#include <stdio.h>
int main()
{
int num, i = 0,pos;
printf(" Enter num \n");
scanf("%d",&num);
for( i = 0; i < 31; i++ )
{
pos = 1 << i;
if ( num & pos )
printf("1");
else
printf("0");
}
printf("\n");
return 0;
}
/*
O/P
Enter Num
12
0011000000000000
*/
But i want to print the o/p as 0000000000001100
So, What are the changes i have to made to get the desired o/p
You're printing the least significant bit first. Change your for loop to count down:
for (int i = 31; i >= 0; i--)
EDIT:
Seem that I'm the one who overlooked desired output. So the solutions provided by others will work for the OP.
I'm surprised people overlooked the fact that endianness usually applies to byte level instead of bit, which make the plain index-based loop fail to provide required output.
for a decimal to big-endian byte, you need :
while (num)
{
big <<= 8;
big |= num & 0xFF;
num >>= 8;
}
so in order to output little-endian integer into big-endian binaries, you need :
// 'num' was 4-byte (int) data in little-endian format
while (num)
{
// select required byte block
unsigned char curByte = num & 0xFF;
// prints the byte binaries
for(int iBit=7; iBit>=0; --iBit)
{
unsigned char theBit = curByte >> iBit;
if (theBit & 0x1)
putchar('1');
else
putchar('0');
}
// shifts to next byte block
num >>= 8;
}
Change your for loop to be more like:
#define BIT(x) (1U << (x))
for (i = 31; i >= 0; --i)
{
if (x & BIT(i)) {
putchar('1');
}
else {
putchar('0');
}
}
Related
Code:
#include <stdio.h>
#include <stdlib.h>
int main()
{
long int x;
x = 1000000;
printf("%ld\n", x);
for(int i = 0; i < 32; i++)
{
printf("%c", (x & 0x80) ? '1' : '0');
x <<= 1;
}
printf("\n");
return 0;
}
This code is supposed to convert a decimal int to binary, but why doesn't it work correctly?
P.S. I solved this problem by replacing 0x80 with 0x80000000. But why was the wrong number displayed at 0x80?
EDIT2:
OP asks "P.S. I solved this problem by replacing 0x80 with 0x80000000. But why was the wrong number displayed at 0x80?"
What was wrong was 0x80 is equal to 0x00000080. 0x80 will never test any bits above b7 (where bits, right to left, are numbered b0 to b31.
The corrected value, 0x80000000, sets the MSB high and can be used (kind of) to 'sample' each bit of the data as the data value is 'scrolled' to the left.
//end edit2
Two concerns:
1) Mucking with the sign bit of a signed integer can be problematic
2) "Knowing" there are 32 bits can be problematic.
The following makes fewer presumptions. It creates a bit mask (only the MSB is set in an unsigned int value) and shifts that mask toward the LSB.
int main() {
long int x = 100000;
printf("%ld\n", x);
for( unsigned long int bit = ~(~0u >> 1); bit; bit >>= 1 )
printf("%c", (x & bit) ? '1' : '0');
printf("\n");
return 0;
}
100000
00000000000000011000011010100000
Bonus: Here is a version of the print statement that doesn't involve branching:
printf( "%c", '0' + !!(x & bit) );
EDIT:
Having seen the answer by #Lundin, the suggestion to insert SP's to improve readability is an excellent idea! (Full credit to #Lundin.)
Below, not only is the long string of bits output divided into "hexadecimal" chunks, but the compile time value is shown in a way to easily see it is 10million. (1e7 would have done, too.)
A new-and-improved version:
#include <stdio.h>
#include <stdlib.h>
int main() {
long int x = 10 * 1000 *1000;
printf("%ld\n", x);
for( unsigned long int bit = ~(~0u >> 1); bit; bit >>= 1 ) {
putchar( '0' + !!(x & bit) );
if( bit & 0x11111111 ) putchar( ' ' );
}
putchar( '\n' );
return 0;
}
10000000
0000 0000 1001 1000 1001 0110 1000 0000
1000000 dec = 11110100001001000000 bin.
80 hex = 10000000 bin.
