1. 
#include <stdio.h>

int main()
{ int x, y, i;
  char ch;

  do
  { scanf("%d", &x);
    ch = getchar();
 
    if (ch != '-' && ch != ' ')
    { printf("Wrong input!\n");
      exit(0);
    }

    if (ch == '-') 
    { scanf("%d", &y);
      if (x > y)
      { printf("Wrong input!\n");
        exit(0);
      }
      for (i=x; i<=y; i++)
        printf("%d ", i);
      ch = getchar();	// read in the next character
    }
    else // (ch == ' ') in this case
      printf("%d ", x);

  }while (ch != '\n');

  putchar('\n');
  return 0;
}

2. This is the version that does not use array

#include <stdio.h>

int main(void)
{ long int dec, oct=0, value=1;

  printf("Enter a decimal number:");
  scanf("%d", &dec);

  printf("(%d)_10 = ", dec);

// the following while loop implement the algorithm given in the hw2.txt
// you can also use an array to keep the reminders
  while( dec != 0 )
  { oct = oct + dec%8*value;
    dec = dec/8;
    value *= 10;
  }

  printf("(%d)_8\n", oct);
  return 0;
}

3. 

#include <stdio.h>

int main()
{ int a, b, power=1, i;

  printf("Enter an integer a and a positive integer b:");
  scanf("%d %d", &a, &b);

  if (b <= 0 )
    printf("b = %d is not positive.\n", b);
  else
    { for (i=0; i<b; i++)
        power *= a;
      printf("Power(%d, %d) = %d\n", a, b, power);
    }

  return 0;
}

4.

#include<stdio.h>

int main(void)
{ int a, b, c;

  do 
  { printf("Enter a binary value for a:");
    scanf("%d", &a);
    if (a!=0 && a!=1)
      printf("Invalid value a = %d\n", a);
  }while ( a!=0 && a!=1);

  do
  { printf("Enter a binary value for b:");
    scanf("%d", &b);
    if (b!=0 && b!=1)
      printf("Invalid value b = %d\n", b);
  }while ( b!=0 && b!=1);

  switch (2*a+b)
  { case 0: 
    case 3: c = 0; break;
    case 1:
    case 2: c = 1; break;
    default: printf("This should never happen!\n");
  }

  printf("c(%d,%d)=%d\n", a, b, c);
  return 0;
}