COMP 1917 Computing 1
Session 2, 2016

Tutorial Solutions - Week 3

1. Write a program that given three numbers first, second and third, finds out if the numbers are in strictly descending order (first > second > third). Is there an alternative way to write this program?

     #include <stdio.h>
    
     int main(int argc, char* argv[]){
        int first, second, third;
        printf("Enter numbers: ");
        scanf("%d %d %d", &num);
        if(first > second && second > third){
           printf("ASCENDING!!!!");
       }else{
           printf("What a shame:(");
       }
   
        return 0;
     }
   

     #include <stdio.h>
    
     int main(int argc, char* argv[]){
        int first, second, third;
        printf("Enter numbers: ");
        scanf("%d %d %d", &num);
        if(first > second){
           
           if(second > third){
               printf("ASCENDING!!!!\n");
           }else{
               printf("What a shame:(\n");       
           }
   
        }else{
           printf("What a shame:(\n");
        }
   
        return 0;
     }
   

2. Modify previous program to check if three numbers are in ascending or descending order.

     #include <stdio.h>
    
     int main(int argc, char* argv[]){
        int first, second, third;
        printf("Enter numbers: ");
        scanf("%d %d %d", &num);
        if(first > second && second > third){
           printf("ASCENDING!!!!\n");
        }else if(third > second && second > first)
           printf("DESCENDING!!!!\n");
        }else{
           printf("I don't know what that order is :/\n");
        }
   
        return 0;
     }
   

3. Write a program that allows the user to enter at integer and then prints out that many stars, each on a new line.

   % ./stars
   Please enter an integer: 5
   *
   *
   *
   *
   *
   

   #include 
   
   int main( void )
   {
      int num;
      int row;
   
      // Obtain input
      printf("Enter number: ");
      scanf("%d", &num);
      for( row = 1; row <= num; row++ ) {
         printf("*\n");
      }
   
      return 0;
   }
   

4. Consider the following code:

    1 #include <stdio.h>
    2
    3 int main( void )
    4 {
    5    int num;
    6    int row, col;
    7
    8    // Obtain input
    9    printf("Enter number: ");
   10    scanf("%d", &num);
   
   11    for(col = 1; col <= num; col++){
   12       printf("*");
   13    }
   14    printf("\n");
   15
   16    for(row = 2; row < num; row++) {
   17        for(col = 1; col <= num; col++) {
   18            if(col == 1 || col == num){
   19                printf("*");
   20            }else{
   21                printf(" ");
   22            }
   23        }
   24        printf("\n"); // Start a new line
   25    }
   26
   27    for(col = 1; col <= num; col++){
   28       printf("*");
   29    }
   30
   31    return 0;
   32 }
   

   What is the output if the user types in the number 4?

   ****
   *  *
   *  *
   ****
   

   Modify the program so that it prints a triangle instead of a square:

   $ ./triangle
   Enter n ? 6
   *
   **
   ***
   ****
   *****
   ******
   

   Change line 12 to:

             for(col = 1; col <= row; col++) {
   

   How would you change your program to print the triangle in different orientations?

   (a) ******   (b)  ******    (c)     *
       *****          *****           **
       ****            ****          ***
       ***              ***         ****
       **                **        *****
       *                  *       ******
   

   (a) change line 12 to:

             for(col = 1; col <= num+1-row; col++) {
   

   (b) replace lines 12-14 with:

             for(col = 1; col < row; col++) {
                 printf(" ");
             }
             while( col <= num ) {
                 printf("*");
                 col++;
             }
   

   (c) replace lines 12-14 with:

             for(col = 1; col <= num-row; col++) {
                 printf(" ");
             }
             while( col <= num ) {
                 printf("*");
                 col++;
             }
   

5. Write a function

   int isPrime( int n )
   

   which accepts a positive integer n and returns 1 if n is prime, 0 otherwise. (Recall that an integer is prime if it has no factors other than itself and 1).

   int isPrime( int n )
   {
     int result;
     int factor = 1; // 1 is always a factor
     int k=2;
     /*
        keep searching until we find a factor larger than 1,
        or we have exhausted all possibilities
     */
     while( k*k <= n && factor == 1 ) {
       if( n % k == 0 ) {
         factor = k;
       }
       k++;
     }
     // not prime if n is 1, or n has a factor larger than 1                       
     if( n == 1 || factor > 1 ) {
       result = 0;  // n is not prime                                
     }
     else {         // no factors                                    
       result = 1;  // n is prime                                    
     }
     return( result );
   }
   

6. Write a program that generates two random numbers between 1 and 107 and check if a smaller number is a factor of a larger.

   #include <stdio.h>
   #include <stdlib.h>
   #include <time.h>
   
   int main(){
       int a, b, temp;
       srand(time(NULL));
       a = rand() % 107 + 1;
       b = rand() % 107 + 1;
   
       if(a < b){
           temp = a;
           a = b;
           b = temp;
       }
   
       if(a % b == 0){
           printf("%d is a factor of %d\n",b, a);
       }else{
           printf("%d is not a factor of %d\n", b, a);
       }
   
       return 0;
   }