If else works but I can't do it with switch

Hi it's me again.

Well I've done this program where it converts roman numerals to numbers but I've used it with if else.

Code:

/*      
    wo0dy at home doing assignment.
    March 24th 2006
    Friday 9am
    D-48-A Cyberia
    
    (5) Write a program that converts a C++ string representing a number in Roman numeral form to
    decimal form. The symbols used in the Roman numeral system and their equivalents are given
    below:
    I 1
    V 5
    X 10
    L 50
    C 100
    D 500
    M 1000
    For example, the following are Roman numbers: XII (12), CII (102), XL (40).
    The rules for converting a Roman number to a decimal number are as follows:
    a. Set the value of the decimal number to zero.
    b. Scan the string containing the Roman character from left to right. If the character is not one
    of the symbols in the numeral symbol set, the program must print an error message and
    terminate. Otherwise, continue with the following steps. (Note that there is no equivalent
    to zero in Roman numerals.)
    
 If the next character is the last character, add the value of the current character to
    the decimal value.
    
 If the value of the current character is greater than or equal to the value of the next
    character, add the value of the current character to the decimal value.
    
 If the value of the current character is less than the next character, subtract the
    value of the current character from the decimal value.
    Solve this problem using a switch statement. Some sample outputs are given below.
*/

  //It works only for letters that correspond to Roman numerals. I think thats what Sir said it should work.
  
#include <iostream>
#include <cstring>
using namespace std;

int main ()
{
	char roman_num[10];
	cout << "Enter a Roman Numeral: ";
	cin >> roman_num;
	int len = (strlen(roman_num)-1);  //I put -1 cos I the lecturer said something about 
	int counter = 0;                  //the program uses an extra space for memory or something
	int number = 0;
	int b4sum = 0;
	int sum = 0;

	for (counter = len; counter >= 0; counter--)
	{
		if (roman_num[counter] == 'M')
			number = 1000;
		else if (roman_num[counter] == 'D')
			number = 500;
		else if (roman_num[counter] == 'C')
			number = 100;
		else if (roman_num[counter] == 'L')
			number = 50;
		else if (roman_num[counter] == 'X')
			number = 10;
		else if (roman_num[counter] == 'V')
			number = 5;
		else if (roman_num[counter] == 'I')
			number = 1;
		else{
			number = 0;
            cout<<"\nOne or more of the inputs entered is invalid. \a"<<endl;
            }
        
		if (b4sum > number)
			sum = b4sum - number;
		else
			sum = sum + number;

		b4sum = number;
	}

	cout << "\nThe Roman Numeral is: " << sum << endl;

    system ("PAUSE");
	return 0;
}

But I'm supposed to use switch.

I've tried it and read my book but it kept saying that switch quantity is not an integer. The examples from the book were also given using integers.

Can anyone explain me the concept so I can understand how to apply this?

Show us an attempt. I successfully changed your program to use switch(), so it's possible. (Your program works well, but doesn't handle odd input, ie, "IIIII" gets "5" back.) I think your error is coming from what you're putting inside the switch( ) statement, so check that. A char can be used in a switch(). So, you can use switch(roman_num[counter])...

About strlen(roman_num) - 1: Say I entered the string "XVII" (roman for 17) strlen() on that will say 4. However, if you say roman_num[4], you won't get "I", you'll get the nul character. This is because in C, all strings are terminated, or end with, and nul character. (It's value is the number 0.) Your string is a character array. And in C, arrays start with index 0, not 1. So, using my example of "XVII", roman_num[0] is 'X', roman_num[1] is 'V', etc. Thus the last, or fourth character is roman_num[3].