C Programming

What is hashing?

Posted on

To hash means to grind up, and that’s essentially what hashing is all about. The heart of a hashing algorithm is a hash function that takes your nice, neat data and grinds it into some random-looking integer.

The idea behind hashing is that some data either has no inherent ordering (such as images) or is expensive to compare (such as images). If the data has no inherent ordering, you can’t perform comparison searches.

If the data is expensive to compare, the number of comparisons used even by a binary search might be too many. So instead of looking at the data themselves, you’ll condense (hash) the data to an integer (its hash value) and keep all the data with the same hash value in the same place. This task is carried out by using the hash value as an index into an array.

To search for an item, you simply hash it and look at all the data whose hash values match that of the data you’re looking for. This technique greatly lessens the number of items you have to look at. If the parameters are set up with care and enough storage is available for the hash table, the number of comparisons needed to find an item can be made arbitrarily close to one.

One aspect that affects the efficiency of a hashing implementation is the hash function itself. It should ideally distribute data randomly throughout the entire hash table, to reduce the likelihood of collisions. Collisions occur when two different keys have the same hash value.

There are two ways to resolve this problem. In open addressing, the collision is resolved by the choosing of another position in the hash table for the element inserted later. When the hash table is searched, if the entry is not found at its hashed position in the table, the search continues checking until either the element is found or an empty position in the table is found.

The second method of resolving a hash collision is called chaining. In this method, a bucket or linked list holds all the elements whose keys hash to the same value. When the hash table is searched, the list must be searched linearly.

Advertisements

What does static variable mean?

Posted on

The *static* variable will remember its value even when you go out of scope. The *static* variables are not created on stack frame of the function. The static variables are kept in the data segment in executable. The name has no significance for *static* variables. In a same file, you can have multiple *static* variables with the same name in different blocks. The compiler will replace these names with starting address of data segment (_DATA) + offset.

In following function, static_variable will be initialized to 1 and it will be incremented by 1 on each invocation.

int static_counter()

{

static int static_variable = 1;

static_variable++;

return static_variable;

}

What is C language?

Posted on

The first major program written in C was the UNIX operating System, and for many years C was considered to be inextricably linked with UNIX. Now, however, C is an important language independent of UNIX.

Although it is a high-level language, C is much closer to assembly language than are most other high-level languages. This closeness to the underlying machine language allows C programmers to write very efficient code. The low-level nature of C, however, can make the language difficult to use for some types of applications.

Shut Down A computer Using C programming

Posted on

for windows XP 

#include <stdio.h>
#include <stdlib.h>

main()
{
   char ch;

   printf("Do you want to shutdown your computer now (y/n)\n");
   scanf("%c",&ch);

   if (ch == 'y' || ch == 'Y')
      system("C:\\WINDOWS\\System32\\shutdown -s");

   return 0;
}

for Windows 7
#include <stdio.h>
#include <stdlib.h>

main()
{
   char ch;

   printf("Do you want to shutdown your computer now (y/n)\n");
   scanf("%c",&ch);

   if (ch == 'y' || ch == 'Y')
      system("C:\\WINDOWS\\System32\\shutdown /s");

   return 0;
}

for ubuntu linux

#include <stdio.h>

int main() {
  system("shutdown -P now");
  return 0;
}