Highlights of this lab:

In this lab, you will

• use a new data type — bool — and recognize the boolean expression
• learn how to use If-Then and If-Then-Else statements
• use If-Then to check the state of an I/O stream

• You should be able to evaluate a boolean expression.
• You need to know how to use the selection control structure.
• You should get a basic introduction to file I/O.

Boolean Data Type and Boolean Expressions

If a testing expression is not of bool type, it is coerced to bool type automaticaly when it is evaluated. A nonzero value is coerced to true and a zero value is coerced to false.

A Boolean expression can be a simple Boolean variable or constant or it can be a more complex expression involving one or more of the relational operators. Relational operators take two operands and test relationships between them. The following table shows the relational operators and the corresponding C++ symbols.

Relational Operators
C++ Symbol	Description
==	Equal To
!=	Not Eqaul To
>	Greater Than
<	Less Than
>=	Greater Than Or Equal To
<=	Less Than Or Equal To

For example, The Boolean expression

	number1 < number2 

When a relational operator is applied between variables of type char, the assertion is in terms of where the two operands fall in the collating sequence of a particular character set.

For example,

	character1 < character2 

A simple Boolean expression is either a Boolean variable or a Boolean constant or an expression involving the relational operators that evaluates to either true or false. These simple Boolean expressions can be combined using logical operations defined on Boolean values. There are three Boolean operators: AND, OR and NOT. Here is a table showing how they are used in C++.

Logical Operators
C++ Symbol	Meaning
&&	AND is a binary Boolean operator. If both operands are true, the result is true. Otherwise, the result is false.
||	OR is a binary Boolean operator. If at least one of the operands is true, the result is true. Otherwise, if both operands are false, the result is false.
!	NOT is a unary Boolean operator. NOT changes value of its operand. If the operand is true, the result is false. If the operand is false, the result is true.

Precedence of Operators

The following table summarizes the precedence of all the C++ operators we have seen so far.

C++ Operator Precedence
++ --	Highest Precedence
! Unary + Unary -	↓
* / %	↓
+ -	↓
<< >>	↓
< <= > >=	↓
== !=	↓
&&	↓
||	↓
=	Lowest Precedence

Operators in the same line in the table have the same precedence. If an expression contains several operators with the same precedence, most of the operators group from left to right.

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If Statement

If-Then Statement

if (Expression)

	Statement

Now, let's look at the following statement:

if (number < 0)
	number = 0;

sum = sum + number;

If-Then-Else Statement

if (Expression)

	Statement1A
else 

	Statement1B

cout << "You are ";
if (age >= 65)
   cout << "a senior ";
else
   cout << "not a senior ";
cout << "citizen." << endl;

There is one thing to point out here: any statement in the if statement could be a block or a compound statement; i.e., more than one simple statement are enclosed in a pair of braces.

Nested If Statement

The following example is a nested If statement.

cout << "You are ";
if (age >= 65)
   cout << "a senior." << endl;
else
   if (age >= 19)
      cout << "an adult." << endl;
   else
      if (age >= 13)
	 cout << "a teenager." << endl;
      else
	 cout << "a child." << endl;
cout << "You are a great person." << endl;

cout << "You are ";
if (age >= 65) {
  cout << "a senior." << endl;
} else {
  if (age >= 19) {
    cout << "an adult." << endl;
  } else {
    if (age >= 13) {
      cout << "a teenager." << endl;
    } else {
      cout << "a child." << endl;
    }
  }
}
cout << "You are a great person." << endl;

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State of an I/O Stream

1. Invalid input data
2. An attempt to read beyond the end of a file
3. An attempt to open a nonexistent file to input

Here is an example program that reads four floating point data values from a file and write to another file in the reverse order.

