Thread: Loop Error with program HELP!!!!!

While loop is not looping the amount of times the user enters and when calculating average grade the output is incorrect. please tell me what I'm doing incorrectly?

Code:

#include <stdio.h>

int main(void)

{

/* Declaration of Variables */
/*--------------------------*/

    int    grade_counter = 0, counter[11], n = 0, i = 2;
    float  grade[100], sum = 0.0, average;
    char c;

/* Output purpose of program being to calculate averages of user input grades */
/*----------------------------------------------------------------------------*/

printf("\nThis program will calculate the average of the grades you enter!\n");

/* Prompt user to enter total number of grades user wishes to average */
/*--------------------------------------------------------------------*/

    while(grade_counter <1 || grade_counter >10)
    {
        counter[i] = -1;
printf("\nTo begin, please enter the total number of grades to process (1-10): ");
        scanf("%d", &grade_counter);
        while((c = getchar() != '\n') && c != EOF);

/* Ensure grad is between 1 and 100. If not reprompt. */
/*----------------------------------------------------*/

        if(grade_counter <1 || grade_counter >10)
printf("*** Invalid Total Entered ***\n");
        else
            ++counter[grade_counter];
    }

/* Instruct user to enter user specified number of grades and then prompt for grade to be averaged */
/*-------------------------------------------------------------------------------------------------*/

printf("\nNow enter the %i grades to be averaged.\n", grade_counter);

printf("\nEnter grade #%i (0-100): ",n+=1);
    scanf("%f", &grade[n]);
    while(( c=getchar() != '\n') && c != EOF);

    while(n = 1, n = grade_counter, ++n, grade_counter++ < 10)
    {
        sum += grade[n];
printf("\nEnter grade #%i (0-100): ", i++);
        scanf("%f", &grade[n]);
        while(( c=getchar() != '\n') && c != EOF);

/* Ensure grad is between 1 and 100. If not reprompt. */
/*----------------------------------------------------*/

        if(grade[n] <0 || grade[n] >100)
        {
printf("*** Invalid Grade Entry ***.\n");
            continue;
        }

        grade_counter++;

    }

    average = sum / grade_counter;
printf("The average of the %i grades entered is %f.", grade_counter, average);

    if(average >= 90)
printf("You have a letter grade of A.\n");
    else if (average >=80)
printf("You have a letter grade of B.\n");
    else if (average >=70)
printf("You have a letter grade of C.\n");
    else if (average >=60)
printf("You have a letter grade of D.\n");
    else
printf("You have a letter grade of E.\n");

    getchar();
}

> counter[i] = -1;
What is i here?
As far as I can see, it's the only place you use i, and it's value is 2.
So one counter is always being reset to -1.

> ++counter[grade_counter];
All the other counters are garbage when you increment them.

TIP: initialise all your arrays to 0 by doing the following.

Code:

int counter[11] = { 0 };

> while(n = 1, n = grade_counter, ++n, grade_counter++ < 10)
Write out in words what you think this is doing.

> That is interesting that the unspecified values are initialized to zero when you only specify the zeroth one.
Initialisers work like this.

Code:

int counter[11];  // you have 11 integers of junk
int counter[11] = { 0 };  // you have 11 integers, all zero
int counter[11] = { 1, 2, 3, 4, 5 };  // the remaining six values are zero.

> But I think the only array he needs is one to hold the grade scores. And since the user will be inputting all those, there is no need to initialize them.
Quite possibly, but the cost of initialising them is tiny compared to the amount of time wasted bug-hunting when things go pear shaped.

Initialising things out of habit makes for a much more predictable programming life later on.

Originally Posted by christophergray

I think it better to only initialize to something that I intend the use. That way the compiler can warn me if I missed something, like in the putchar(b). In the putchar(a), I have hidden my error from the compiler.

The compiler can tell you that you initialized things you didn't use as well:

Code:

#include <stdio.h>

int main(void) {
    int a = 'a', b = 'b';
    putchar(a);
    return 0;
}

C:\Users\jk>gcc -c -Wall  sandbox.c
sandbox.c: In function 'main':
sandbox.c:4:15: warning: unused variable 'b' [-Wunused-variable]
  int a = 'a', b = 'b';

I guess you could say that there are always ways to hide your mistakes from the compiler. To offer a different perspective on this though, initialization leads to predictable code, which leads to bugs that can be reproduced and subsequently fixed.

Not nulling pointers for example can lead to double free exploits...

> I think it better to only initialize to something that I intend the use.
How about only declaring things you intend to use?

If you didn't intend to use either a or b, then at least you would always get a consistent \0 in your output.

Only you (the programmer) can decide that putchar(a) is actually wrong, but at least the result will be consistently wrong. That much gives you something to work with when trying to debug the damn thing.

Anybody going to the trouble of using -Wall -Wextra -Wpedantic wouldn't be writing that kind of sloppy code to begin with.

whiteflags wrote:
>initialization leads to predictable code, which leads to bugs that can be reproduced and subsequently fixed.


Salem wrote:
>but at least the result will be consistently wrong


Hmm. Maybe I haven't had problems debugging because I haven't worked on any sizeable programs yet. I have only written little snippets of programs that are exercises in my textbook. It certainly makes sense that to fix a bug, you have to reproduce it.


It turns out that because of my habit of putting my variable declarations close to the first use of the variable, that I am usually initializing them. For example:

Code:

 
  // Prompt for grades to be averaged
  printf("\nNow enter the %d grade", GRADE_COUNTER);
  printf("%s", 1 == GRADE_COUNTER ? "": "s");
  printf(" to be averaged.\n\n");
  float get_valid_score(int);
  float grade[GRADE_COUNTER];
  for (int n = 0; n < GRADE_COUNTER; n++) 
    grade[n] = get_valid_score(n);

  // Compute average
  float sum = grade[0];
  for (int n = 1; n < GRADE_COUNTER; n++)
    sum += grade[n];
  assert((int)sum <= 100 * GRADE_COUNTER);
  const float AVERAGE = sum / GRADE_COUNTER;