How to calculate with characters in C - c

Question

Sorry if the title is not clear about what I want to achieve.
So let me explain:
I'm trying to make a calculator that can enable the use of variables that are each labeled with a lowercase letter. All lowercase letters should be usable (a-z, each with value storage). The following should be possible:
Assignment of a value in a variable, example: a = 1
Use of a variable in an invoice with direct output, example: 3 + a
Assignment of an invoice result into a variable, examples: b = 3 + 1 or c = 6 * b
For clarification again, for example the following inputs should be possible:
x=3 // Variable x receives value 3
d=9 // Variable d gets value 9
3*5 // Output result
h=9/3 // Variable h gets value Result of calculation 9/3
x * h // Output result of calculation x*h, because x=3 and h=3, output 9
i=d/x // variable i gets value from calculation, so 9/3, output result 3
I'm not looking for code snipets per se but more for a way to apprehend the problem.
Thank you in advance for any help!
This is what I did so far:
int main(void) {
int weiter = 1;
do {
char eingabe[21];
int zahl1 = 0;
int zahl2 = 0;
int ergebnis = 0;
char op = ' ';
int retval = 0;
retval = scanf("%s", &eingabe);
printf("retval = %d\n", retval);
printf("eingabe[0] = %c\n", eingabe[0]);
// fall2 zahl opo char
char *peingabe = &(eingabe[0]);
printf("peingabe = %c\n", *peingabe);
char *temp = strpbrk(peingabe, "+-*/=");
op = *temp;
printf("Op = %c\n", op);
temp = strtok(peingabe, "+-*/");
printf("1.temp = %s\n", temp);
zahl1 = atoi(temp);
printf("Zahl_1 = %d\n", zahl1);
temp = strtok(NULL, "+-*/");
printf("2.temp = %s\n", temp);
zahl2 = atoi(temp);
printf("Zahl_2 = %d\n", zahl2);
// fall3 char gleich zahl
// fall4 char op zahl
// fall5 char op char
// fall6 char=zahl op zahl
// fall7 char=char op zahl
// fall8 char=zahl op char
// fall9 char=char op char
// fall10 char gleich char
switch (op) {
case '+':
ergebnis = zahl1 + zahl2;
printf("Sum = %d\n", ergebnis);
break;
case '-':
ergebnis = zahl1 - zahl2;
printf("Sum = %d\n", ergebnis);
break;
case '*':
ergebnis = zahl1 * zahl2;
printf("Sum = %d\n", ergebnis);
break;
case '/':
ergebnis = zahl1 / zahl2;
printf("Sum = %d\n", ergebnis);
break;
default:
printf("Error\n");
weiter = 0;
break;
}
} while (weiter > 0);
return 0;
}

Answer 1

Your calculator appears to be processing a line of text and so it will need to sample each character in order to determine how to handle it, maybe by use of a switch statement.
If you read a numerical character e.g., in the range 0 to 9, then you append or add it to a temporary storage char array so that you can convert it to a numerical value later.
If you detect a letter within the ASCII range a to z, then that is your variable.
Just remember to clear the temporary storage locations when no longer required.
To handle your variables, I would create an array of the largest signed values that you expect to calculate then index these based upon the ASCII value of the variable letter starting with a being an index of zero, z being 25. ASCII a has a value of 97, z has a value of 122