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Same program but different output?

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bhadmanathan

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Same program but different output ?

Hi ,
int main()
{
char a=1;
printf("%d\n",a<<32);
printf("%d\n",1<<32);
return 0;
}

OUTPUT:
1
0

I am using 32 bit compiler. When i shift beyond the width (0:31) , what is actually happening.

Even though both the printf statements are meant to same, i am getting different output ?


Thanks in advance
 

Re: Same program but different output ?

In the printf you are asking it to print a decimal number but you are supplying it with a char.

What is the expected number if you shift char 1 32 bits to the left?

In the second printf, 1 is taken as a number and not a char. That does make a difference.
 

Re: Same program but different output ?

char is a signed 8 bit number, the second (constant) expression can be expected to be evaluated as int according to the default compiler format (I presume int32).

According to C standard, the overflow behavior of signed numbers is undefined, so any result would be allowed in both cases, not necessarily the same.

The result of E1 << E2 is E1 left-shifted E2 bit positions; vacated bits are filled with zeros. If E1 has an unsigned type, the value of the result is E1 × 2E2, reduced modulo one more than the maximum value representable in the result type. If E1 has a signed type and nonnegative value, and E1 × 2E2 is representable in the result type, then that is the resulting value; otherwise, the behavior is undefined.
Click to expand...
 

Re: Same program but different output ?

int main()
{
char a=1;
unsigned char b=1;
int c=1;
unsigned int d=1;
printf("%d\n",a<<32);
printf("%d\n",b<<32);
printf("%d\n",c<<32);
printf("%d\n",d<<32);

printf("%d\n",1<<31);
return 0;
}

O/P;
1
1
1
1
0

Here i am using 32 bit processor, so it can process with 32 bits at a time.When i shift numbers from any variable then it gets shifted in rotational order i.e

a<<30 - fine
a<<31 - fine
a<<32 - gets shifted 0 times
a<<33 - gets shifted 1 time
a<<34 -gets shifted 2 times
......etc

But i shift direct constant beyond the limit ,the answer is always zero.
 

Re: Same program but different output ?

The compiler doesn't seem to fully comply with the C standard. At least for shift of unsigned numbers beyond it's range, the result has to be zero.

Shift left "<<" is not rotate left.

What's the compiler under test?
 

Re: Same program but different output ?

I used
Geeksforgeeks IDE (online)
Dev cpp

In both the compilers i got the same result....
 

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