What is the use of cerr?

[Anmol444]

In the following Code:

// EX6_10.cpp
// A program to implement a calculator

#include <iostream>                   // For stream input/output
#include <cstdlib>                    // For the exit() function
#include <cctype>                     // For the isdigit() function
using std::cin;
using std::cout;
using std::cerr;
using std::endl;

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

const int MAX(80);                    // Maximum expression length, 
                                      // including '\0'
int main()
{
   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;
      try
      {
        cout << "\t= " << expr(buffer)      // Output value of expression
             << endl << endl;
      }
      catch( const char* pEx)
      {
        cerr << pEx << endl;
        cerr << "Ending program." << endl;
        return 1;
      }
   }
}


// Function to eliminate spaces 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 space
   return;
}

// Function to evaluate an arithmetic expression
double expr(char* str)
{
  double value(0.0);                   // Store result here
  int index(0);                        // Keeps track of current character position

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

  for(;;)                              // Indefinite 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 is junk
         char message[38] = "Expression evaluation error. Found: ";
         strncat_s(message, str + index - 1, 1);  // Append the character
         throw message;
         break;
    }
  }
}

// Function to get the value of a term
double term(char* str, int& index)
{
  double value(0.0);                   // 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(true)
  {

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

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

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

  // There must be at least one digit...
  if(!isdigit(*(str + index)))
  { // There's no digits so input is junk...
    char message[31] = "Invalid character in number: ";
    strncat_s(message, str+index, 1);  // Append the character
    throw message;
  }

  while(isdigit(*(str + index)))       // Loop accumulating leading digits
    value = 10*value + (*(str + index++) - '0');

                                       // 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) - '0')*factor;   // Add decimal place
  }

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

On line 43 & 44 what is the use of cerr? I can use cout and it compiles perfectly fine. Is it there just to tell another programmer it outputs a error rather then just normal output? If not why? Also is there any functionality difference between cout cerr and clog?

May someone please explain this to me? Thank you.

Also it takes about needing that separate header file for the "exit function"? Where is this function in the code? o.O

[MiiNiPaa]

http://stackoverflow.com/a/2404545
Main idea that you can redirect streams. So you can have cout on your screen, clog in some file, and cerr displayed on your screen and written to a file. Or printed. Or sent to logging server. Or something

separate header file for the "exit function"? Where is this function in the code?

Well looks like he didn't need it.

[Anmol444]

But it says at the top he added this:

#include <cstdlib> // For the exit() function

What function. Its nowhere to be found...

[Zhuge]

That's exactly it. It isn't there, so it was probably something that was changed and the comment was never updated.

[Anmol444]

I see thank you very much.

But the thing is its from my book "Ivor Horton's Beginning C++ 2012"

Although I have found many typos in the book, this is just a fail. f7