Chapter 4: Basic Control Flow

Chapter Goals

• To be able to implement decisions using if statements
• To understand statement blocks
• To learn how to compare integers, floating-point numbers, and strings
• To develop strategies for processing input and handling errors
• To understand the Boolean data type
• To avoid infinite loops and off-by-one errors

The if Statement

• The if statement is used to implement a decision.
• It has two parts: a test and a body.
• Example:

  if (area < 0)
     cerr << "Error: Negative area.\n";

• Multiple statements can be grouped together in a block statement by enclosing them in braces { }:

  if (area < 0)
  {
     cerr << "Error: Negative area.\n";
     return 1;
  }

The if Statement (Syntax 4.1 : if Statement)

if (condition) statement

if (x >= 0) y = sqrt(x);

The if Statement (Syntax 4.2 : Block Statement)

{ 
   statement1
   statement2
   ...
   statementn
}

{
   double length = sqrt(area);
   cout << area << "\n";
}

The if Statement (area1.cpp)

The if/else Statement

• The if/else consists of a condition of two alternatives.
  • The first condition is performed is the condition is true.
  • The second condition is performed if the condition is false.

  if (area >= 0)
     cout << "The side length is " << sqrt(area) << "\n";
  else
     cout << "Error: Negative area.\n";

• The if/else statement is a better choice than a pair of if statements with complementary conditions.

  if (area >= 0) /* complementary conditions */
     cout << "The side length is " << sqrt(area) << "\n";
  if (area < 0) /* complementary conditions */
     cout << "Error: Negative area.\n";

The if/else Statement (Syntax 4.3 : if/else Statement)

if (condition) statement1 else statement2

if (x >= 0) y = sqrt(x); else cout << "Bad input\n";

The if/else Statement (Syntax 4.3 : if/else Statement)

Relational Operators

• C++ has six relational operators to implement conditions:

  C++	Description	Example	Notes
  >	greater than	a > 5
  >=	greater than or equal to	x >= 5	Be careful to not write => Remember that the symbols appear in the order you say them
  <	less than	x < 10
  <=	less than or equal to	x <= 100	Be careful to not write =< Remember that the symbols appear in the order you say them
  ==	equal to	a == 5	Don't confuse with = which is assignment.
  !=	not equal	a != 5	The ! is supposed to be the line that "crosses through" the equal sign.

Relational Operators (comparing strings)

• The relational operators listed above can also be used to compare strings using lexicological comparison.
• "car" is less than "cargo"

  c

  a

  r

  c

  a

  r

  g

  o

  "cargo" is less than "cathode"

  c

  a

  r

  g

  o

  c

  a

  t

  h

  o

  d

  e

Input Validation

• Input validation is an application of the if statement that verifies the user has given reasonable input.
• Example

  double area;
  cin >> area;

  user types "five" and hits return! (Causing cin to fail).
• After every input a good program will test:

  if (cin.fail())
  

• The actual input should also be validated, even if the type is correct.

  if (area < 0)

• Other strategies:

  • if (cin)
       /* the stream did not fail */
    else
       /* the stream failed */

  • if (cin >> x) ...

Input Validation (area3.cpp)

Simple Loops

• A loop is a block of code that can be performed repeatedly.
• A loop is controlled by a condition that is checked each time through the loop.
• The while statement makes a check before each execution of the code.

  /* how long does it take an investment to double? */
  
  while( balance < 2 * initial_balance)
  {
     balance = balance * ( 1 + rate / 100);
     year++;
  }

Simple Loops (Syntax 4.4 : while Statement)

while (condition) statement

while (x >= 10) x = sqrt(x);

Processing a Sequence of Inputs (Sentinels)

• Whenever you read a sequence of input values, you need to have some method of terminating the input.
• A number used to signal termination is called a sentinel.
• Sentinels only work if there is some restriction on the input.
• Common sentinel values are 0 or -1.

Processing a Sequence of Inputs (sentinel.cpp)

Processing a Sequence of Inputs (Causing the Stream to Fail)

• When reading input from the console, you can close the stream manually.
  • Ctrl + Z in Windows
  • Ctrl + D in UNIX
• Reading from a closed stream causes the stream to enter the failed state.

Processing a Sequence of Inputs (maxtemp.cpp)

Using Boolean Variables

• The bool type can hold exactly two values, denoted false and true.
• Boolean variables are named after George Boole (1815-1864), a pioneer in the study of logic.
• Example:

  bool more = true;
  while (more)
  {
     cin >> next;
     if (cin.fail())
        more = false;
     else
     {
         // process next
     }
  }

• Don't:

  while(more == false) /* don't */
  
  while(more != false) /* don't */