If statements

COP 3330 – Object Oriented Programming in Java

Boolean Expressions and the If Statement

Boolean Expressions

Definition: In Java, boolean (in lowercase) is a type that can hold one of two values:
- Literals:
- true
- false
Example:
- boolean ok = true;
- This statement initializes a variable named ok to true.

Relational Operators

Definition: Boolean expressions can be constructed using relational operators that compare primitive quantitative types.
List of Operators:
- Greater Than: >
- Greater Than or Equal To: >=
- Less Than: <
- Less Than or Equal To: <=
- Equal To: ==
- Not Equal To: !=
Comparison with C and Python:
- In both C and Java, the operators behave similarly.
- In C, the result is an integer (0 or 1) rather than a boolean.
- For example, if age is set to 40:
- age >= 21 evaluates to true.
- age < 40 evaluates to false.

Boolean Operators

Use: To create complex boolean expressions, multiple simple boolean expressions can be combined using boolean operators.
List of Operators:
- Logical AND: &&
- Logical OR: ||
- Logical NOT: !
Comparison with Other Languages:
- The operators in Java correspond with C.
- Python uses keywords (e.g. and, or, not) instead of symbols.
Example:
- If age is set to 17, the expression age >= 16 && age <= 18 evaluates to true.
- Note: 16 <= age <= 18 does not compile in Java, but works in C because it evaluates 16 <= age first, yielding 1 (true), which is then compared to whether it's <= 18. In Python, it gets interpreted implicitly with and.
Advantages of Java:
- The strict rules in Java catch more errors at compile time, reducing debugging time for the programmer.

If Statement

General Description:
- The if statement functions similarly across C, Python, and Java, with minor syntax differences.
Block of Statements:
- Python: Indentation signifies a block.
- C and Java: Curly braces {} are used to denote a block of statements inside a control structure.
- Styles:
  - Placing the opening brace on the same line as the control structure.
  - Placing the opening brace on the next line left-justified.
- Closing braces should be left-justified to match the start of the control structure.
Single Statement Rule:
- All three languages allow only one statement officially within an if statement, but a block of statements is treated as a single statement.
- In Java and C, failing to use braces means only the next statement is executed within the if.
- In Python, indentation clarifies what belongs to the control structure.

