Invalid conversion from "char" to "char *" - c

Question

This is the program to convert a Roman number (for example VI) to a decimal.
The algorithm is writing a function that recognize each element of the input string and return the corresponding value in decimal.
We got char Roman_num[20]
For each element, sum+=value(Roman_num[i]).The function prototype is int value (char digit).It results in the 'Invalid conversion from char to char *' error.
However, when passing each element's address &a[i] to the function and changing the prototype to int value (char *digit), it doesn't repeat this error but leads to another error in this switch-case (inside the function) : switch (*digit) gives an error of 'digit' cannot appear in a constant-expression
My question is I was not clear that: in this example, do we have to pass only the address to the funcion? If we want to pass the value, a single character value, then how? Last but not least, *digit is actually a single character, then why it cannot appear in a constant-expression in case()?
I will be grateful for your help. On the other hand, can you please recommend me some books for deep understanding in C? I'm now using C : How To Program book, and I hardly know about how the variables, functions working on the inside for a deeper understanding.
This is the code:
int value (char *digit);
int main (void)
{
char a[100];
int length,i,sum=0;
printf("Enter your Roman number: ");
fflush(stdin);
gets(a);
printf("\nThe Roman number that you have entered is %s",a);
length=strlen(a);
for (i=0;i<length;i++)
{
sum+=value(&a[i]);
}
printf("\nthen it is: %d",sum);
getch();
return 0;
}
int value (char *digit)
{
int num;
case ( *digit ){
case 'M':
num=1000;
break;
case 'D':
num=500;
break;
case 'C':
num=100;
break;
case 'L':
num=50;
break;
case 'X':
num=10;
break;
case 'V':
num=5;
break;
case 'I':
num=1;
break;
}
return num;
}

Answer 1

Since you will not post code, here some code for getting roman numerals that always increase. The subtraction part is left to you to figure out. I only post this as it sounds like you are self teaching yourself which is comendable:
int romanToValue(const char c)
{
switch(c) // Only works with upper-case as lower case means different things.
{
case 'I' : return 1;
case 'V' : return 5;
case 'X' : return 10;
case 'L' : return 50;
case 'C' : return 100;
case 'D' : return 500;
case 'M' : return 1000;
default: printf("Bad value in string %c\n", c); break;
}
return 0;
}
int romanToInt(const char *str)
{
int value = 0;
int i;
for(i=0; str[i]; i++) // Dangerous way to do strings, but works for a C example.
{
value += romanToValue(str[i]);
}
return value;
}
int main(void)
{
const char cstr[] = "VIII";
printf("value:%d\n", romanToInt(cstr));
return 0;
}
Notice the switch statement is working off of char values.