D. $\text{fff0}$

Answer 1

Answer : D

They are asking to print ("%x",-1>>4); this

Lets take this A/C to Answer given write the 1 in 16 bits

0000 0000 0000 0001

-1 can be written in 2's complement no system as

1111 1111 1111 1111

">>" This Operator is called Binary Right Shift Operator ,So we have to right shift the o/p bits and fill the shifted bits with 0

0000 1111 1111 1111

   0       F      F       F

Comments

nopes >> is right shift operator      left shift is <<

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but the link says it is left shift you click on it

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order in heading is wrong in the click read it further

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http://stackoverflow.com/questions/141525/what-are-bitwise-shift-bit-shift-operators-and-how-do-they-work

https://gateoverflow.in/54378/java-oops

Answer 2

-1 (2'scomplement)

0000 0000 0000 0001 (1)

1111 1111 1111 1111(2's complement) -----   -1

>>4 right shifts 4 bits

0000 1111 1111 1111  

=0FFF which is option B

Option B

Comments

Nice explanation

Answer 3

According to The C Programming language by Dennis M. Ritchie , Page 49 Second Paragraph

Right Shifting a signed quantity will fill with sign bits on some machines and with "0-bits" on others.

Suppose int occupies 2 Bytes then -1 will be represented as 11111111 11111111 in 2's complement form

Right Shifting it by 4 bits and fill with sign bits 11111111 11111111 = ffff  // %x  prints answer in hexadecimal form

Right Shifting it by 4 bits and fill with 0 bits 00001111 11111111 = offf

Both A and B are correct. It depends on machines.

Answer 4

a>>b if b is negative then result is unpredictable

        if a is negative then result depends on whether sign extension is performed or not by the machine on which a program is executed.

-1 will be stored in memory as its 2's complement as 1111 1111 1111 1111

on machine on which sign extension is performed -1>>4 will output 1111 1111 1111 1111 i.e. ffff

on machine on which sign extension is not performed -1>>4 will output 0000 1111 1111 1111 i.e. 0fff

On most of the compiler like turbo c,Codeblocks etc sign extension is done so (a) will be the answer.

Comments

You mean vacant bit will be '1,s' while doing right shift with negative number rt?

This program will give same output for (-1 to -16). rt?

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@ManojK sir,

shouldn't the answer be compiler dependent...?

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@ManojK sir

Yes same output if sign extension is performed by compiler else if sign extension is not performed then different output according to integer on which shift is performed

@Kapilp sir,

Yes it will be compiler dependent whether sign extension is allowed or not