malloc, calloc and realloc

malloc is used for only allocating the memory of any given size and return a void pointer .
for e.g.
void *malloc(size);  
p = malloc(4);        //syntax of malloc
But malloc only allocates memory it does not initialize the allocated memory. So, for initializing the allocated memory we use calloc. it initialize the allocated memory to zero.
void *calloc(num_of_blocks, size_of_each_block);
And when we require to reallocate the memory of previously allocated memory then we use realloc for reallocating memory.

void *realloc(void *ptr, size_t size);

Extern Variable in 'C'

When we declare a variable without using extern then the variable is declared as well as memory is allocated to the variable. for e.g.
   int a;
This statement tell that there is a variable named 'a'. i.e. it is declared and memory of 4 bytes is allocated to 'a' i.e. it is defined. But if we use extern to declare a variable then it only declare the variable but does not allocate memory to it.for e.g.
   extern int a;
This statement only declare the variable 'a'. for allocating memory to variable 'a' we have to first initialize it. we can't use it directly in any statement.

Java Basics

What is "out" on System.out.println() ?

In the System class "out" is an static final variable of class type i.e. it's an object of the "PrintStream" class. And this printsream class contains a definition of a public method called "println()".

here is the actual representation of the classes:

//the System class belongs to java.lang package
class System {
  public static final PrintStream out;
  //...
}

//the Prinstream class belongs to java.io package
class PrintStream{
public void println();
//...
}

Pair of Numbers

C program to find presence of  pair of elements in array whose sum is equal to a given number.

#include<stdio.h>

int cmp(const void *a,const void *b)

{

return (*(int*)a-*(int*)b);

}

int main()

{

int n,i,x,j,flag=0;

printf("Enter the no. of elements in array:");

scanf("%d",&n);

int a[n];

printf("Enter the elements of array:");

for(i=0;i<n;i++)

scanf("%d",&a[i]);

printf("Enter the sum of Numbers:"); 

scanf("%d",&x);

qsort(a,n,sizeof(int),cmp);

i=0;j=n-1;

while(i<j)

{

if((a[i]+a[j])==x)

{

flag=1;

break;

}

else if((a[i]+a[j])>x) j--;

else if((a[i]+a[j])<x) i++;

}

if(flag)

printf("\nPresent");

else

printf("\nNot Present");

return 0;

}

Distinct Characters

C program to find number of distinct characters in a string.

#include<stdio.h>
int main()
{
char a[100001];

unsigned int b[26],i=0,c=0;

memset(b,0,26*sizeof(unsigned int));

printf("Enter the String: ");

scanf("%s",a);

while(a[i]!='\0')

{

b[a[i]-'a']++;

i++;

}

for(i=0;i<26;i++)

{

if(b[i]!=0) c++;

}

printf("There are total %u characters in string.\n",c);

return 0;

}

Large Factorials

 C program to find factorial of a number N <= 100 .

#include<stdio.h>
int main()
{
    int t;
    scanf("%d",&t);
    while(t--)
    {
        int a[200],m=0,i,k,n,temp=0;
    scanf("%d",&n);
        a[0]=1;
        m=1;
        while(n>1)
        {
            for(i=0;i<m;i++)
            {
                k=((a[i]*n)+temp);
                a[i]=k%10;
                temp=k/10;
            }
            while(temp>0)
            {
                a[m++]=temp%10;
                temp/=10;
            }
            n--;
        }
        for(i=m-1;i>=0;i--)
            printf("%d",a[i]);
        printf("\n");
    }
    return 0;
}