Control Structures: Selection

Mithat Konar

Introduction

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

Control structures in C++

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”

Single-entry/single-exit structures

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.

Selection structures

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

if selection structure

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.

if syntax

C++ syntax:
```
if (<expression>)
  <statement>;
```

Example:

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

complete example

Relational operators

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++ relational operators

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!

Order of precedence

operator	associativity	type
()	left to right	parenthesis
* / %	left to right	multiplicative
+ -	left to right	additive
<< >>	left to right	stream insertion/extraction
< <= > >=	left to right	relational
== !=	right to left	equality
=	right to left	assignment

if/else selection structure

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.

if/else syntax

C++ syntax:

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

Example:

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

complete example

Nested if/else selection structure

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

Nested if/else pseudocode

“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"

Nested if/else pseudocode

“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"

Nested if/else syntax

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

Compound statements

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;
}

Compound statement example

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.

Blocks

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

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

C++ logical operators

&& (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: ! → && → ||

Truth table for logical AND

expression	result
`false && false`	false
`false && true`	false
`true && false`	false
`true && true`	true

Truth table for logical OR

expression	result
`false \|\| false`	false
`false \|\| true`	true
`true \|\| false`	true
`true \|\| true`	true

Truth table for logical NOT

expression	result
`!false`	true
`!true`	false

Examples

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

expression	result
`(x > y) && (y > z)`	true
`(x > y) && (z > y)`	false
`(x <= z) \|\| (y == z)`	false
`(x <= z) \|\| (y != z)`	true
`!(x >= z)`	false

complete example

Mithat Konar (the wiki)

Control Structures: Selection

Introduction

Control structures in C++

Single-entry/single-exit structures

Selection structures

if selection structure

if syntax

Relational operators

C++ relational operators

Order of precedence

if/else selection structure

if/else syntax

Nested if/else selection structure

Nested if/else pseudocode

Nested if/else pseudocode

Nested if/else syntax

Compound statements

Compound statement example

Blocks

Logical operators

C++ logical operators

Truth table for logical AND

Truth table for logical OR

Truth table for logical NOT

Examples

Short-circ