C Basics : Grade Book Using Array Storing and Printing, Plus Looping - c

Question

Yes, hello. I'm really new to programming. I genuinely need help because I would like to both understand what I'm doing wrong and also pass my class.
I'm in a Intro to Programming class and my assignment is to create a program that uses an array to store percentage grades in the range from 0 to 100 (inclusive). The program should allow the user to indicate when they are done entering grades. When the user is done entering grades, the program should print out the grades entered.
I have a running code, compiled in Code::Blocks. But my questions/problems are:
A.) What's the best way to tell C when the user is done? Should I
Leave the code as is and let any key be hit?
Add a specific variable like 'done'?
Do something else?
B.) How do I print only the grades that have been entered without it going through all 100 slots? I can't for the life of me find the solution.
Any and all suggestions welcome and appreciated!!!
int i = 0;
float percentScore[100];
for (i = 0; i < 10; i++) {
printf("Grade %d: ", i + 1);
scanf("%f", &percentScore[i]);
}
for (i = 0; i < 10; i++) {
printf("\n%.2f%%", percentScore[i]);
}
return 0;

Answer 1

Your choices for A) are not mutually exclusive; the first is something the user can do, the second is a way to represent that in the code. As such, you could reasonably do both.
As for B), you need a way to represent the number of grades entered (hint: a variable); this can then be used to control how many get printed.

Answer 2

Here is a simple approach for your problem:
for each input, read a full line from the user ;
try and convert this line as a number with sscanf() ;
if the conversion fails, consider the input exhausted ;
if the value is out of bounds, restart this input.
The user can thus signal the end of the list by entering a blank line.
Here is a example:
#include <stdio.h>
#define GRADE_NB 100
int main(void) {
int i, n;
float grade, percentScore[GRADE_NB];
char buf[100];
for (n = 0; n < GRADE_NB;) {
printf("Grade %d: ", n + 1);
if (!fgets(buf, sizeof buf, stdin)) {
/* end of file reached: input exhausted */
break;
}
if (sscanf(buf, "%f", &grade) != 1) {
/* not a number: input exhausted */
break;
}
if (grade >= 0 && grade <= 100) {
/* value is OK, grade accepted */
percentScore[n] = grade;
n++;
} else {
printf("Invalid grade: %f\n", grade);
}
}
for (i = 0; i < n; i++) {
printf("%.2f%%\n", percentScore[i]);
}
return 0;
}
If the potential number of inputs is not limited, you must allocate the array from the heap and reallocate it as more inputs are gathered. Here is a simple solution for this where the array is reallocated for each input:
#include <stdio.h>
#include <stdlib.h>
int main(void) {
int i, n;
float grade, *percentScore;
char buf[100];
for (n = 0, percentScore = NULL;;) {
printf("Grade %d: ", n + 1);
if (!fgets(buf, sizeof buf, stdin)) {
/* end of file reached: input exhausted */
break;
}
if (sscanf(buf, "%f", &grade) != 1) {
/* not a number: input exhausted */
break;
}
if (grade >= 0 && grade <= 100) {
/* value is OK, grade accepted */
percentScore = realloc(percentScore, (n + 1) * sizeof(*percentScore));
if (percentScore == NULL) {
printf("allocation failed\n");
return 1;
}
percentScore[n] = grade;
n++;
} else {
printf("Invalid grade: %f\n", grade);
}
}
for (i = 0; i < n; i++) {
printf("%.2f%%\n", percentScore[i]);
}
free(percentScore);
return 0;
}