C Program Limit the integer input range - c

Question

I am a new C programming learner, I am trying to make a program which would calculate the GPA of a student on the basis of grade marks input and credits of a subject.
The problem I am having is I want to limit the number of subjects input from 2 to 6 only.
Another problem is I want to limit the user to input integer from 1 to 100 only, instead of any other keywords, special characters (EOF)
I have put the "###" in comment line where I require these modifications.
#include <stdio.h>
int main(void) {
// input user input -- hopefully a number
// temp used to collect garbage characters
// status did the user enter a number?
// counter for keeping track of loop repetition
// no no. of subjects to be entered by user.
// credits credits per subject
// grades grades acheived in each subject (1 to 100).
// grade_value for holding the value of each subject grade (for ex; 80 to hundred is 4.0)
// grade_points Grade points for each subject (credits * grade_value)
// sum sum of total grade points
int counter = 1, subjects, no, credits, grades, status, temp;
float grade_value, grade_point, sum;
printf("Enter number of subjects you took for current semester: ");
status = scanf("%d", & no);
// ### I want to limit this integer input to be >=2 && <=6.
while (status != 1) {
while ((temp = getchar()) != EOF && temp != '\n');
if ((temp < 2) && (temp > 6));
break;
printf("Invalid input... please enter the number of subject again: ");
status = scanf("%d", & no);
}
// ### I want to be this input to block other character inputs than integer from 1 to hundred.
while (counter <= no) {
printf("\nEnter Subject %d grades separated with credits \n", counter);
scanf("%d %d", & grades, & credits);
if ((grades > 0) && (grades <= 29)) {
grade_value = 0;
printf("Grade value for subject %d is: %.2f", counter, grade_value);
grade_point = credits * grade_value;
printf("\nGrade point for subject %d is: %.2f", counter, grade_point);
sum = sum + grade_point;
++counter;
} else if ((grades >= 30) && (grades <= 34)) {
grade_value = 0.67;
printf("Grade value for subject %d is: %.2f", counter, grade_value);
grade_point = credits * grade_value;
printf("\nGrade point for subject %d is: %.2f", counter, grade_point);
sum = sum + grade_point;
++counter;
} else if ((grades >= 35) && (grades <= 39)) {
grade_value = 1;
printf("Grade value for subject %d is: %.2f", counter, grade_value);
grade_point = credits * grade_value;
printf("\nGrade point for subject %d is: %.2f", counter, grade_point);
sum = sum + grade_point;
++counter;
} else if ((grades >= 40) && (grades <= 44)) {
grade_value = 1.33;
printf("Grade value for subject %d is: %.2f", counter, grade_value);
grade_point = credits * grade_value;
printf("\nGrade point for subject %d is: %.2f", counter, grade_point);
sum = sum + grade_point;
++counter;
} else if ((grades >= 45) && (grades <= 49)) {
grade_value = 1.67;
printf("Grade value for subject %d is: %.2f", counter, grade_value);
grade_point = credits * grade_value;
printf(" \nGrade point for subject %d is: %.2f", counter, grade_point);
sum = sum + grade_point;
++counter;
} else if ((grades >= 50) && (grades <= 54)) {
grade_value = 2;
printf("Grade value for subject %d is: %.2f", counter, grade_value);
grade_point = credits * grade_value;
printf("\nGrade point for subject %d is: %.2f", counter, grade_point);
sum = sum + grade_point;
++counter;
} else if ((grades >= 55) && (grades <= 59)) {
grade_value = 2.33;
printf("Grade value for subject %d is: %.2f", counter, grade_value);
grade_point = credits * grade_value;
printf("\nGrade point for subject %d is: %.2f", counter, grade_point);
sum = sum + grade_point;
++counter;
} else if ((grades >= 60) && (grades <= 64)) {
grade_value = 2.67;
printf("Grade value for subject %d is: %.2f", counter, grade_value);
grade_point = credits * grade_value;
printf("\nGrade point for subject %d is: %.2f", counter, grade_point);
sum = sum + grade_point;
++counter;
} else if ((grades >= 65) && (grades <= 69)) {
grade_value = 3;
printf("Grade value for subject %d is: %.2f", counter, grade_value);
grade_point = credits * grade_value;
printf("\nGrade point for subject %d is: %.2f", counter, grade_point);
sum = sum + grade_point;
++counter;
} else if ((grades >= 70) && (grades <= 74)) {
grade_value = 3.33;
printf("Grade value for subject %d is: %.2f", counter, grade_value);
grade_point = credits * grade_value;
printf("\nGrade point for subject %d is: %.2f", counter, grade_point);
sum = sum + grade_point;
++counter;
} else if ((grades >= 75) && (grades <= 79)) {
grade_value = 3.67;
printf("Grade value for subject %d is: %.2f", counter, grade_value);
grade_point = credits * grade_value;
printf("\nGrade point for subject %d is: %.2f", counter, grade_point);
sum = sum + grade_point;
++counter;
} else if ((grades >= 80) && (grades <= 100)) {
grade_value = 4;
printf("Grade value for subject %d is: %.2f", counter, grade_value);
grade_point = credits * grade_value;
printf("\nGrade point for subject %d is: %.2f", counter, grade_point);
sum = sum + grade_point;
++counter;
}
// To print a message if user doesnt enter an integer varying from 1 to 100.
else {
printf("\n Error Grade input, Please Key in Again. (1 to 100 only.)");
}
}
printf("\n");
printf("\n");
printf("\nThe GPA is: %.2f", sum);
if (sum <= 49) {
printf("\nYou can register for 2 subjects for next semester.");
} else if ((sum >= 50) && (sum >= 79)) {
printf("\nYou can register for 5 subjects for next semester.");
} else if ((sum >= 80) && (sum <= 100)) {
printf("\nYou can register for 6 subjects for next semester.");
}
printf("\n");
printf("\n");
printf("\n_______________________________________________________");
printf("\nEnd of program");
return 0;
}

