Test Scores Program

Hi All,

I believe its going to be a long night. This is a pretty long program (for me) and Im only about 1/2 way thru. Right now Im stuck because somehow one of my variables is getting changed and I cant figure out how. Ive been using this site as a resource for awhile now as I struggle thru my C courses, desperately trying to understand. I knew it was only a matter of time before I turned here for help. So, here goes...

NOTE: You will see a lot of printf statements that are left justified. These are for debugging. Sorry! But I left them in so it is also easier to tell you where the problem is. Somehow between printf("NumElems6:) and printf("NumElems7:) my numElems variable is getting changed from 10 to 4. Im using an input file (obviously) and will try to upload it, as I imagine it will probably help you see what is going on.

Thanks so much in advance for your help!

Description: Program reads from input file # of test
answers, correct test answers, Student ID, and student's
answers, checks to see if student got answer right, and
then prints the results.

Code:

---


#include<stdio.h>

int readAnswerInfo (FILE *infilep, int numElems);
int readStudentData(FILE *infilep, int numElems, int answerKeyArray[], int frequencyArray[], FILE *outfilep);


int main (void)
{
    int numElems = -1; /*number of test questions and
                    return variable from readAnswerInfo (function 1)*/
    FILE *infile; /*declares infile as a file variable*/
    FILE *outfile; /*declares outfile as a file variable*/
    int answerKeyArray[] = {0};
    int frequencyArray[] = {0};

printf("NumElems1: %d\n", numElems);

    infile=fopen("test_input1.txt", "r");
    if( infile == NULL )
    {
        printf("Input file didn't open\n");
        return 100; //exit main
    } //end

printf("NumElems2: %d\n", numElems);

    outfile=fopen("output.txt", "w");
    if(outfile==NULL)
    {
        printf("Output file didn't open\n");
        return 101; //exit main
    } //end

printf("NumElems3: %d\n", numElems);

    numElems = readAnswerInfo(infile, numElems);

printf("NumElems4: %d\n", numElems);

    if(numElems > 0 || numElems <= 100)
    {
        readStudentData(infile, numElems, answerKeyArray,
                        frequencyArray, outfile);
    }
    else
        printf("Error with input file: input1.txt\n");

printf("\nNumElems8: %d\n", numElems);
printf("\nAnswer Key: ");
for( int i = 0; i < numElems ; i++ )
printf("%d", answerKeyArray[i]);


    return 0;
}

int readAnswerInfo (FILE *infilep, int numElems)
{

    fscanf(infilep,"%d", &numElems);
printf("NumElems5: %d\n", numElems);

    if( numElems > 0 || numElems <= 100 )
        return numElems;
    else
        return 0;
}

int readStudentData (FILE *infilep, int numElems, int answerKeyArray[], int frequencyArray[], FILE *outfilep)
{
    int i;
printf("NumElems6: %d\n", numElems);

    for( i = 0; i < numElems ; ++i )
        fscanf(infilep,"%d", &answerKeyArray[i]);

printf("\nNumElems7: %d\n", numElems);
printf("Answer Key: ");
for( i = 0; i < numElems ; i++ )
printf("%d", answerKeyArray[i]);

    return 0;
}

---

> int answerKeyArray[]
C doesn't have self-expanding arrays.
At the moment, all you have is an array with one element (from your initialiser list).

Try something like this, which is 20 elements all zero.

Code:

---

    int answerKeyArray[20] = { 0 };

---

HAHA... omg, that worked! I actually used [100], because I think I read that somewhere in the instructor's looong instructions (that I spared you from, for the moment) and sure enough numElems stayed at 10, just like it's supposed to. Any idea how it was getting changed or where it was getting the '4' from?

Thank you for your quick reply and help!

P.S. You'll likely be hearing from me again with another problem in about, oh 20 mins? lol

> numElems variable is getting changed from 10 to 4.

Code:

---

//    answerKeyArray[3] goes here
    int numElems = -1; /*number of test questions and
                    return variable from readAnswerInfo (function 1)*/
//    answerKeyArray[2] goes here
    FILE *infile; /*declares infile as a file variable*/
//    answerKeyArray[1] goes here
    FILE *outfile; /*declares outfile as a file variable*/
//    answerKeyArray[0] goes here
    int answerKeyArray[] = {0};

---

So if you wrote 1 2 3 4 into the first 4 'elements' of your undersized array, then the excess elements would start overwriting other variables.

Understand that this is just a quirk of how your compiler arranged the variables in memory in this particular instance. A different compiler might arrange the variables in another order, or may even pad the ends of your array with a "dead zone", and you might never even notice (or only notice something else later on).

