Control Structures: Selection

Mithat Konar

• Control structures are used to affect how statements are executed.
  • Sometimes also called flow control.

• Three kinds of control structures are used in C++ and many other languages:
  • Sequence
    • Statements are executed one after the other.
    • This is C++'s default behavior!
  • Selection structures
    • Used to choose among alternative courses of action.
    • “Making decisions”
  • Repetition structures
    • Used to repeat a set of instructions.
    • “Looping”

• Control structures that are entered from one point and exited from one point.
• Connect the exit point of one control structure to entry point of the next: control-structure stacking.
• Makes programs easy to build.

• Four selection structures available in C++:
  • if
  • if/else
  • nested if/else
  • switch
• Also has a ternary conditional operator that performs selection.

• Used to do something if some condition is “true”.
• Pseudocode example:

  if student’s grade is greater than 60
    print "Passed"

  If the condition is true the print statement is executed and program goes on to next statement. If the condition is false the print statement is ignored and the program goes onto the next statement.

• The if structure is a single-entry/single-exit structure.

• C++ syntax:

  if (<expression>)
    <statement>;

• Example:

  if (grade > 60)
    cout << "Passed" << endl;

• complete example

• Relational operators are used to compare values and return a true or false value.
• A value that is either true or false is a Boolean value.
  • bool type in C++

• C++ supports the following relational operators:
  • > greater than
  • < less than
  • >= greater than or equal to
  • <= less than or equal to
  • == equal to (equality)
  • != not equal to (inequality)
  • Note: Do not confuse the == equality operator with the = assignment operator. This is a common mistake and can cause hard-to-find errors!

• Used to do one thing if some condition is “true”, something else if the condition is “false”.
• Pseudocode example:

  if student’s grade is greater than or equal to 60
    print “Passed”
  else
    print "Failed"

  If the condition is true the “Passed” print statement is executed and the program goes on to next statement. If the condition is false the “Failed” print statement is executed and the program goes onto the next statement.

• The if/else structure is a single-entry/single-exit structure.

• C++ syntax:

  if (<expression>)
    <statement>;
  else
    <statement>;

• Example:

  if (grade >= 60)
    cout << "Passed" << endl;
  else
    cout << "Failed" << endl;

• complete example

• Placing if/else selection structures inside if/else selection structures can be used to test for multiple cases.

• “Logical” formatting:

  if student’s grade is greater than or equal to 90
    print "A"
  else
    if student’s grade is greater than or equal to 80
      print "B"
    else
      if student’s grade is greater than or equal to 70
        print "C"
      else
       if student’s grade is greater than or equal to 60
         print "D"
       else
         print "F"

• “Readability” formatting:

  if student’s grade is greater than or equal to 90
    print "A"
  else if student’s grade is greater than or equal to 80
    print "B"
  else if student’s grade is greater than or equal to 70
    print "C"
  else if student’s grade is greater than or equal to 60
    print "D"
  else
    print "F"

• C++ syntax:

  if (grade >= 90)
    cout << "A" << endl;
  else if (grade >= 80)
    cout << "B" << endl;
  else if (grade >= 70)
    cout << "C" << endl;
  else if (grade >= 60)
    cout << "D" << endl;
  else
    cout << "F" << endl;

• complete example

• A simple statement in C++ is any statement that ends with a semicolon:

  cout << "This is a simple statement." << endl;

• A compound statement is a set of simple statements placed between curly brackets:

  {
    x = 3 * y;
    cout << "The magic number is: " << x << endl;
  }

• A compound statement can be used anywhere a simple statement can be used, for example in an if statement:

  if (grade >= 60)
    cout << "Passed." << endl;
  else
  {
    cout << "Failed." << endl;
    cout << "You must take this course again." << endl;
  }

• Without the brackets, the statement

  cout << "You must take this course again." << endl;

  would be executed no matter what.

• A compound statement with variable declarations is called a block.
• C++ example:

  if (grade >= 60)
    cout << "Passed." << endl;
  else
  {
    char yourGrade;
    yourGrade = 'F';
    cout << "You received a grade of " << yourGrade << endl;
    cout << "You must take this course again." << endl;
  }

• Logical operators are used to implement standard logical operations.
• Like relational operators, logical operators return Boolean values (i.e., they form Boolean expressions).

• && (logical AND)
  • Takes two operands
  • Returns true if both operands are true, returns false otherwise.
• || (logical OR)
  • Takes two operands
  • Returns true if either operand is true, returns false otherwise.
• ! (logical NOT, logical negation)
  • Takes one operand
  • Returns true when its operand is false, false otherwise.
• Precedence: ! → && → ||

assume: int x = 12, y = 5, z = -4;

• complete example

• short-circuit evaluation: evaluating only as much as necessary (from left to right) to determine the result.

  int x = 5, y = 6;
  if ( (x < 0) && (-6 > y-1) )  // (-6 > y-1) is not evaluated
    ...
  if ( (x > 0) || (-6 > y-1) )  // (-6 > y-1) is not evaluated
    ...

• Keep this in mind for later

• In C++:
  • The value 0 (the integer zero) is considered false,
  • anything else is considered true.
• Thus, the following example will print foo:

  if (6)
    cout << "foo" << endl;
  else
    cout << "bar" << endl;

• and the following will print bar:

  if (0)
    cout << "foo" << endl;
  else
    cout << "bar" << endl;

• Comparing characters example
• See Gaddis 4.12 (programs Pr4-20.cpp and Pr4-21.cpp).

• simple switch example
• See Gaddis 4.14 (programs Pr4-23.cpp to Pr4-27.cpp).

• C++ has a ternary conditional operator
  • Similar to an if/else control structure.
  • Unlike if/else, the conditional operator returns a value.
  • Syntax:expr1 ? expr2 : expr3;
  • Semantics: “If expr1 is true, return the value of expr2, otherwise return the value of expr3.”
  • Examples:

    y = x<0 ? -1.0*x : x;
    w = x<0 ? y=10 : z=20;

• See Gaddis 4.15 (programs Pr4-28.cpp to Pr4-30.cpp).