And this doesn't make much sense at all:
11110100001001000000
& 10000000
Instead fix the loop body to something like this:
#include <stdio.h>
#include <stdlib.h>
int main (void)
{
long int x;
x = 1000000;
printf("%ld\n", x);
for(int i = 0; i < 32; i++)
{
unsigned long mask = 1u << (31-i);
printf("%c", (x & mask) ? '1' : '0');
if((i+1) % 8 == 0) // to print a space after 8 digits
printf(" ");
}
printf("\n");
return 0;
}
Without using an integer counter to see what digit is at the ith position, you can instead use an unsigned variable which is equal to 2^i at the ith iteration. If this variable is unsigned, when it overflows it will become zero. Here is how the code would look like. It displays the number in reversed order (first position means the coefficient of 2^0 in the polynomial decomposition of the number).
int
main()
{
int x;
x = 1000000;
printf("%lx\n", x);
for(unsigned b = 1; b; b<<=1)
printf("%c", x & b ? '1':'0');
printf("\n");
return 0;
}
I would use functions
void printBin(long int x)
{
unsigned long mask = 1UL << (sizeof(mask) * CHAR_BIT - 1);
int digcount = 0;
while(mask)
{
printf("%d%s", !!(x & mask), ++digcount % 4 ? "" : " ");
mask >>= 1;
}
}
int main(void)
{
printBin(0); printf("\n");
printBin(1); printf("\n");
printBin(0xf0); printf("\n");
printBin(-10); printf("\n");
}
Dear all C programmer:
X = 1 << N; (left shift)
how to recover N from X ?
Thanks
N in this case is the bit position where you shifted in a 1 at. Assuming that X here only got one bit set. Then to find out what number that bit position corresponds to, you have to iterate through the data and mask with bitwise AND:
for(size_t i=0; i<sizeof(X)*8; i++)
if(X & (1<<i))
printf("%d", i);
If performance is important, then you'd make a look-up table with all possible results instead.
In a while loop, keep shifting right until X==1, record how many times you have to shift right and the counter will give you N.
int var = X;
int count = 0;
while (var != 1){
var >>= 1;
count++;
}
printf("N is %d", count);
Try this (flsl from here which is available from string.h on macOS) :
int flsl(long mask)
{
int bit;
if (mask == 0) return (0);
for (bit = 1; mask != 1; bit++)
mask = (unsigned long)mask >> 1;
return (bit);
}
unsigned char binlog(long mask) { return mask ? flsl(mask) - 1 : 0; }
int x = 1 << 20;
printf("%d\n", binlog(x)); ===> 20
I am reviewing for an exam and have a practice problem that I'm stuck on.
I need to write the function find_sequence(unsigned int num, unsigned int patter) {}.
I have tried comparing num & (pattern << i) == (pattern << i) and other things like that but it keeps saying there is a pattern when there isn't. I see why it is doing that but I can not fix it.
The num I'm using is unsigned int a = 82937 and I'm searching for pattern unsigned int b = 0x05.
Pattern: 00000000000000000000000000000101
Original bitmap: 00000000000000010100001111111001
The code so far:
int find_sequence(unsigned int num, unsigned int pattern)
{
for (int i=0; i<32; i++)
{
if ((num & (pattern << i)) == (pattern << i))
{
return i;
}
}
return -9999;
}
int
main()
{
unsigned int a = 82937;
unsigned int b = 0x05;
printf("Pattern: ");
printBits(b);
printf("\n");
printf("Original bitmap: ");
printBits(a);
printf("\n");
int test = find_sequence(a, b);
printf("%d\n", test);
return 0;
}
Here is what I have so far. This keeps returning 3, and I see why but I do not know how to avoid it.
for (int i=0; i<32; i++)
{
if ((num & (pattern << i)) == (pattern << i))
is bad:
- it works only when pattern consists of 1 entirely
- you generate at the end of the loop pattern << 31 which is 0 when pattern is even. Condition will hold every time then.
Knowing the length of the pattern would simplify the loop above; just go until 32 - size. When not given by the API, the length can be calculated either by a clz() function or manually by looping over the bits.