Having variables change "mysteriously" is usually a sign of messing up on the sizes of arrays (or later on, allocating memory through malloc).

[edit]Salem has beaten me :-) [/edit]

Quote:

---

Originally Posted by dangerous :P

Any idea how it was getting changed or where it was getting the '4' from?

---

Since you have written into memory outside of your array, you have probably overwritten the value of the variable which was stored in a memory block following the array.

Bye, Andreas

Oooh, my instructor warned against doing this and said we would be marked down, but I didnt realize NOT declaring the size of the array could have the same result. Thanks to both of you for the explanation AND helping to save my grade from unnecessary deductions! :-)

Don't you fret, AndiPersti, Ill have another question soon... you can get the next one ;-) lol

I think I am finally just about done with this program, believe it or not. Im just having trouble with one last piece of it. If theres anyone out there that is able to to help me, Id be most appreciative. :-)

The first elements of the answerKeyArray and frequencyArray print as 0 and as a result, the student scores for student 1234 and student 9012 are one number too low for Q/A #1. I did have it working just fine at one point, then I changed something to fix another issue and cant remember how to get the old working code back. lol :P famous last words, right? lol

I was going to truncate the code, but Im not sure where the problem is and what code is safe to remove or not from this post. Still using same input file I attached for the first post.

Thanks so much in advance for your help,

Code:

---


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


int readAnswerInfo (FILE *infilep, int numElems, int answerKeyArray[]);
int readStudentData (FILE *infilep, int numElems, int answerKeyArray[],
                    int frequencyArray[], FILE *outfilep);
int fillArray (FILE *infilep, int numElems, int array[]);
int correctAnswers (int answerKeyArray[], int numElems, int studentAnswersArray[],
                    int studentID, int frequencyArray[], int score);
void printStudentScores (int numElems, int answerKeyArray[],
                        int frequencyArray[], FILE *outfilep);

int main (void)
{
    //number of test questions and return variable
    int numElems = -1;        //from readAnswerInfo
    FILE *infile;                  //declares infile as a file variable
    FILE *outfile;                  //declares outfile as a file variable
    int answerKeyArray[100] = {0}; //declares & initializes array to 0
    int frequencyArray[100];        //declares array to 100 elements

    infile = fopen("test_input1.txt", "r"); //test to open input file
    if( infile == NULL )
    {
        printf("Input file didn't open\n");
        exit(100); //exit main
    } //end

    outfile = fopen("output.txt", "w"); //test to open output file
    if(outfile == NULL)
    {
        printf("Output file didn't open\n");
        exit(101); //exit main
    } //end

    //function to read in numElems
    numElems = readAnswerInfo(infile, numElems, answerKeyArray);

    //test for numElems
    if(numElems > 0 || numElems <= 100)
    {
        //function to process student data
        readStudentData(infile, numElems, answerKeyArray, frequencyArray, outfile);
    }
    else
        printf("Error with input file: input1.txt\n");

    //function to print student data and scores
    printStudentScores(numElems, answerKeyArray, frequencyArray, outfile);

    fclose(infile);                    //closes input file
    fclose(outfile);                    //closes output file

    return 0;
} // end main


int readAnswerInfo (FILE *infilep, int numElems, int answerKeyArray[])
{
    //scanning numElems
    fscanf(infilep,"%d", &numElems);

    //function to store numElems and fill array
    fillArray(infilep, numElems, answerKeyArray);

    if( numElems > 0 || numElems <= 100 )
        return numElems;
    else
        return 0;
}

int readStudentData (FILE *infilep, int numElems, int answerKeyArray[], int frequencyArray[], FILE *outfilep)
{
    int i;
    int studentID = 0;                  //student ID variable
    int studentAnswersArray[100] = {0}; //student answer array
    int score = 0;

    //for loop to initialize array elements to 0
    for(int i = 0; i <= 100 ; i++)
    {
      frequencyArray[i] = 0;
    }

    //prints headers
    printf("\nStudent ID    Score    Student Answers\n");
    for( i = 0; i < numElems; i++ )
        printf("%4d\n", studentID);  //prints studentIDs

    //loop to run function to fill student answer array
    //loop to run function to correct student answers
    //loop to print studentIDs and student answer arrays
    while (fscanf(infilep,"%d", &studentID) != EOF)
    {
        fillArray(infilep, numElems, studentAnswersArray);
        score = correctAnswers(answerKeyArray, numElems, studentAnswersArray, studentID, frequencyArray, score);

        printf("%7d\t\t%d\t", studentID, score);

        for( i = 0; i < numElems; i++)
        {
            printf("%4d", studentAnswersArray[i]);
        }
        printf("\n");
    }
    return 0;
}

int fillArray (FILE *infilep, int numElems, int array[])
{
    int i;
    //for loop to fill array
    for( i = 0; i < numElems ; i++ )
        fscanf(infilep,"%d", &array[i]);

    return 0;
}

int correctAnswers (int answerKeyArray[], int numElems, int studentAnswersArray[],
                    int studentID, int frequencyArray[], int score)
{
    int i;

