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here is alittle excercise i wrote..its sorta a calculator that solves math equations

Code:

#include <iostream>                 // For stream input/output
#include <cstdlib>                  // For the exit() function
#include <cctype>                   // For the isdigit() function
#include <string>                   // For the strcpy function
using namespace std;

void eatspaces(char* str);          // Function to eliminate blanks
double expr(char* str);             // Function evaluating an expression
double term(char* str, int& index);   // Function analyzing a term
double number(char* str, int& index); // Function to recognize a number
char* extract(char* str, int& index); //Function to extract a substring

const int MAX = 80;         // Maximum expression length including '\0'

int main(void)
{
   char buffer[MAX] = {0};    // Input area for expression to be evaluated

   cout << endl
        << "Welcome to your friendly calculator."
        << endl
        << "Enter an expression, or an empty line to quit."
        << endl;

   for(;;)
   {
      cin.getline(buffer, sizeof buffer);   // Read an input line
      eatspaces(buffer);                    // Remove blanks from input

      if(!buffer[0])                      // Empty line ends calculator
         return 0;

      cout << "\t= " << expr(buffer)      // Output value of expression
           << endl << endl;
   }
}


// Function to eliminate blanks from a string
void eatspaces(char* str)
{
   int i=0;         // 'Copy to' index to string
   int j=0;         // 'Copy from' index to string

   while((*(str+i) = *(str+j++)) != '\0')   // Loop while character
                                            // copied is not \0
      if(*(str+i) != ' ')                   // Increment i as long as
         i++;                               // character is not a blank
   return;
}


// Function to evaluate an arithmetic expression
double expr(char* str)
{
   double value = 0;          // Store result here
   int index = 0;             // Keeps track of current character position

   value = term(str, index);  // Get first term

   for(;;)                    // Infinite loop, all exits inside
   {
      switch(*(str+index++))  // Choose action based on current character
      {
         case '\0':                 // We're at the end of the string
            return value;           // so return what we have got

         case '+':                         // + found so add in the
            value += term(str, index);     // next term
            break;

         case '-':                         // - found so subtract
            value -= term(str, index);     // the next term
            break;

         default:                       // If we reach here the string
            cout << endl                // is junk
                 << "Arrrgh!*#!! There's an error"
                 << endl;
            exit(1);
      }
   }
}


// Function to get the value of a term
double term(char* str, int& index)
{
   double value = 0;              // Somewhere to accumulate the result

   value = number(str, index);    // Get the first number in the term

   // Loop as long as we have a good operator
   while((*(str+index)=='*')||(*(str+index)=='/'))
   {

      if(*(str+index)=='*')                  // If it's multiply,
         value *= number(str, ++index);      // multiply by next number

      if(*(str+index)=='/')                  // If it's divide,
         value /= number(str, ++index);      // divide by next number
   }
   return value;             // We've finished, so return what we've got
}


// Function to recognize an expression in parentheses
// or a number in a string
double number(char* str, int& index)
{
   double value = 0.0;              // Store the resulting value

   if(*(str+index) == '(')             // Start of parentheses
   {
      char* psubstr = 0;               // Pointer for substring
      psubstr = extract(str, ++index); // Extract substring in brackets
      value = expr(psubstr);           // Get the value of the substring
      delete[]psubstr;                 // Clean up the free store
      return value;                    // Return substring value
   }

   while(isdigit(*(str+index)))     // Loop accumulating leading digits
      value=10*value + (*(str+index++) - 48);

                                     // Not a digit when we get to here
   if(*(str+index)!='.')             // so check for decimal point
      return value;                  // and if not, return value

   double factor = 1.0;              // Factor for decimal places
   while(isdigit(*(str+(++index))))  // Loop as long as we have digits
   {
      factor *= 0.1;                 // Decrease factor by factor of 10
      value=value + (*(str+index)-48)*factor;   // Add decimal place
   }

   return value;                     // On loop exit we are done
}


// Function to extract a substring between parentheses 
// (requires string)
char* extract(char* str, int& index)
{
   char buffer[MAX];         // Temporary space for substring
   char* pstr=0;             // Pointer to new string for return
   int numL = 0;             // Count of left parentheses found
   int bufindex = index;     // Save starting value for index

   do
   {
      buffer[index-bufindex] = *(str+index);
      switch(buffer[index-bufindex])
      {
         case ')':
            if(numL==0)
            {
               buffer[index-bufindex] = '\0';  // Replace ')' with '\0' 
               ++index;
               pstr = new char[index-bufindex];
               if(!pstr)
               {
                  cout << "Memory allocation failed,"
                       << " program terminated.";
                  exit(1);
               }
               strcpy(pstr,buffer);  // Copy substring to new memory
               return pstr;          // Return substring in new memory
            }
            else
               numL--;           // Reduce count of '(' to be matched
            break;

         case '(':
            numL++;              // Increase count of '(' to be matched
            break;
      }
   } while(*(str+index++) != '\0');// Loop - don't overrun end of string

   cout << "Ran off the end of the expression, must be bad input."
        << endl;
   exit(1);
   return pstr;
}

hope that helps

Honestly I haven't read the entire program yet but good work. It's nice to see young programmers putting so much effort into it. One comment - now try modularizing operations that are common and repetitive, or even just error-prone. Not nitpicking, just an aside...