Now, you can generate the mask as mask = (1u << length) - 1u (note: you have to handle the length == 32 case in a special way) and write
for (int i=0; i < (32 - length); i++)
{
if ((num & (mask << i)) == (pattern << i))
or
for (int i=0; i < (32 - length); i++)
{
if (((num >> i) & mask) == pattern)
((num & (pattern << i)) == (pattern << i)) won't give you the desire results.
Let's say you pattern is 0b101 and the value is 0b1111, then
0101 pattern
1111 value
& ----
0101 pattern
Even though the value has not the pattern 0b101, the check would return true.
You've got to create a mask where all bits of the pattern (until the most
significant bit) are 1 and the rest are 0. So for the pattern 0b101 the mask
must be b111.
So first you need to calculate the position of the most significant bit of the pattern, then create
the mask and then you can apply (bitwise AND) the mask to the value. If the
result is the same as the pattern, then you've found your pattern:
int find_sequence(unsigned int num, unsigned int pattern)
{
unsigned int copy = pattern;
// checking edge cases
if(num == 0 && pattern == 0)
return 0;
if(num == 0)
return -1;
// calculating msb of pattern
int msb = -1;
while(copy)
{
msb++;
copy >>= 1;
}
printf("msb of pattern at pos: %d\n", msb);
// creating mask
unsigned int mask = (1U << msb + 1) - 1;
int pos = 0;
while(num)
{
if((num & mask) == pattern)
return pos;
num >>= 1;
pos++;
}
return -1;
}
Using this function I get the value 14, where your 0b101 pattern is found in
a.
In this case you could make a bitmask that 0's out all the spaces you aren't looking for so in this case
Pattern: 00000000000000000000000000000101
Bitmask: 00000000000000000000000000000111
So in the case of the number you are looking at
Original: 00000000000000010100001111111001
If you and that with this bitmask you end of with
Number after &: 00000000000000000000000000000001
And compare the new number with your pattern to see if equal.
Then >> the original number
Original: 00000000000000010100001111111001
Right shifted: 00000000000000001010000111111100
And repeat the & and compare to check the next 3 numbers in the sequence.
I wrote this function to remove the most significant bit in every byte. But this function doesn't seem to be working the way I wanted it to be.
The output file size is always '0', I don't understand why nothing's been written to the output file. Is there a better and simple way to remove the most significant bit in every byte??
In relation to shift operators, section 6.5.7 of the C standard says:
If the value of the right operand is negative or is greater than or
equal to the width of the promoted left operand, the behavior is
undefined.
So firstly, remove nBuffer << 8;. Even if it were well defined, it wouldn't be an assignment operator.
As people have mentioned, you'd be better off using CHAR_BIT than 8. I'm pretty sure, instead of 0x7f you mean UCHAR_MAX >> 1 and instead of 7 you meant CHAR_BIT - 1.
Let's just focus on nBuffer and bit_count, here. I shall comment out anything that doesn't use either of these.
bit_count += 7;
if (bit_count == 7*8)
{
*out_buf++ = nBuffer;
/*if((write(out_fd, bit_buf, sizeof(char))) == -1)
oops("Cannot write on the file", "");*/
nBuffer << 8;
bit_count -= 8;
}
nBuffer = 0;
bit_count = 0;
At the end of this code, what is the value of nBuffer? What about bit_count? What impact would that have on your second loop? while (bit_count > 0)
Now let's focus on the commented out code:
if((write(out_fd, bit_buf, sizeof(char))) == -1)
oops("Cannot write on the file", "");
Where are you assigning a value to bit_buf? Using an uninitialised variable is undefined behaviour.
