Input Validation in C When Calculating Grades - c

Question

I have a problem that needs to calculate the grade needed on a final exam in order to get the letter grade that they desire. I have the code correct for the calculations but I need to account for all invalid user input. I have accounted for negative grades and letter grades that don't exist but I can't figure out how to make sure the percentages they are inputting don't contain letters or other characters. For example if I ask for their current grade in the class they cannot input something like 95.6asi!. This is the code I have right now but there are many errors.
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <ctype.h>
void calcGradeNeededOnFinal() {
double percentWanted = 0.0;
double currentGrade, finalWeight;
char gradeWanted;
printf("Enter the grade you want in the class: ");
scanf("%c", &gradeWanted);
if (gradeWanted == 'A' || gradeWanted == 'a'){
percentWanted = 90.0;
}
else if (gradeWanted == 'B' || gradeWanted == 'b'){
percentWanted = 80.0;
}
else if (gradeWanted == 'C' || gradeWanted == 'c'){
percentWanted = 70.0;
}
else if (gradeWanted == 'D' || gradeWanted == 'd'){
percentWanted = 60.0;
}
else if (gradeWanted == 'F' || gradeWanted == 'f'){
percentWanted = 0.0;
}
else {
printf("Unknown Grade Received: %c. Ending Program.\n", gradeWanted);
exit(0);
}
printf("Enter your current percent in the class: ");
scanf("%lf", &currentGrade);
if(currentGrade < 0) {
printf("The number you last entered should have been positive. Ending program.\n");
exit(0);
}
char gradeString = (char)currentGrade;
for(int i=0; i < strlen(gradeString); ++i) {
if(!(isdigit(gradeString[i])) && (strcmp(gradeString[i], '.') != 0))) {
printf("Invalid formatting. Ending program.\n");
exit(0);
}
}
printf("Enter the weight of the final: ");
scanf("%lf", &finalWeight);
if(finalWeight < 0) {
printf("The number you last entered should have been positive. Ending program.\n");
exit(0);
}
char weightString = (char)finalWeight;
for(int i=0; i < strlen(weightString); ++i) {
if(!(isdigit(weightString[i])) && (strcmp(weightString[i], '.') != 0))) {
printf("Invalid formatting. Ending program.\n");
exit(0);
}
}
// this calculates the grade need on the final test to get the desired grade of the user
double gradeNeededOnFinal = (percentWanted - (currentGrade * (1.0 - finalWeight/100.0))) / (finalWeight/100.0);
printf("You need to get at least %.2lf%% on the final to get a %c in the class.\n", gradeNeededOnFinal, gradeWanted);
}
int main() {
calcGradeNeededOnFinal();
return 0;
}

Answer 1

For example if I ask for their current grade in the class they cannot input something like 95.6asi!.
You have declared grade as char so there is no possibility that it can store 95.6asi!
char gradeWanted;
What you need is : isalpha or isupper and islower function(s) from ctype.h
isalpha() checks for an alphabetic character; in the standard "C" locale, it is equivalent to (isupper(c) || islower(c)). In some locales, there may be additional characters for which isalpha() is true-letters which are neither upper case nor lower case.
Just check if your gradeWanted is an alphabet or not.
Using
if (isalpha( gradeWanted ))
or
if (isupper( gradeWanted ) || islower( gradeWanted ))
is enough.
All ctype functions takes an int and returns an int, you can declare char gradeWanted; as int gradeWanted;

Answer 2

you could read the characters entered and, using the ascii encoding, check (perhaps with a switch) which character is inserted and manage it.
This way you have full control of the entries.
you can get char to ascii conversion with:
int a_as_int = (int)'a';