    //loop to correct student scores
    for( i = 0, score = 0; i < numElems; i++)
    {
            if (answerKeyArray[i] == studentAnswersArray[i])
            {
                score++;
                frequencyArray[i]++;
            }
    }
    return score;
}

void printStudentScores (int numElems, int answerKeyArray[],
                        int frequencyArray[], FILE *outfilep)
{
    int i;
    int num;

    printf("\nQuestion #: \t\t");
    for( num = 1; num <= numElems ; num++ )
    printf("%4d", num);
    printf("\n\n");

    printf("Answer Key: \t\t");
    for( i = 0; i < numElems ; i++ )
    printf("%4d", answerKeyArray[i]);
    printf("\n\n");

    printf("Frequency: \t\t");
    for( i = 0; i < numElems ; i++ )
    printf("%4d", frequencyArray[i]);
    printf("\n\n");

    return;
}

---

> for(int i = 0; i <= 100 ; i++)
You're overrunning the ends of your arrays (again).

Do this

Code:

---

#define MAX_SIZE 100

---

Now, everywhere you have 100 in your code, you write MAX_SIZE

And it's always going to be
for ( i = 0 ; i < MAX_SIZE ; i++ )
when looping over array elements.

> int correctAnswers (int answerKeyArray[], int numElems, int studentAnswersArray[],
> int studentID, int frequencyArray[], int score)
You should make score a local variable. There is no point passing in junk, only to initialise it to 0 almost immediately.

:P :P :P :P :P :P :P :P sorry

i had a feeling thats what was going on.

Okay, so i think thats how the problem with the two values changing to 0 started; i decided to define 100 and had gone thru and changed all the 100s to SIZE. I wasnt convinced it was working and when i suspected that change was the culprit of my values now being 0, i thought maybe i better change it back; guess i was better off leaving it, though.

So, ive set the #define and changed all the 100s, and changed score to local.

Couple of questions, please?:

- Should i also switch out numElems for MAX_SIZE on lines 101, 114 and 126 above? Any others?
- How do i get my two values that got changed to 0 back?
- Im having some trouble understanding this array boundary concept (obviously). In the future, how do i make sure to stay within bounds? I had set the array size to be 100 which was much larger than I needed, but was probably also pointless after using numElems with a value of 10 when actually working with the arrays.

Thanks for your help again, Salem!

> - Should i also switch out numElems for MAX_SIZE on lines 101, 114 and 126 above? Any others?
No, those you should leave at numElems, because at some earlier point, you determined that there were so many specific elements.

It's maximum value should be MAX_SIZE, and this you need to enforce in readAnswerInfo and/or fillArray.

> if( numElems > 0 || numElems <= 100 )
This should be &&, not ||
1234567 and -9999999 would also pass the 'or' test.

> - How do i get my two values that got changed to 0 back?
Don't know - I assumed it was down to array overrun.
But if you've fixed all the loops to start at 0, and run to < (not <=), then I suggest you post your latest code.

I only actually start running code off the board when I can't spot any more problems from just reading it.

> - Im having some trouble understanding this array boundary concept (obviously). In the future, how do i make sure to stay within bounds?
Well given
type array[SIZE];
the idiomatic loop is
for ( i = 0 ; i < SIZE ; i++ )

But if you're counting the elements, then it's always <= (as you almost managed in readAnswerInfo)

Okay, i think i finally found it :P

I tried to print the Student IDs twice -- once in the for loop on lines 88-89 and then again in the while loop (line 99). Somehow, i guess was rewriting the first fscanf with 8 - 0s, maybe since i had student ID = 0?, so the first 8 lines of my output file was just a 0\n0\n0\n... and THEN it printed the file correctly, the way it was supposed to. Now that i found the reason for all the 0s, I think Im good!

I just have to say, Im sure this looks like an easy program, but it just might be the most complicated and longest one Ive ever written. Gotta say Im feelin a teensy-weensy bit proud at the moment. :D

Thanks so much for your help, Salem and AndiPersti... seriously couldn't have done it without you. This forum (and your blazing fast replies) are a lifesaver!

Im out for now... but Ill be back... guaranteed!

L8!