Instead of going through all of the bits to find the high one, this goes through only the 1 bits. high() returns the high bit of the argument, or zero if the argument is zero.
inline int high(int n)
{
int k;
do {
k = n ^ (n - 1);
n &= ~k;
} while (n);
return (k + 1) >> 1;
}
inline int drop_high(int n)
{
return n ^ high(n);
}
unsigned char remove_most_significant_bit(unsigned char b)
{
int bit;
for(bit = 0; bit < 8; bit++)
{
unsigned char mask = (0x80 >> bit);
if( mask & b) return b & ~mask;
}
return b;
}
void remove_most_significant_bit_from_buffer(unsigned char* b, int length)
{
int i;
for(i=0; i<length;i++)
{
b[i] = remove_most_significant_bit(b[i]);
}
}
void test_it()
{
unsigned char data[8];
int i;
for(i = 0; i < 8; i++)
{
data[i] = (1 << i) + i;
}
for(i = 0; i < 8; i++)
{
printf("%d\r\n", data[i]);
}
remove_most_significant_bit_from_buffer(data, 8);
for(i = 0; i < 8; i++)
{
printf("%d\r\n", data[i]);
}
}
I won't go through your entire answer to provide your reworked code, but removing the most significant bit is easy. This comes from the fact that the most significant bit can easily be found by using log base 2 converted to an integer.
#include <stdio.h>
#include <math.h>
int RemoveMSB(int a)
{
return a ^ (1 << (int)log2(a));
}
int main(int argc, char const *argv[])
{
int a = 4387;
printf("MSB of %d is %d\n", a, (int)log2(a));
a = RemoveMSB(a);
printf("MSB of %d is %d\n", a, (int)log2(a));
return 0;
}
Output:
MSB of 4387 is 12
MSB of 291 is 8
As such, 4387 in binary is 1000100100011 with a most significant bit at 12.
Likewise, 291 in binary is 0000100100011 with a most significant bit at 8.
I'm trying to simply convert a byte received from fget into binary.
I know the value of the first byte was 49 based on printing the value. I now need to convert this into its binary value.
unsigned char byte = 49;// Read from file
unsigned char mask = 1; // Bit mask
unsigned char bits[8];
// Extract the bits
for (int i = 0; i < 8; i++) {
// Mask each bit in the byte and store it
bits[i] = byte & (mask << i);
}
// For debug purposes, lets print the received data
for (int i = 0; i < 8; i++) {
printf("Bit: %d\n",bits[i]);
}
This will print:
Bit: 1
Bit: 0
Bit: 0
Bit: 0
Bit: 16
Bit: 32
Bit: 0
Bit: 0
Press any key to continue . . .
Clearly, this is not a binary value. Any help?
The problem you're having is that your assignment isn't resulting in a true or false value.
bits[i] = byte & (mask << i);
This gets the value of the bit. You need to see if the bit is on or off, like this:
bits[i] = (byte & (mask << i)) != 0;
Change
bits[i] = byte & (mask << i);
to
bits[i] = (byte >> i) & mask;
or
bits[i] = (byte >> i) & 1;
or
bits[i] = byte & 1;
byte >>= 1;
One way, among many:
#include <stdio.h>
#include <limits.h>
int main(void) {
int i;
char bits[CHAR_BIT + 1];
unsigned char value = 47;
for (i = CHAR_BIT - 1; i >= 0; i -= 1) {
bits[i] = '0' + (value & 0x01);
value >>= 1;
}
bits[CHAR_BIT] = 0;
puts(bits);
return 0;
}
You may notice that your output has a couple 1's and 0's, but also powers of 2, such as 32. This is because after you isolate the bit you want using the mask, you still have to bit-shift it into the least-significant digit so that it shows up as a 1. Or you could use what other posts suggested, and instead of bit-shifting the result (something like 00001000 for example), you could simply use (result != 0) to fetch either a 1 or 0, since in C, false is 0, and comparisons such as != will return 1 as true (I think).
#include<Stdio.h>
#include <limits.h>
void main(void) {
unsigned char byte = 49;// Read from file
unsigned char mask = 1; // Bit mask
unsigned char bits[8];
int i, j = CHAR_BIT-1;
// Extract the bits
for ( i = 0; i < 8; i++,j--,mask = 1) {
// Mask each bit in the byte and store it
bits[i] =( byte & (mask<<=j)) != NULL;
}
// For debug purposes, lets print the received data
for (int i = 0; i < 8; i++) {
printf("%d", bits[i]);
}
puts("");
}
This addition in place of that will work:
bits[i]= byte & (mask << i);
bits[i] >>=i;