// Program iodemo.cpp demonstrates how to use files #include <iostream> #include <fstream> using namespace std; int main() { cout << fixed << showpoint; float val1, val2, val3, val4; // declares 4 variables ifstream inData; // declares input stream ofstream outData; // declares output stream // binds program variable inData to file "Data.In" inData.open("Data.In"); //Testing the state of the stream //true means the last I/O operation on that stream succeeded //false means the last I/O operation on that stream failed if (!inData) { cout << "Can't open the input file successfuly." << endl; return 1; } // binds program variable outData to file "Data.Out" outData.open("Data.Out"); //Testing the state of the stream if (!outData) { cout << "Can't open the output file successfuly." << endl; return 2; } inData >> val1 >> val2 >> val3 >> val4; // inputs 4 values outData << val4 << endl; outData << val3 << endl; outData << val2 << endl; outData << val1 << endl; // outputs 4 values return 0; }

Notice that inData and outData are identifiers in the program; "Data.In" and "Data.Out" are character strings. Data.In is the name of the input data file that we have created; Data.Out is the name of the file where the answers are stored.

If the input file Data.In cannot be found, 1 is returned to the operating system. If the output file Data.Out cannot be opened or created, 2 is returned to the operating system. If there is no input and output error, 0 is returned to the operating system. Notice that the main() is exited as soon as a value is returned. Therefore, Returning 0 value means normal completion of a program; returning any other value signals an error. When you write your own program, you may choose the value to return to indicate different error conditions.

You can use the emacs text editor to create the input data file according to the requirement of the data type and format in your program. Input data file must exist and contain correct data. Otherwise, you will get input failure.

For example, in the preceding iodemo.cpp program, the input file should look like this:

5.5
6.6
7.7
8.8

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Lab Exercise — Seclection Control Structure

Exercise 1:

• Type the following code into a file named convert.cpp using Emacs (emacs convert.cpp &).

  // Program convert.cpp converts a temperature in Fahrenheit to // Celsius or a temperature in Celsius to Fahrenheit // depending on whether the user enters an F or a C.. #include <iostream> using namespace std; int main() { char letter; // Place to store input letter float tempIn; // Temperature to be converted float tempOut; // Converted temperature cout << "Input Menu" << endl << endl; cout << "F: Convert from Fahrenheit to Celsius" << endl; cout << "C: Convert from Celsius to Fahrenheit" << endl; cout << "Type a C or an F, then press return." << endl << endl; cout << "Enter your option: "; cin >> letter; cout << "Type a temperature: "; cin >> tempIn; if (letter == 'C') tempOut = (9 * tempIn / 5) + 32; else tempOut = 5 * (tempIn - 32) / 9; cout << endl << endl; cout << "Temperature to convert: " << tempIn << endl; cout << "Converted temperature: " << tempOut << endl; return 0; }

  This program demonstrates how to use If-Then-Else statement.

• Compile and run this C++ program. Use input value (C 100), (F 32), (c 0), (d 0), and (F 0).
• Explain why the last three sets of the input yield the same output?
• Fix the program to deal with (C 100), (c 100), (F 32), and (f 32).
• Print error message "Wrong letter" if the input letter is not C, c, F , or f.

Exercise 2:

• Type the following code into a file named ioexercise.cpp using Emacs (emacs ioexercise.cpp &).

  // Program ioexercise.cpp demonstrates how to use files #include <iostream> #include <fstream> using namespace std; int main() { cout << fixed << showpoint; float val1, val2, val3, val4; // declares 4 variables ifstream inData; // declares input stream ofstream outData; // declares output stream // Bind program variable inData to file "Data.In" inData.open("Data.In"); // Bind program variable outData to file "Data.Out" outData.open("Data.Out"); inData >> val1 >> val2 >> val3 >> val4; // inputs 4 values outData << val4 << endl; outData << val3 << endl; outData << val2 << endl; outData << val1 << endl; // outputs 4 values return 0; }

  This program demonstrates how to use File I/O. It reads four floating point values from the input data file and output these numbers to an output file in reverse order.

• Add in "testing the state of the input stream". Hint: You might want to use if ( !inData )
• Compile and run the program. Fix the program according to the error messages if there are any mistakes.

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Hand In: This lab handout with the answers filled in attached to a listing of your final exercises
(use the enscript command from your Programming Style Sheet to print it out:
enscript -E -G -2rj -M Letter -PECT2_PS <filename> or a2ps <filename>).