Answer 1

A do-while loop is perfect for loops that need to be called at least once and will get rid of your duplicated code. Reading unstructured input, possibly from the keyboard, (one could call, ./a.out < text.txt,) is actually something that is tricky to get right.
Luckily, the C FAQ has lots of advice, eg, http://c-faq.com/stdio/scanfprobs.html. It's going to be very arduous without functions, though. From limited testing, I'm pretty sure that this is a solid way to read the first variable.
#include <stdio.h> /* fgets sscanf */
#include <stdlib.h> /* EXIT_ printf fprintf */
#include <string.h> /* strlen */
int main(void) {
int no;
/* Input number, no \in [2, 6], and make sure that the read cursor is on
the next line. */
do {
char buffer[80];
size_t len;
printf("Enter number of subjects you took for current semester, [2, 6]: ");
/* On the advice of http://c-faq.com/stdio/scanfprobs.html, this reads
into a buffer first. */
if(!fgets(buffer, sizeof buffer, stdin)) {
if(feof(stdin)) {
fprintf(stderr, "Premature EOF.\n");
} else {
/* On IEEE Std 1003.1-2001-conforming systems, this will print
a meaningful error. */
perror("stdin");
}
/* Can't really do anything interactive once stdin has a read
error. */
return EXIT_FAILURE;
}
/* This is always going to be true, but segfaults if not. Paranoid. */
if(!(len = strlen(buffer))) continue;
/* Normally fgets stores a '\n' at the end; check. */
if(buffer[len - 1] != '\n') {
/* Check if the length of the buffer is big enough to hold input. */
if(len >= sizeof buffer - 1) {
int c;
fprintf(stderr, "Line too long.\n");
/* Flush whole line. http://c-faq.com/stdio/stdinflush2.html */
while((c = getchar()) != '\n') {
if(c != EOF) continue;
if(feof(stdin)) fprintf(stderr, "Premature EOF.\n");
else perror("stdin");
return EXIT_FAILURE;
}
continue;
} else {
/* Non-POSIX lines, eg, file without '\n' terminating. */
fprintf(stderr, "Note: line without line break detected.\n");
}
}
/* Parse buffer for a number that's between [2, 6]. Ignore the rest. */
if(sscanf(buffer, "%d", &no) != 1 || no < 2 || no > 6) {
fprintf(stderr, "Invalid input.\n");
continue;
}
/* Now no \in [2, 6]. */
printf("no: %d\n", no);
break;
} while(1);
return EXIT_SUCCESS;
}
One may be able to get away with a subset of this if your teacher enters only well-formed numbers.

Answer 2

Your code has some semantic mistakes. You must never put a semicolon after a if statement. Also you can write a more legible code by spacing blocks of code. Even better, you could refactor some blocks and encapsulate it into functions. Aside of that, see this solutions:
For the first problem, it can be solved with few lines of code. It's a good practice that the user know the restrictions of inputs before enter his input:
/* ### I want to limit this integer input to be >=2 && <=6. */
#include <stdio.h>
int main()
{
int no;
do {
printf("Enter number of subjects you took for current semester (2~6): ");
scanf("%d", &no);
} while (no < 2 || no > 6);
return 0;
}
The other issue can be solved by using the Standard C Library. Just #include <string.h> and #include <ctype.h>
#include <stdio.h>
#include <string.h> /* strlen() */
#include <ctype.h> /* isdigit() */
int main()
{
int n, invalidInput, status;
char strInput[5];
int i; /* for loop */
do {
invalidInput = 0;
printf("Enter a **number** between 1 and 100: ");
fflush(stdin);
status = scanf("%4s", strInput); /* read at maximum 4 chars from stdin */
if (status > 3) {
continue; /* try again */
}
for (i = 0; i < strlen(strInput); ++i) { /* ensure that all characters are numbers */
if (!isdigit(strInput[i])) {
invalidInput = 1;
break;
}
}
if (!invalidInput) {
n = atoi(strInput); /* now your input is a integer */
if (n < 1 || n > 100) {
printf("Error: the input must be between 1 and 100.\n");
invalidInput = 1;
}
}
} while (invalidInput); /* repeat until all chars are digits and they are in the required interval */
return 0;
}