If Statement Versions

Version 1: Basic If Statement
- Syntax:
```java
if (boolean_expression) stmt;

  - For multiple statements inside, use:

java
if (boolean_expression) {
stmt1;
…
stmtk;
}

  - **Behavior**: If the boolean expression evaluates to `true`, execute statements inside; otherwise, skip them.  

- **Version 2**: If-Else Statement  
  - **Syntax**:

java
if (boolean_expression) stmt; else stmt;

  - Using blocks:

java
if (boolean_expression) {
stmta1;
…
stmtak;
} else {
stmtb1;
…
stmtbk;
}

  - **Behavior**: Executes statements based on the evaluation of the boolean expression; one block runs, the other is skipped.  

- **Version 3**: Else-If Ladder  
  - **Syntax**:

java
if (booleanexpression1) stmt1;
else if (booleanexpression2) stmt2;
else if (booleanexpression3) stmt3;
…
else if (booleanexpressionk) stmtk;

  - **Behavior**: Evaluates expressions in order; executes the block of the first `true` expression; if no expressions are true, none are executed.  
  - **Note**: In Python, the keyword `elif` is used instead of `else if`.  

- **Version 4**: Else-If with Default Case  
  - **Syntax**:

java
if (booleanexpression1) stmt1;
else if (booleanexpression2) stmt2;
else if (booleanexpression3) stmt3;
…
else if (booleanexpressionk) stmtk;
else stmtn;

  - **Behavior**: A default action `stmtn` is executed if all previous boolean expressions evaluate to `false`.  

### If Statement Design/Caveats  
- **Flexibility**: With four constructs, a programmer can create a vast variety of programs.  
- **Logical Nuances**: The structure and placement of statements can significantly impact execution and meaning.  
- **Common Error**: Beginners often feel compelled to use `else` with every `if`; however, not every logical task requires an `else`.  

### Example Using Framework #1  
- **Context**: Determine item cost with or without tax.  
- **Program**:

java
import java.util.*;
public class SalesTax {
final public static double SALESTAXRATE = 0.065;
public static void main(String[] args) {
// Get cost.
Scanner stdin = new Scanner(System.in);
System.out.println("How much did your item cost?");
double cost = stdin.nextDouble();
// Get if it's taxed or not.
System.out.println("Is it taxed?(0=no,1=yes)");
int tax = stdin.nextInt();
// Add tax only if necessary.
if (tax == 1) cost += (cost * SALESTAXRATE);
// Print final cost.
System.out.printf("Your final cost is $%.2f.\n", cost);
}
}

- **Notes**:  
  - One statement inside the `if` does not require a block of statements.  
  - Indentation is essential for readability, even if Java allows flexibility in indents.  
  - Experimentation: Change `==` to `=` to observe outcomes and learn about common programming errors.  

### Example Using Framework #2  
- **Context**: Temperature conversion between Celsius and Fahrenheit.  
- **Program**:

java
import java.util.*;
public class Temp {
public static void main(String[] args) {
Scanner stdin = new Scanner(System.in);
// Get the desired conversion.
System.out.println("Would you like to…");
System.out.println("1. Convert from Celsius to Fahrenheit.");
System.out.println("2. Convert from Fahrenheit to Celsius.");
int ans = stdin.nextInt();
double fahr, cel;
// cel -> fahr.
if (ans == 1) {
// Read in the input temperature.
System.out.println("What is the temperature in Celsius?");
cel = stdin.nextDouble();
// Calculate and output temperature in Fahrenheit.
fahr = 1.8 * cel + 32;
System.out.print("The converted temperature is " + fahr);
System.out.println(" degrees Fahrenheit.");
}
// fahr -> cel
else {
// Read in the input temperature.
System.out.println("What is the temperature in Fahrenheit?");
fahr = stdin.nextDouble();
// Calculate and output temperature in Celsius.
cel = 5 * (fahr - 32) / 9;
System.out.print("The converted temperature is " + cel);
System.out.println(" degrees Celsius.");
}
}
}

- **Notes**:  
  - Menu presents two choices.  
  - Each choice has its own block for the corresponding task.  
  - Calculations and print statements change based on selected task.  

### Example Using Framework #3  
- **Context**: Calculate user's age based on birth date.  
- **Concept**: Subtract birth year from the current year and adjust based on birthday.  
- **Program**:

java
import java.util.*;
public class Age {
final public static int CURYEAR = 2026;
public static void main(String[] argv) {
Scanner stdin = new Scanner(System.in);
System.out.println("Enter the month of your birthday:");
int month = stdin.nextInt();
System.out.println("Enter the day of your birthday:");
int day = stdin.nextInt();
System.out.println("Enter the year of your birthday:");
int year = stdin.nextInt();
System.out.println("Enter today's month:");
int curmonth = stdin.nextInt();
System.out.println("Enter today's day:");
int curday = stdin.nextInt();
int age = CURYEAR - year;
if (curmonth < month) age--;
else if (curmonth == month && curday < day) age--;
System.out.println("You are " + age + " years old.");
}
}

- **Notes**:  
  - The program calculates age correctly by adjusting for the actual birthday.  
  - Logic is handled with clear `if` and `else if` structures.

### Example Using Framework #4  
- **Context**: Convert numeric grades to letter grades (A, B, C, D, F).  
- **Program**:

java
import java.util.*;
public class ClassGrade {
public static void main(String[] args) {
// Get input.
Scanner stdin = new Scanner(System.in);
System.out.println("What percent(integer) did you get?");
int grade = stdin.nextInt();
char letter;
// Criterion for an A.
if (grade >= 90) letter = 'A';
else if (grade >= 80) letter = 'B';
else if (grade >= 70) letter = 'C';
else if (grade >= 60) letter = 'D';
else letter = 'F';
// Show letter grade.
System.out.println("Your letter grade is " + letter + ".");
}
}
```

Notes:
- The program contains logical branches to evaluate and assign letter grades based on numeric input.
- The order of these conditions is crucial to ensure proper grading.
- If conditions are not mutually exclusive, errors may result (e.g., if checking for D were done before C).