Worksheet: C0

Worksheets are self-guided activities that reinforce lectures. They are not graded for accuracy, only for completion. Worksheets are due by Sunday night before the next lecture.

• Github Classroom Link: https://classroom.github.com/a/DNgH-Fa9

Questions

1. Write a program called youshotme.c that prints out the following message:

   You shot me! Ugh. Oh. Ow. Ow.
   .
   .
   .
    .-.
   (o o)
   | O \
    \   \
     `~~~'
   

   Add the program to your repo.

   Reveal Solution

   //youshoutme.c
   #include <stdio.h>
   int main(){
       printf("You shot me! Ugh. Oh. Ow. Ow.\n");
       printf(".\n");
       printf(".\n");
       printf(".\n");
       printf(".-.\n");
       printf("(o o)\n");
       printf("| O \\\n");
       printf(" \\   \\\n");
       printf("  `~~~' \n");
   }
   

2. Compile the program above with gcc.

   • What gcc command did you use?
   • Did you have any errors? If so, note them and how you fixed them.
   • If you had no errors, introduce one (like removing a ;) and note the error message.

   Reveal Solution

   • gcc youshoutme.c -o youshoutme
   • Possible errors could be that the \ needed to be escaped \\ to actually print \
   • UNIQUE TO YOU

3. The following program should not compile… fix it and explain why.

   int main(){
     printf("Why oh why does this program fail to compile?!?!\n");
   }
   

   Reveal Solution

   Failed to include `#inlude <stdio.h>`, this is required for `printf()`
   

4. We were expecting the following output

   They're not equal :-| 
   They're not equal :-| 
   They're not equal :-| 
   They're not equal :-| 
   They're not equal :-| 
   They're equal :-) 
   They're not equal :-| 
   They're not equal :-| 
   They're not equal :-| 
   They're not equal :-| 
   

   But it’s not working :( … Can you fix the program below to get it to work?

   int main(){
     int a = 10;
     int b = 5;
     while(a > 0){
       if(a = b){
         printf("They're equal :-) \n");
       }else{
         printf("They're not equal :-| \n");
       }
       a-=1;
     }
   }
   

   Describe what was wrong?

   Reveal Solution

   a=b is an assignment, not an equality check. Since a=b is assignment, the value of the assignment is returned, and since it’s not 0, it’s true, so the statement They're equal :-) always print.

5. What value(s) is/are true in C? What value(s) is/are false in C?

   Reveal Solution

   Values that are not 0 are true. There are many types of 0, all false. For example, integer 0, character \0, and floating point NULLx.

6. Convert the following code snippet into a function deceleration and definition.

   int minus(int a, int b){return a-b;}
   

   Reveal Solution

   int minus(int,int);
   //..some code, like main()
   
   int minus(int a, int b){ return a-b;}
       
   

7. The following three programs will not compile when trying

   gcc main.c one.c two.c
   

   Fix the issues. The programs are below:

   //funcs.h
   void printOne();
   void printTwo();
   

      //one.c
      void printOne(){
        printf("one\n");
      }
   

      //two.c
      void printTwo(){
        printf("two\n");
      }
   

      //main.c
   
      int main(){
       for(int i=0;i<10;i++){
          if(i%2){
            printOne();
          }else{
            printTwo();
          }
       }
      }
   

   Reveal Solution

   Missing the #include statements

        //main.c
        #include "funcs.h"
   

   Note that it needs to be in quotes "funcs.h"

8. Below are Java print statements. Write the C equivalent print statement.

   int a = 5;
   int b = 2;
   float c = 4.4;
   System.out.println("a = "+a+" b="+b+" c="+c);
   

   Reveal Solution

   //main.c
   int a = 5;
   int b = 2;
   float c = 4.4;
   printf("a = %d b=%d c=%f\n",a,b,c);
   

9. What is the differences between fopen() file mode w and w+?

   Reveal Solution

   • w opens in write mode, and will create the file if it doesn’t exist
   • w+ open a file for reading and writing mode, and will create the file if it doesn’t exist

10. The C program below doesn’t write anything to a file.

    int main(){
      FILE * stream = fopen("helloworld.txt", "r");
      fprintf(stream, "Hello World!\n");
      fclose(stream);
    }
    

    Fix the program and describe the error(s).

    Reveal Solution

    The file needs to be open in write mode.

    FILE * stream = fopen("helloworld.txt", "w"); //<--
    

11. What is the output of this C program?

    int main(){
       float f = 3.14;
       int n = 10;
       printf("%d %f\n",f,n);
    }
    

    Was it what you expected or not? Try and describe the process by which the output was achieved.

    Reveal Solution

    The output is 3 10.0 because the float f is converted to an integer, rounded down to 3 and the integer n is converted to a float 3.0 for printing. That’s because we used %d and %f.

12. Consider the format directive %.3g, use man 3 printf to describe what the output would be if the input was 3.141592

    Reveal Solution

    The %g format formats a floating point number, like %f. The .3 says to only include three significant digits (the . is ignored in %g), and so that means 3.15 prints (three sig digits).

13. You’re opening a file, and you get an error!?! Provide two preferred ways to report the error to stderr, as in …

    if( fopen( /* ... */) == NULL){
      // WHAT GOES HERE?!
    }
    

    Reveal Solution

    You could use fprintf()

    fprintf(stderr,"fopen: ERROR Cannot open file\n"); 
    

    Or use perror()

    perror("fopen");
    

14. Fix and describe the error in the code below.

    printf("Enter a number: ");
    
    int num;
    scanf("%d", num);
        
    printf("You entered %d\n", num);
    

    Reveal Solution

    It’s missing an & to indicate to store the scanned value at the address of num

    scanf("%d", &num);
    

15. Fix and describe the error in the code below.

    printf("Enter a number: ");
    
    int num;
    scanf("%f", &num);
        
    printf("You entered %d\n", num);
    

    Reveal Solution

    scanf("%d", &num);