#include<stdio.h>
int main(){
  FILE *p,*q;
  char file1[20],file2[20];
  char ch;
  printf("\nEnter the source file name to be copied:");
  gets(file1);
  p=fopen(file1,"r");
  if(p==NULL){
      printf("cannot open %s",file1);
      exit(0);
  }
  printf("\nEnter the destination file name:");
  gets(file2);
  q=fopen(file2,"w");
  if(q==NULL){
      printf("cannot open %s",file2);
      exit(0);
  }
  while((ch=getc(p))!=EOF)
      putc(ch,q);
  printf("\nCOMPLETED");
  fclose(p);
  fclose(q);
 return 0;
}

#include<stdio.h>

int main(){

  int a[10][10],i,j,sum=0,m,n;

  printf("\nEnter the row and column of matrix: ");
  scanf("%d %d",&m,&n);

  printf("\nEnter the elements of matrix: ");
  for(i=0;i<m;i++)
      for(j=0;j<n;j++)
           scanf("%d",&a[i][j]);
  printf("\nThe matrix is\n");

  for(i=0;i<m;i++){
      printf("\n");
      for(j=0;j<m;j++){
      printf("%d\t",a[i][j]);
      }
 }
 for(i=0;i<m;i++){
     for(j=0;j<n;j++){
          if(i==j)
              sum=sum+a[i][j];
     }
 }
 printf("\n\nSum of the diagonal elements of a matrix is: %d",sum);

 return 0;
}

Sample output:

Enter the row and column of matrix: 3 3
Enter the elements of matrix:
2
3
5
6
7
9
2
6
7
The matrix is
2       3       5
6       7       9
2       6       7
Sum of the diagonal elements of a matrix is: 16

#include <stdio.h>

int main() {

  int a[5][5], b[5][5], c[5][5], i, j, k, sum = 0, m, n, o, p;

  printf( "\nEnter the row and column of first matrix" );

  scanf( "%d %d", &m, &n );

  printf( "\nEnter the row and column of second matrix" );

  scanf( "%d %d", &o, &p );

  if(n!=o) {

      printf( "Matrix mutiplication is not possible" );

      printf( "\nColumn of first matrix must be same as row of second matrix" );

  }

  else {

      printf( "\nEnter the First matrix" );

      for( i=0; i<m; i++ )

      for( j=0; j<n; j++ )

           scanf( "%d", &a[i][j] );

      printf( "\nEnter the Second matrix" );

      for( i=0; i<o; i++ )

      for( j=0; j<p; j++ )

           scanf( "%d", &b[i][j] );

      printf( "\nThe First matrix is\n" );

      for( i=0; i<m; i++ ) {

      printf("\n");

      for( j=0; j<n; j++ ) {

           printf( "%d\t", a[i][j] );

      }

      }

      printf( "\nThe Second matrix is\n" );

      for( i=0; i<o; i++ ) {

      printf("\n");

      for( j=0; j<p; j++ ) {

           printf( "%d\t", b[i][j] );

      }       

      }

      for( i=0; i<m; i++ )

      for( j=0; j<p;j++ )

           c[i][j] = 0;

      for( i=0; i<m; i++ ) { //row of first matrix

      for( j=0; j<p; j++ )  {  //column of second matrix

           sum = 0;

           for( k=0; k<n; k++ )

               sum = sum + a[i][k]*b[k][j];

           c[i][j] = sum;

      }

      }

  }

  printf( "\nThe multiplication of two matrix is\n" );

  for( i=0; i<m; i++ ) {

      printf("\n");

      for( j=0; j<p; j++ ) {

           printf( "%d\t", c[i][j] );

      }

  }

  return 0;

}

 #include<stdio.h>
int main(){
  int r,i,j,k;
  printf("Enter the number range: ");
  scanf("%d",&r);
  for(i=1;i<=r;i++){
      for(j=1;j<=10;j++)
           printf("%d*%d=%d ",i,j,i*j);
      printf("\n");
  }
  return 0;
}

Sample Output:
Enter the number range: 5

1*1=1 1*2=2 1*3=3 1*4=4 1*5=5 1*6=6 1*7=7 1*8=8 1*9=9 1*10=10
2*1=2 2*2=4 2*3=6 2*4=8 2*5=10 2*6=12 2*7=14 2*8=16 2*9=18 2*10=20
3*1=3 3*2=6 3*3=9 3*4=12 3*5=15 3*6=18 3*7=21 3*8=24 3*9=27 3*10=30
4*1=4 4*2=8 4*3=12 4*4=16 4*5=20 4*6=24 4*7=28 4*8=32 4*9=36 4*10=40
5*1=5 5*2=10 5*3=15 5*4=20 5*5=25 5*6=30 5*7=35 5*8=40 5*9=45 5*10=50

 #include<stdio.h>
int main(){
    int k,r;
    long int i=0l,j=1,f;

    //Taking maximum numbers form user
    printf("Enter the number range:");
    scanf("%d",&r);

    printf("FIBONACCI SERIES: ");
    printf("%ld %ld",i,j); //printing firts two values.

    for(k=2;k<r;k++){
         f=i+j;
         i=j;
         j=f;
         printf(" %ld",j);
    }
 
    return 0;
}

Sample output:
Enter the number range: 15
FIBONACCI SERIES: 0 1 1 2 3 5 8 13 21 34 55 89 144 233 377

#include
int main( int count,char * argv[] ) {
    int i;
    FILE *ptr;
    char *str;
    char ch;
    if( count == 1) {

         printf( "The syntax of the command is incorrect.\n" );
    }
    for( i=1;i<cout;i++ ){
         ptr=fopen(argv[i],"r");
         if(ptr==NULL){
             printf("The system cannot find the file specified.");
             if(count>2)
                 printf("\nError occurred while procesing : %s.\n",argv[i]);
         }
         else {
             if(count>2) {
                 printf("%s\n\n",argv[i]);
             }
             while((ch=getc(ptr))!=-1)
                 printf("%c",ch);
         }
         fclose(ptr);
    }
    return 0;
}

Save the above file as open.c, compile and execute the go to command mode (current working directory) and write: open xy.c (xy.c any file present in that directory)
To run the open command in all directories and drive you will have to give the path of current working directory in command mode. Write:
C:tc\bin>PATH c:\tc\bin
Now press enter key. Now your open command will work in all directory and drive.

Size of any array in c cannot be constantan variable.

We cannot use special character – in the variable name.
Variable name can have only underscore.