Control Structures - Intro, Selection

Flow of Control:

Flow of control through any given function is implemented with three basic types of control structures:

Sequential: default mode. Sequential execution of code statements (one line after another) -- like following a recipe
Selection: used for decisions, branching -- choosing between 2 or more alternative paths. In C++, these are the types of selection statements:
- if
- if/else
- switch
Repetition: used for looping, i.e. repeating a piece of code multiple times in a row. In C++, there are three types of loops:
- while
- do/while
- for

The function construct, itself, forms another way to affect flow of control through a whole program. This will be discussed later in the course.

Some useful tools for building programs or program segments

pseudocode - helps "think" out a problem or algorithm before trying to code it
flowcharting - graphical way to formulate an algorithm or a program's flow
stepwise refinement (top-down design) of algorithms

True and False

Selection and repetition statements typically involve decision steps. These steps rely on conditions that are evaluated as true or false
C++ has a boolean data type (called bool) that has values true and false. Improves readability.
Most functions that answer a yes/no question (or a true/false situation) will return a boolean answer (or in the case of user-defined functions, they should be coded that way)
Important: ANY C++ expression that evaluates to a value (i.e. any R-value) can be interpreted as a true/false condition. The rule is:
- If an expression evaluates to 0, its truth value is false
- If an expression evaluates to non-zero, its truth value is true

Logical Operators:

The arithmetic comparison operators in C++ work much like the symbols we use in mathematics. Each of these operators returns a true or a false.

  x == y        // x is equal to y
  x != y        // x is not equal to y
  x < y         // x is less than y
  x <= y        // x is less than or equal to y
  x > y         // x is greater than y
  x >= y        // x is greater than or equal to y

We also have Boolean operators for combining expressions. Again, these operators return true or false

  x && y        // the AND operator -- true if both x and y are true
  x || y        // the OR operator -- true if either x or y (or both) are true
  !x            // the NOT operator (negation) -- true if x is false

These operators will be commonly used as test expressions in selection statements or repetition statements (loops).

Examples of expressions

  (x > 0 && y > 0 && z > 0)     // all three of (x, y, z) are positive
  (x < 0 || y < 0 || z < 0)     // at least one of the three variables is negative

  ( numStudents >= 20 && !(classAvg < 70))
        // there are at least 20 students and the class average is at least 70
  
  ( numStudents >= 20 && classAvg >= 70)
        // means the same thing as the previous expression

Short Circuit Evaluation:

The && and || operators also have a feature known as short-circuit evaluation.

In the Boolean AND expression (X && Y), if X is false, there is no need to evaluate Y (so the evaluation stops). Example:

  (d != 0 &&  n / d > 0)
  
  // notice that the short circuit is crucial in this one.  If d is 0,
  //  then evaluating (n / d)  would result in division by 0 (illegal). But
  //  the "short-circuit" prevents it in this case.  If d is 0, the first
  //  operand (d != 0) is false.  So the whole && is false.

Similarly, for the Boolean OR operation (X || Y), if the first part is true, the whole thing is true, so there is no need to continue the evaluation. The computer only evaluates as much of the expression as it needs. This can allow the programmer to write faster executing code.

Selection Statements

The `if/else` Selection Statement

The most common selection statement is the if/else statement. Basic syntax:
```
   if (expression)
      statement
   else
      statement
```
The else clause is optional, so this format is also legal:
```
   if (expression)
      statement
```
The expression part can be any expression that evaluates a value (an R-value), and it must be enclosed in parintheses ( ).
- The best use is to make the expression a Boolean expression, which is an operation that evaluates to true or false
- For other expressions (like (x + y), for instance):
  - an expression that evaluates to 0 is considered false
  - an expression that evalautes to anything else (non-zero) is considered true
The statement parts are the "bodies" of the if-clause and the else-clause. The statement after the if or else clause must be either:
- an empty statement
```
    ;
```
- a single statement
```
    expression;
```
- a compound statement (i.e. a block). Can enclose multiple code statements. Remember, a compound statement is enclosed in set braces { }
Appropriate indentation of the bodies of the if-clause and else-clause is a very good idea (for human readability), but irrelevant to the compiler

Examples

   if (grade >= 68)
      cout << "Passing";

// Notice that there is no else clause. If the grade is below 68, we move on.

   if (x == 0)
      cout << "Nothing here";
   else
      cout << "There is a value";

// This example contains an else clause. The bodies are single statements.

   if (y != 4)
   {
      cout << "Wrong number";
      y = y * 2;
      counter++;
   }
   else
   {
      cout << "That's it!";
      success = 1;
   }

Multiple statements are to be executed as a result of the condition being true or false. In this case, notice the compound statement to delineate the bodies of the if and else clauses.

Be careful with ifs and elses. Here's an example of an easy mistake to make. If you don't use { }, you may think that you've included more under an if condition than you really have.

// What output will it produce if val = 2? Does the "too bad" statement really go with the "else" here?

   if (val < 5)
      cout << "True\n";
   else
      cout << "False\n";
      cout << "Too bad!\n";

* Indentation is only for people! It improves readability, but means nothing to the compiler.

Example links

Some common errors

What's wrong with these if-statements? Which ones are syntax errors and which ones are logic errors?

if (x == 1 || 2 || 3)
   cout << "x is a number in the range 1-3";

if (x > 5) && (y < 10)
   cout << "Yahoo!";

if (response != 'Y' || response != 'N')
   cout << "You must type Y or N (for yes or no)";

The `switch` statement

A switch statement is often convenient for occasions in which there are multiple cases to choose from. The syntax format is:

switch (expression)
{ 
    case constant: 
        statements 
    case constant: 
        statements 

    ...  (as many case labels as needed) 

    default:            // optional label 
        statements 
}

The switch statement evaluates the expression, and then compares it to the values in the case labels. If it finds a match, execution of code jumps to that case label.
The values in case labels must be constants, and may only be integer types, which means that you
- This means only integer types, type char, or enumerations (not yet discussed)
- This also means the case label must be a literal or a variable declared to be const
- Note: You may not have case labels with regular variables, strings, floating point literals, operations, or function calls
If you want to execute code only in the case that you jump to, end the case with a break statement, otherwise execution of code will "fall through" to the next case
Examples:
- Recall this example -- an if/else structure to determine a letter grade
- Switch Example 1 -- An attempt to convert the letter grade example into a switch. Syntactically correct, but has a logic flaw. What is it?
- Switch Example 2 -- corrected version of Example 1
- Switch Example 3 -- Uses a switch statement to process a menu selection, using both upper and lower case options

The Conditional Operator

There is a special operator known as the conditional operator that can be used to create short expressions that work like if/else statements.

Format:

  test_expression ? true_expression : false_expression

How it works:
- The test_expression is evaluated for true/false value. This is much like the test expression of an if-statement
- If the test expression is true, the operator returns the true_expression
- If the test expression is false, the operator returns the false_expression
Note that this operator takes three operands. It is the one ternary operator in the C++ language

Example 1:

  cout << (x > y ? "x is greater than y" : "x is less than or equal to y");

  // Note that this expression gives the same result as the following
  if (x > y)
     cout << "x is greater than y";
  else
     cout << "x is less than or equal to y";

Example 2:

  (x < 0 ? value = 10 : value = 20);

  // this gives the same result as:
  value = (x < 0 ? 10 : 20);

  // and also gives the same result as:
  if (x < 0)
     value = 10;
  else
     value = 20;