Looking for help understanding how this example program flows and works. Can you

Question

Looking for help understanding how this example program flows and works. Can you write comments next to each line of code explaining what it does? Why does it use a switch? why not set the roman numerals as constants instead of a switch? Thank you!!

//CPP program for converting roman into integers

#include <iostream>
#include <fstream>
#include<cctype>
#include<cstdlib>
#include<string.h>
using namespace std;

int value(char roman)
{
switch(roman)
{
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 : return -1;
}
}

int get(const string& input)
{
int total=0; char prev='%';
for(int i=(input.length()-1); i>=0; i--)
{
if(value(input[i])==-1)
return -1;
if(value(input[i])<total && (input[i]!=prev))
{ total -= value(input[i]);
prev = input[i];
}
else
{
total += value(input[i]);
prev = input[i];
}
}
return total;
}

int main()
{
char str[255];
int val;
ifstream in("Roman.txt");

if(!in) {
cout << "Cannot open input file. ";
return 1;
}

while(in) {
in.getline(str, 255); // delim defaults to ' '
  
if(in)
{
val=get(str);
if(val==-1)
cout << "Invalid character"<<endl;
else
cout << str << " = "<<val<<endl;
  
}
  

}

in.close();

return 0;
}

Explanation / Answer

//CPP program for converting roman into integers
#include <iostream>
#include <fstream>
#include<cctype>
#include<cstdlib>
#include<string.h>
using namespace std;
// Takes a roman character as input, and will return its integer equivalent,
// given the input is a valid roman character, and returns -1 otherwise.
int value(char roman)
{
    switch(roman)
   {
       case 'I':return 1;       //If the input is: I, returns 1.
       case 'V':return 5;       //If the input is: V, returns 5.
       case 'X':return 10;       //If the input is: X, returns 10.
       case 'L':return 50;       //If the input is: L, returns 50.
       case 'C':return 100;   //If the input is: C, returns 100.  
       case 'D':return 500;   //If the input is: D, returns 500.
       case 'M':return 1000;   //If the input is: M, returns 1000.
       default : return -1;   //If neither of the above characters, returns -1.
   }
}

//Takes a roman number string as input, and returns its equivalent integer as output.
int get(const string& input)
{
   int total=0;   
   char prev='%';
   for(int i=(input.length()-1); i>=0; i--)   //From the last character in the input string, down till the first character.
   {
       if(value(input[i])==-1)   //If the character in the input string is not a valid roman literal.
           return -1;           //Return -1.
       if(value(input[i])<total && (input[i]!=prev)) //If the current literal value is less than tha total,
                               //and current literal is not same as the previous literal.
       {
          total -= value(input[i]);   //Subtract the current literal value, from the total.
           prev = input[i];           //Mark the current literal as prev, and move forward.
       }
       else                   //If that condition is not satisfied.
       {
           total += value(input[i]);   //Add the current literal value, to the total.
           prev = input[i];           //Mark the current literal as prev, and move forward.
       }
   }  
   return total;           //After the characters in string are exhausted, return the total value.
}

int main()
{
char str[255];           //Declares an array of characters.
int val;               //Declares an integer.
ifstream in("Roman.txt");   //Reads the file Roman.txt as input file stream.
if(!in) {       //If file is not opened successfully.
cout << "Cannot open input file. ";   //Push a message to screen.
return 1;                   //And stop execution.
}
  
while(in) {       //Keep reading line by line of input from the file, till end of file is reached.
in.getline(str, 255); // delim defaults to ' '  
  
if(in)               //If read value is not end of file.
{
val=get(str);   //Call the function get, which return the integer equivalent of roman string, and stores the returned value in val.
if(val==-1)       //If the read string is invalid.
cout << "Invalid character"<<endl;   //Show this message to screen.
else           //If valid.
cout << str << " = "<<val<<endl;   //Show the integer value to screen.
  
}
  
}
in.close();   //After reading all the strings from file, close the file.
return 0;   //Stop execution.
}

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