Topics covered on Monday, 2010-08-30

Variable Scopes

All variables defined in C++ have a scope. The scope of a variable is the portion of the program within which it exists.

In general, the scope of a variable is the smallest block that includes the definition of the variable.

For instance, in the program below, two variables are defined with the same name. The scope of the first variable is the block from lines 14 to 27. The scope of the second variable is the block from lines 19 to 24. Note how changing the value of the second variable within its scope doesn't affect the value of the first variable.

Demonstrate variable scope (scope1.cpp)

     1  //
     2  // scope1.cpp
     3  //
     4  // Program to demonstrate the scope of variables.
     5  //
     6  // Copyright 2010, Raj Mathur
     7  //
     8  #include <iostream>
     9  #include <cstdlib>
    10
    11  using namespace std;
    12
    13  int main( int argc, char *argv[] )
    14  {
    15    int
    16      variable;
    17    variable = 10;
    18    cout << "Before the block, the value of variable is " << variable << endl;
    19    {
    20      int
    21        variable;
    22      variable = 20;
    23      cout << "In the block, the value of variable is " << variable << endl;
    24    }
    25    cout << "After the block, the value of variable is " << variable << endl;
    26    exit(0);
    27  }

Functions

The program below is a modification of compute1.cpp. We move the common portion (prompting the user for the operation) out into its own function get_operation that may be called any number of times.

Add or subtract two numbers using a function (compute2.cpp)

     1  // compute2.cpp
     2  //
     3  // Prompt user for 2 numbers and and operation (+ or -) and
     4  // print out result of the operation.  Use a fixed function to
     5  // get the operation from the user.
     6  //
     7  // Copyright (C) 2010, Raj Mathur
     8  #include <iostream>
     9  #include <cstdlib>
    10  #include <cstring>
    11
    12  using namespace std;
    13  // Functions (prototypes)
    14  char get_operation(void);
    15
    16  int main(int argc, char *argv[])
    17  {
    18    char
    19      op;
    20    int
    21      number_1 = 0;
    22    int
    23      number_2 = 0;
    24    int
    25      result;
    26    // Get operation type (p == plus, m == minus)
    27    op = get_operation();
    28    while( op != 'q' )
    29    {
    30      // Get values to be used for computation
    31      cout << "What is the first number? ";
    32      cin >> number_1;
    33      cout << "What is the second number? ";
    34      cin >> number_2;
    35      // Check operation and do it
    36      if( op == 'p' )
    37      {
    38        result = number_1 + number_2;
    39      }
    40      else if( op == 'm' )
    41      {
    42        result = number_1 - number_2;
    43      }
    44      // Display it
    45      cout << "The result is: " << result << endl;
    46      // Get operation type (p == plus, m == minus)
    47      op = get_operation();
    48    }
    49    cout << "OK, exiting..." << endl;
    50    exit(0);
    51  }
    52  //
    53  // get_operation
    54  //
    55  // Get 'p', 'm' or 'q' from the user and return that.
    56  //
    57  char get_operation(void)
    58  {
    59    char
    60      value = ' ';
    61    while( value != 'p' && value != 'm' && value != 'q' )
    62    {
    63      cout << "Enter (p)lus, (m)inus or (q)uit: ";
    64      cin >> value;
    65    }
    66    return( value );
    67  }

Line 14 declares the function get_operation. All functions must be declared before they are used. Line 14 intimates the compiler that get_operation takes no arguments and returns one char value to the calling function.

Lines 57 through 67 define the function get_operation. All function definitions make a new block. The value given in the return statement is made available to the calling function.

Add or subtract two numbers using a more flexible function (compute3.cpp)

     1  // compute3.cpp
     2  //
     3  // Prompt user for 2 numbers and and operation (+ or -) and
     4  // print out result of the operation.  Use a flexible function to get
     5  // the operation from the user.
     6  //
     7  // Copyright (C) 2010, Raj Mathur
     8  #include <iostream>
     9  #include <cstdlib>
    10  #include <cstring>
    11
    12  using namespace std;
    13  //
    14  // Functions (prototypes)
    15  char get_operation(char *prompt, char *valid_chars);
    16
    17  int main(int argc, char *argv[])
    18  {
    19    char
    20      op;
    21    int
    22      number_1 = 0;
    23    int
    24      number_2 = 0;
    25    int
    26      result;
    27    // Get operation type (p == plus, m == minus)
    28    op = get_operation("Enter (p)lus, (m)inus, (t)imes or (q)uit: ", "pmtq");
    29    while( op != 'q' )
    30    {
    31      // Get values to be used for computation
    32      cout << "What is the first number? ";
    33      cin >> number_1;
    34      cout << "What is the second number? ";
    35      cin >> number_2;
    36      // Check operation and do it
    37      if( op == 'p' )
    38      {
    39        result = number_1 + number_2;
    40      }
    41      else if( op == 'm' )
    42      {
    43        result = number_1 - number_2;
    44      }
    45      // Display it
    46      cout << "The result is: " << result << endl;
    47      // Get operation type (p == plus, m == minus)
    48      op = get_operation("Enter (p)lus, (m)inus or (q)uit: ", "pmq");
    49    }
    50    cout << "OK, exiting..." << endl;
    51    exit(0);
    52  }
    53  //
    54  // get_operation
    55  //
    56  // Get 'p', 'm' or 'q' from the user and return that.
    57  //
    58  char get_operation(char *prompt, char *valid_chars)
    59  {
    60    char
    61      value = ' ';
    62    while( strchr(valid_chars, value) == NULL )
    63    {
    64      cout << prompt;
    65      cin >> value;
    66    }
    67    return( value );
    68  }

Assignment

Make compute4.cpp based on compute1.cpp so that it takes 1, 2 or 3 command-line arguments.

• If one argument is given, it is treated as the operation code. The program should then prompt for the first and second numbers, do the operation and display the result.
• If two arguments are given, they are treated as the operation and the first number. The program should then prompt for the second number, do the operation and display the result.
• If three arguments are given, they are treated as the operation, the first number and the second number. The program should then not prompt for anything, do the operation and display the result.