C++ Calculator Prog Mod 5 with input checking for valid number

// kalk-05.cpp
//
// Calculator program that runs in console.
// Type M for menu.
//
// This is the original from PSW book Using C++
// I have used it as a basis to learn C++
//
// I have kept is at the following modifications will
// be easier to understand as the program becomes more sophisticated.
//
// Mods
//   01 Change command short cuts to alpha characters so that they are simple to
//      key. Trivial change.
//   02 Add extra functions again trivial.
//   03 Add Memory functions
//      MEM IN .. loads memory with current value
//      MEM PLUS .. adds current value to memory
//           Note: Mem Minus achieved by typing n (negative) to make current
//           value negative and MEM PLUS. If required redo n to make current
//           value positive.
//      MEM SWITCH switches current value and memory.
//   03a Added code to change negative current value to positive when
//      Power function AND exponent is less than 1 (e.g. 0.5 for square root)
//      as even roots of negative numbers are imaginary
//   04 Add capability to use mem value in function, e.g. to subtract value in
//      in memory from current value, but keep memory value.
//      This was a bit complicated and required writing function to replace CIN
//      and use of STRING to DOUBLE function
//   05 Improve by checking that values entered are numbers and not garbage.
//
// Planned mods
//   06 Capability to undo up to last 5 commands. Code with Vector?
//   07 Log comands to file so that they can be audited.
//      Have default log KALKLOG.DAT in C:|MyDATA   which is backed up to
//      KALKLOG.BAK start of new program, overwriting previous version.
//   08 Allow user to change log file name and locaton.
//   09 Take data file in LOG format and run as batch program.
//
//  Also plan to use as test for compiling in Linux and with other systems.
//  May try to convert to Java and Ruby??
//  When I have learned widows programming may try to convert to windows type.
//
//  I know it is all old hat but it helps me to learn and may even help others.
//

#include <iostream>
#include <iomanip>
#include <cmath>
#include <cctype>
#include <string>
using namespace std;

void display_menu();
// int is_menu_choice(char choice);
void process_choice (char choice,     // IN
                    double& num,     // IN-OUT
                    double& mem);    // IN-OUT
void cin_number(double& num,    // IN-OUT
               double& mem);   // IN-OUT
char RT_Num_Check(string& digit);

int main()
{
  char choice;     // menu choice
  double curr_val; // current value of calculator
  double mem;      // Calculator Mem function

  display_menu();
  cout << setprecision(15);
  curr_val = 0;
  mem = 0;

  do {
     cin >> choice;
     choice = toupper(choice);
     process_choice (choice, curr_val, mem);
      }
  while (choice != 'Q');
  return 0;
}

void display_menu()
{
  cout << "Calculator functions:\n"
       << "  (A)add, (S)ubtract X(mult), (D)ivide , (N)egative,\n"
       << "  (R)eciprocal, (I)nitialize, (P)ower [positive only], Q)uit,\n"
       << "  (M)em in, mem p(L)us, mem (O)ut, mem s(W)itch \n"
       << "  use mem to refer to memory in calculations"
       << "  (H)elp show this menu again.\n\n";
}

void process_choice (char choice,      // IN
                    double& curr_val, // IN-OUT
                    double& mem)      // IN-OUT

{
//  double mem;
  double work;    // required for memory sWitch function
  double num;     // numeric value entered by user
  num = 0;
//   mem=0;
  switch (choice)
     {
     case 'A': cin_number(num, mem);
               curr_val += num;
               break;
     case 'D': cin_number(num, mem);
               curr_val = curr_val/num;
               break;
     case 'H': break;
     case 'I': cin_number(num, mem);
               curr_val = num;
               break;
     case 'L': mem += curr_val;
               break;
     case 'M': mem = curr_val;
               break;
     case 'N': curr_val *= (-1);
               break;
     case 'O': curr_val = mem;
               break;
     case 'P': cin_number(num, mem);
               if (num < 1) curr_val = fabs(curr_val);
                    //stops possibilty roots of negative values causing errors
               curr_val = pow(curr_val, num);
               break;
     case 'Q': break;
     case 'R': curr_val = 1/curr_val;
               break;
     case 'S': cin_number(num, mem);
               curr_val -= num;
               break;
     case 'W': work = curr_val;
               curr_val = mem;
               mem = work;
               break;
     case 'X': cin_number(num, mem);
               curr_val *= num;
               break;
     default:  cout << "\tInvalid operation!\n";
     } // end switch
  if (choice=='H') display_menu();
  if (choice != ('Q' || 'H' ))
     cout << setw(16) << ' ' << curr_val
          << endl;
}

void cin_number(double& num,
               double& mem)      // IN-OUT
{
 string input;
 char ValidNumber = 'N';
 while (ValidNumber == 'N')
 {     
   cin >> input;
   if (input == "mem")
      {
        num = mem;
        ValidNumber = 'Y';
      }
   else
      {
        ValidNumber=RT_Num_Check(input);
        if (ValidNumber=='Y')
           num = strtod(input.c_str(), NULL);
        else 
         {
          cout << input <<" Not a proper number\n Re-enter number\n";
         }
      } 
   }   
 return;

}
char RT_Num_Check(string& digit)
{
  char NumOK;               // result
  int p;                    // digit position  
  char decimal = 'N';       // used to make sure not more than 1 decimal point
  string sign = "0";        // used to check for negative
//    cout << "The length of str is " << digit.length() << " characters.\n"; 
if (digit.substr(0,1) == "-") // sign -1
 {
   digit=digit.substr (1,(digit.length()-1));  // strip sign
   sign= "-";                                  // to check so that neg restored at end
 }                             
for (p=0;p<digit.length();p++)
  {
  
      if ((digit.substr(p,1) >="0") && (digit.substr(p,1) <="9"))
          NumOK='Y';
      else
        {
           if ((digit.substr(p,1) ==".") && (decimal=='N'))
             {
               NumOK='Y';
               decimal='Y';               // so that later positions checked for erraneous 2nd decimal point
             }
           else                     
             {
               NumOK='N';                 // if we reach here must be an incorrect number
               break;
             }
        }    
//         cout <<"p is " <<p<< " number is "<< digit.substr(p,1)<<endl;            
  }  

     if (sign == "-")
         digit = sign+digit;             // put back negative if necessary
     return (NumOK);
  
}


/*A typical run:

Calculator functions:\n"
 (A)add, (S)ubtract X(mult), (D)ivide , (N)egative,\n"
 (R)eciprocal, (I)nitialize, (P)ower, Q)uit,\n"
 (H)elp show this menu again.\n\n";

I 2.5
               2.5
P 3
            15.625
R
             0.064
x 100
               6.4
n
              -6.4
a 10
               3.6
s 1.12
              2.48
H
Calculator functions:\n"
 (A)add, (S)ubtract X(mult), (D)ivide , (N)egative,\n"
 (R)eciprocal, (I)nitialize, (P)ower, Q)uit,\n"
 (H)elp show this menu again.\n\n";

              2.48
d 2.5
             0.992
i 4
                 4
p 0.5
                 2
n
                -2
p 0.5
   1.4142135623731
i10
              10
m
              10
r
             0.1
w
              10
o
             0.1
x1000
             100
l
             100
l
             100
o
           200.1
i 2
               2
m
               2
i5
               5
amem
               7
p mem
              49
d mem
            24.5
a 5..2
5..2 Not a proper number
Re-enter number
5.2
             29.7            
q

------------------------

Note you do not need to leave space,
e.g. you can enter "d 2.5" or "d2.5"

*/