Selection Statements (if/else)

Control Structures - C++ has three types of control structures

1. sequence structures - built into C++; unless directed otherwise, computer executes each and every C++ statement one after another, top to bottom (this is all we've done so far)
2. selection structures - used to chose among alternative courses of action
3. iteration structures - allow a set of instructions to be repeated until a certain condition is reached; a condition may be open-ended as in a "while" and "do/while" loops or it may be predefined as in the "for" loop (we will cover this later)

Selection Structures

• "if" structure is single selection structure; it selects or ignores a single action; if the condition is true, the statement is executed; if the condition is false, the statement is skipped (indention is optional but improves readability of program)

if (condition)
     statement;

• "else" statement used to perform an alternative action; if the condition is true, the statement1 is executed; if the condition is false, the statement2 is executed

if (condition)
     statement1;
else
     statement2;

• a "condition" in an "if" structure can be formed by using two types of operators; these operators evaluate their operands to produce a true or false result; all relational operators have the same level of precedence and associate left to right; the equality operators have the same level of precedence (lower than relational operators) and associate left to right
• relational     >     <     >=     <=
• equality     ==     !=
• Use relational operators to form conditional expressions whose result is either true (1) or false (0)
  ( 4 < 5 ) evaulates to true (1)
  ( 2 * 4 == 3 ) evaluates to false (0)
• Conditional Expressions can involve both variables and constants
  e.g. x > 4        x <= y      y = = z
• "nested" if statements can be used to implement decisions with several alternatives; an else statement is executed when the nearest preceding if condition evaluates as false

if (condition 1 is true)
    if (condition 2 is true)
        statement a;
    else
        statement b;
else
    statement c;

Decision Tree



• to include several statements in the body of an if or an else(instead of one statement as shown previously), you include the statements in braces, { and } This is called a compound statement - each statement executes on after another

if (condition is true)
{
    statement a1;
    statement a2;
    statement a3;
}

//Program to determine age status
#include <iostream.h>
int main()
{
    int age;

    //Prompt user for age
    cout << endl << "Enter age in years: ";
    cin >> age;

    if (age > 40)
        cout << endl << "A " << age << "year old person is old";
    else
        cout << endl << "A " << age << "year old person is young";

    return 0;
}

Lab 4 Core 1 & Core 2 background & warmup (show debugger also)
show if( i > 10); does nothing

Logical Operators
Nested ifs are useful for double or overlapping conditions
e.g.
if (age >= 0)
    if (age > 40)
        cout << endl << "A " << age << "year old is old";
    else
        cout << endl << "A " << age << "year old is young";

Another way to do this is to use logical operators:
!      NOT (changes true to false, false to true)
&& AND (both conditions must be true)
||      OR (at least one of the condtions must be true)

e.g.
if (age >= 0) && (age > 40)
    cout << endl << "A " << age << " year old person is old";
else
    if (age >= 0) && (age <= 40)
        cout << endl << "A " << age << " year old person is young";

Lab 4 Core 3 background & warmup

Boolean expressions - Operators needed for Lab 4 and their precedence (highest to lowest)
( )
! (NOT)
*, /, %
+, -
<, <=, >, >= (relational operators)
==, != (equality operators)
&& (AND)
||(OR)
= (assignment)

Complete ACTIVITY program using the following specification:
Write a program that displays a suggested activity based on the outdoor temperature (in fahrenheit) and whether or not it is raining/snowing. Use the following table for suggested activities. Show both with nested loops and with logical operators.

"switch" structure ("switch" or "case" statement) - is a multiple selection structure (selects among many different actions); it can only be used for evaluation a constant integral condition (any combination of character constants (single character in quotes) and integer constants (integer value) that evaluates to a constant integer value); the result of the switch statement's expression is compared with each of the case labels; if the comparison with label1 is true, the statement following this case is executed; if the comparison is false, the next case is evaluated and so on; if no match occurs, the default statement (optional) is executed; it is recommended that you end each case structure with a break statement which ensures that only one statement sequence is executed (otherwise "fall-through" occurs with a match and the remaining statements after the match are executed

switch (expression)
{
case label1:
    statement1;
    break;
case label2:
    statement2;
    break;
case label3:
    statement3;
    break;
default:
    defaultStatement;
} 

// Sample program using switch statement.
#include <iostream.h>
int main ()
{
    char letterGrade;

    cout << endl << "Enter a letter grade: ";
    cin >> letterGrade;

    // Execute the selected test.
    switch ( letterGrade )
    {
        case 'A' : cout << "1st case" << endl; break;
        case 'B' : cout << "2nd case" << endl; break;
        case 'C' : cout << "3rd case" << endl; break;
        default : cout << "Invalid input" << endl;
    }

    return 0;
}

Lab 4 Core 4 background & warmup, discuss pseudo-code for application exercise

Copyright Matthew Bauer, Computer Science, Illinois Institute of Technology, Fall 2000