Thread: Question about Arrays and program flow

My challenge from C Programming of for the Absolute Beginner (which accurately describes me)

Create a student GPA average calculator. The program should
prompt the user to enter up to 30 GPAs, which are stored in a
single-dimension array. Each time he or she enters a GPA, the
user should have the option to calculate the current GPA
average or enter another GPA. Sample data for this program is
shown below.
GPA: 3.5
GPA: 2.8
GPA: 3.0
GPA: 2.5
GPA: 4.0
GPA: 3.7
GPA Average: 3.25
Hint: Be careful to not calculate empty array elements into
your student GPA average.

I have really no clue what I am doing. A friend of mine who worked for a decade as a computer programmer gave me this advice, which I tried to code:

I asked him:

I am trying to write a program that does the following:

Code:

While !inputdone
{
Accepts inputs into an array
Checks for digits and rejects alpha
Once there are more than two inputs stored in the array
    {
    offer the option to average inputs, or
    continue entering input
    }

}

He said:

Code:

While !inputdone
{
Ask for grade
Check for numeric
If numeric {
    Put input in array
    increment array index
    If two or more numbers have been entered {
        ask if user wants to average
        if answer is yes {
            average
            print result
            return(0);
        }
    }
}

And I have this so far (note I have not yet used iInputDone, and I don't even really know how I would do so - please be gentle, I know it is riddled with mistakes, but although a competent and logical person, I am crushed under the feet of computer programming)

Code:

#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>

int checkisdigit(int), i, iInput;
char cInput;

void main(void)
{
    int iGrades[30] = {0}, iInput = 0, i = 0, iDoneInput = 0;
    char cAverage;

    printf("ENTER STUDENT GRADES\n");
    system("pause");
    system("cls");


    while (!iDoneInput)
    {
        entergrade:
        system("cls");
        iInput = 0;
        printf("\nPlease enter a grade: ");
        scanf("%c", &cInput);

        if (checkisdigit(cInput))
        iInput = cInput - '0';
        else
        goto entergrade;

        for (i = 1; i <= 30; i++);
        iGrades[i] = iInput;

        {
            if (i >= 2)
            {
                calculateaverage:
                printf("\nCalculate (A)verage?");
                scanf("%c", &cAverage);
            {
                if (!checkisdigit(cAverage))
                {
                    if ((cAverage == 'A') || (cAverage == 'a'))
                    {
                        printf("\nAverage is %d", iGrades[i] += 0 / i);
                        return 0;
                } // end if (cAverage == A)

                else
                goto entergrade;
            }  // end if (!checkisdigit)

                else
                goto calculateaverage;
        } // end calculateaverage

    } // end if i>2

} // end for

} // end while

} // end main


/*******\
Functions
********/

int checkisdigit(int iMyDigit)
{
    if ((iMyDigit >= '0') && (iMyDigit <= '9'))
    return 1;
    else
    return 0;
} // end checkisdigit

The main problem is this: In runtime I enter a value but it displays the (A)verage? question for a split second, even after the first entry. And I have no idea how to check for a digit, then accept the value into the array.


Please teach me! Thanks in advance.

David

Checking for digits is easy... just include ctype.h and use the standard isdigit() library function...

Code:

#include <ctype.h>

int main (void)
  { int x, y; 
     char input[80];
     int number = 0;

    // get stuff from user... (fgets() works well)

     // check for all digits.
     y = strlen(input)
     for (int x = 0; x < y; x++)
       { if (! isdigit(input[x]))
           { printf ("Entry Error!  Numbers only!"); 
              exit (1); } }

     // here we assume it's all digits
     sscanf(input, "%d", number)
     // process number here

     return 0; }

It would seem your primary issue here is not so much writing code, you seem to have a pretty decent idea of that... Where your problem appears to lie is with program design. Before you start coding you should sit down with pencil and paper and work out a step by step procedure to solve the problem. Never mind trying to write C code... just figure out how to solve the problem. Next you should try to simplify your solution to it's minimum nuber of steps... Once you can write the solution in your native language, then you should begin designing your C code...

Look for standard library functions whenever you can (no need to reinvent the wheel) and write functions for anything that either A) needs private variables or B) is repeated in your code. In your case getting the input is an excellent candiate for a function... you could make it read a string, validated it as all digits, extract an integer (or float, as you need) from the string and actually return the number back to your main program.

So, summarizing, two bits of advice...

1) Understanding the problem is always the first step in creating a solution... There is no programmer, anyplace on this planet, no matter how experienced or skilled, who can successfully code the solution to a problem he or she does not understand.

2) Think in smaller blobs. A program is not one continuous run of code. It is the sum of many smaller bits of code that work together to act out the solution to your problem.

My, my....where to begin:

Code:

#include <stdio.h>
#include <stdlib.h>
#include <ctype.h>

int checkisdigit(int), i, iInput;
char cInput;

You should move your variables inside of your main function. There is no need for global scope here.

Code:

void main(void)

The main function always needs to return an int. Read about why void main is wrong

Code:

system("pause");
system("cls");

I wouldn't worry about this "fancy" stuff until you learn some basic C.

There is never a reason to use goto. Sit down, think about what you want to do and restructure your code to prevent this. I believe the psuedocode your friend suggested is close enough to go with.

Why did you include the ctype library if you aren't going to use anything in it. Hint: There is already a built in function, maybe you should take a look at isalpha() and isdigit() functions. The FAQs on this site are cool. Also I would read up on atoi().

Look through that stuff and maybe even some of the other tutorials on this site and then repost some revised code. I am sure we will all help. What we will not do is to take spaghetti code that is broken and do a complete rewrite.

Fair enough.......I have only ever seen it with error trapping. Which as you pointed out is not our case here. Thanks for keeping me honest.

Isn't that the truth....

I enjoy that in the atmosphere here though, it helps satisfy my OCD.

We look forward to seeing it. Here is another tutorial to take a look at.

-Happing Coding

I am glad to have helped. Feel free to post questions when you need to.


-Happy Coding.