How can I integrate modularity into my code in C?

Question

I don't know much about modularity except it's basically dividing up your code into smaller groups of functions (this question is for the C language).

I'm doing an assignment where I can only see my source code working with one giant nested loop in "main" (no if statements are allowed for the assignment). In other words, if I try and use functions to cut up the code, I don't see how the necessary nested loop will work. But the assignment requires an attempt at modularity.

Thus, can anyone tell me how I might break up the following code into smaller modules without messing up its actual execution?

#include <stdio.h>

int main(void)
{
   int counter = 0;
   int marknum = 0;
   int sectioncode, studentnumber;
   int dummyvariable = 0;
   int index;
   int marks;
   int total = 0;

   do
   {
      printf("Enter a Section Code: ");
      scanf("%d", &sectioncode);

      while(sectioncode > 4 || sectioncode < 1)
      {
         printf("Invalid value entered. Must be 1 to 4, please re-enter: ");
         scanf("%d", &sectioncode);
      }

      do
      {
         printf("Enter the Student's ID: ");
         scanf("%d", &studentnumber);

         while (studentnumber < 1 || studentnumber > 999999999)
            {
               printf("Invalid value entered. Must be 1 to 999999999. Please re-enter: ");
               scanf("%d", &studentnumber);
            }

         while (sectioncode != 0)
        {
           while (counter < 5)
           {
              counter++;
              marknum++;
              printf("Enter mark%d: ", marknum);
              scanf("%d", &marks);
              total = total + marks;
           }

           printf("%09d's total mark is %d\n", studentnumber, total);
           counter = 0;
           marknum = 0;
           sectioncode = 0;
        }

      dummyvariable = 1;

   } while (dummyvariable = 0);

} while (sectioncode != 0);

   return 0;
}

Also, how would I incorporate modularity for this one (same question basically):

#include <stdio.h>

int main(void)
{
   int num;                                                      //User inputted number
   int i;                                                        //Increment variable
   char ch;                                                      //Check for characters variable

      do                                                         //Begin "do while" loop
      {
      printf("\nEnter a number:");                               //User prompt
      scanf ("%d", &num);                                        //Scan for user inputted integer
      while ( (ch = getchar()) != '\n')                          //Scan for character, check for non-numeric input
         {
            printf("Invalid number entered. Please re-enter: "); //Error message and prompt for invalid user input
            scanf ("%d", &num);                                  //Scan for user inputted integer
         }                                                       //Repeat loop if condition becomes true again

      for (i=0; i<num; i++)                                      //Begin "for" loop; condition prints asterisks equal to user number; increment i by 1
         {
            printf("*");                                         //Prints a single asterisk each loop until i is less than num
         }
      } while (num!=0);                                          //Stop "do while" loop if user enters 0

return 0;
}

Answer 1

Normally I'd suggest that you ask your instructor instead of asking homework questions here, but as Daniel points out in the comments, the use of loops and extra variables just to avoid having if statements in the code is stupid, and I'm not sure telling you to get advice from an instructor who thought that was a good idea would be entirely responsible behavior on my part. So, having said that:

What you want to look for in cases like this is multiple chunks of similar code, or chunks of code that conceptually do a single thing. Then see if you can split those chunks out into a function.

In the first example, you display a prompt, read user input, and verify the input for both sectioncode and studentnumber. That process could be split into a separate function. (Everything from printf("Enter ...") through the end of the while loop.) Then in the main function, you just have something like

sectioncode = readval("Enter a Section Code: ", 1, 4);
studentnumber = readval("Enter the Student's ID: ", 1, 999999999);

For the second example, that input/validation code isn't duplicated, but it's still probably worth splitting out into a function, since it does a single well-defined thing, and spans enough lines that splitting it out into a function might help make the logic of the remaining code more clear. But it's less important in this case than in the first.

---

Also, an unrelated issue: At the end of one of the do-whiles, you have while (dummyvariable = 0);. Note the single equal sign. You're assigning 0 to dummyvariable, not comparing.

In this particular case, it works anyway, since the whole expression evaluates to 0 (i.e. false), just like (dummyvariable == 0) would have. But if that constant had been anything else, or if you hadn't just set dummyvariable to 1 prior to the end of loop, that'd be a bug.

I strongly recommend always putting the constant on the left hand side of expressions like that in order to catch bugs of that sort at compilation. while (dummyvariable = 0) silently does something unexpected; while (0 = dummyvariable) will give an error at compile-time, and you can fix it.