Changing an integer grade into a character grade using a switch statement - c

Question

Using the switch statement, write a program that converts a numerical grade into a letter
grade:
Enter numerical grade: 84
Letter grade: B
Use the following grading scale:
A = 90-100, B = 80-89, C = 70-79, D = 60-69, F = 0-59.
Print an error message if the grade is larger than 100 or less than 0.
Hint: Break the grade into two digits, then use a switch statement to test the ten's digit.
How can I change the code to let 100 become A? I input 100 and the output was F.
#include <stdio.h>
int main(void)
{
int front_number, back_number;
printf("Enter numerical grade: ");
scanf("%1d%1d", &front_number, &back_number);
printf("Letter grade: ");
switch (front_number){
case 0: case 1: case 2: case 3: case 4: case 5:
printf("F");
break;
case 6:
printf("D");
break;
case 7:
printf("C");
break;
case 8:
printf("B");
break;
case 9:
printf("A");
break;
}
return 0;
}

Answer 1

In a real world program you would likely create a struct per grade, then search through a table of read-only elements. Example:
#include <stdio.h>
typedef struct
{
int low_limit;
int high_limit;
char grade;
} grade_t;
int main (void)
{
const grade_t grade[] =
{
{ 0, 59, 'F' },
{ 60, 69, 'D' },
{ 70, 79, 'C' },
{ 80, 89, 'B' },
{ 90, 100, 'A' },
};
int input = 82;
for(size_t i=0; i<sizeof grade/sizeof *grade; i++)
{
if(input <= grade[i].high_limit)
{
printf("%d gives %c", input, grade[i].grade);
break;
}
}
}
This is a naive linear search method but since the amount of items is so limited, it's likely quite efficient. Alternatively you could use standard C bsearch, in case you have a larger amount of sorted data.
Notably, we don't even need to know the lower limit since that's implicit by the previous item in the array.

Answer 2

It is better to not read two digits, because:
This tries to read the input and process this input in one step, while these are separate processes.
If the input is 100, it will leave the last 0 character in the input stream.
The cases 10 and 100 are indistinguishable.
The hint to "break the grade into two digits" is misleading for similar reasons.
The (probably) intended solution
Here is, what I assume to be, more or less the intended solution. It reads the entire (unsigned) integer, and uses a switch for its value divided by 10. The one not-so-elegant part is the default case (more of an "else" here) which has to distinguish between valid 3-digit input (i.e. 100 and invalid input.
Technically, part of the input is now being handled by an if-statement instead of a switch, but this is the most sensible solution given the problem statement. A bit better would be to catch the edge case grade == 100 at the top of function and leave the default case to only return an error value.
#include <stdio.h>
#include <stdlib.h>
char gradeUintToChar(unsigned int grade);
int main(void)
{
printf("Enter numerical grade: ");
fflush(stdout);
unsigned int gradeUint;
if (scanf("%u", &gradeUint) != 1)
{
fprintf(stderr, "Invalid input\n");
exit(EXIT_FAILURE);
}
char gradeChar = gradeUintToChar(gradeUint);
if (gradeChar == 0)
{
fprintf(stderr, "gradeUintToChar: Invalid parameter\n");
exit(EXIT_FAILURE);
}
printf("Letter grade: %c\n", gradeChar);
}
char gradeUintToChar(unsigned int grade)
{
switch (grade / 10U)
{
case 0: case 1: case 2: case 3: case 4: case 5:
return 'F';
case 6:
return 'D';
case 7:
return 'C';
case 8:
return 'B';
case 9:
return 'A';
default:
if (grade == 100U)
return 'A';
else
return (char) 0; // Indicates failure
}
}
The GNU case ranges solution
There is a GNU extension called case ranges which allows for this to be implemented rather directly using a switch. Since this is non-standard C, this is generally not a recommended method.
Code example:
#include <stdio.h>
#include <stdlib.h>
char gradeUintToChar(unsigned int grade);
int main(void)
{
printf("Enter numerical grade: ");
fflush(stdout);
unsigned int gradeUint;
if (scanf("%u", &gradeUint) != 1)
{
fprintf(stderr, "Invalid input\n");
exit(EXIT_FAILURE);
}
char gradeChar = gradeUintToChar(gradeUint);
if (gradeChar == 0)
{
fprintf(stderr, "gradeUintToChar: Invalid parameter\n");
exit(EXIT_FAILURE);
}
printf("Letter grade: %c\n", gradeChar);
}
char gradeUintToChar(unsigned int grade)
{
switch (grade)
{
case 0 ... 59:
return 'F';
case 60 ... 69:
return 'D';
case 70 ... 79:
return 'C';
case 80 ... 89:
return 'B';
case 90 ... 100:
return 'A';
default:
return (char) 0; // Indicates failure
}
}