Multiply by 8 (left shift by 3 bits) and then subtract the number.
(x << 3) - x
Reversing a number
int n=12345,m=0;
cout«"Orginal No:“<
while(n>0)
{
m *= 10;
m += n%10;
n /= 10;
}
cout«”\nRevesed No:“<
Recursive function
int reverse\_num(int n,int m)
{
if(n==0)
return m; //base (exit condition)
m\*=10;
m+=n%10;
return reverse\_num(n/10,m); //recursive call
}
```To reverse the bits in an integer
Method1
`
unsigned int num; // Reverse the bits in this number.
unsigned int temp = num; // temp will have the reversed bits of num.
int i;
for (i = (sizeof(num)*8-1); i; i–)
{
temp = temp | (num & 1);
temp «= 1;
num »= 1;
}
temp = temp | (num & 1);
`
Method2
In this method, we use a lookup table.
`
const unsigned char ReverseLookupTable[] =
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Check if the 20th bit of a 32 bit integer is on or off?
AND it with x00001000 and check if its equal to x00001000
if((num & x00001000)==x00001000)
Note that the digits represented here are in hex.
`
0 0 0 0 1 0 0 0
^
|
x0000 0000 0000 0000 0001 0000 0000 0000 = 32 bits
^ ^ ^
| | |
0th bit 20th bit 32nd bit
`
Count bits set in an integer
Question: Write a function that determines the number of bits set to 1 in the binary representation of an integer.
Method 1 - Simple method (Right shift integer until it becomes 0)
Going by the brute force approach, it would be easy to come up with the following solution. If the least significant bit of a number is 1, it will return a result of 1 when we AND it with 1.
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File Handling in c
/* Program to create a file and write some data the file */
#include
#include
main( )
{
FILE *fp;
char stuff[25];
int index;
fp = fopen(“TENLINES.TXT”,“w”); /* open for writing */
strcpy(stuff,“This is an example line.");
for (index = 1; index <= 10; index++)
fprintf(fp,"%s Line number %d\n”, stuff, index);
fclose(fp); /* close the file before ending program */
}
FILE * fopen(char * filename, char * mode)
The mode value in the above example is set to ‘r’, indicating that we want to read from the file.
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Syntax in C
qualifier:
volatile
const
storage-class:
auto extern
static register
type:
void char short
int long float
double signed unsigned
enum-specifier
typedef-name
struct-or-union-specifier
What is a dangling pointer? What are reference counters with respect to pointers?
A pointer which points to an object that no longer exists. Its a pointer referring to an area of memory that has been deallocated. Dereferencing such a pointer usually produces garbage.
Using reference counters which keep track of how many pointers are pointing to this memory location can prevent such issues. The reference counts are incremented when a new pointer starts to point to the memory location and decremented when they no longer need to point to that memory.
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How to find out if a machine is 32 bit or 64 bit?
n = sizeof(void *);
if n==8, than 64 bit
if n==4, than 32 bit.
Operator Precedence in C & CPP
Operator Precedence Chart Operator Type Operator Associativity
1. Primary Expression Operators () [] . -> expr++ expr– left-to-right
2. Unary Operators * & + - ! ~ ++expr –expr right-to-left
(typecast) sizeof() 3. Binary Operators * / % left-to-right
+ -
» «
< > <= >=
== !=
&
^
|
&&
||
4. Ternary Operator ?: right-to-left
5. Assignment Operators = += -= *= /= %= »= right-to-left
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