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Branching - Controlling the Flow of a Program (in Java)

Two roads diverged in a yellow wood,

And sorry I could not travel both

…

-Robert Frost, The Road Not Taken

1. Overview
2. Flow Charts
3. If/Else if/Else
4. Switch/Case
5. The Ternary Operator
6. Conclusions

Overview

Concept

The flow of a program can be controlled such that it deviates from its ‘normal’ path.

• What is a program’s ‘normal’ path?

• By default, a program will execute statements in the order written, from top to bottom.

• Some branches are unconditional - the control will always break out of this sequential flow every time the program is executed.

• Other branches are conditional - certain contextual conditions must be met if the control of the program is to break out of its usual flow.

• Conditional branches are our focus here.

Flow Charts

Concept

Understanding the flow of a program’s control can be visualized easily with diagrams, irrespective of the programming language and code.

• Writing code is precarious, laborious, and error-prone.

• Creating such flow diagrams can help understand and communicate the problem being solved without such burden.

• The Unified Modeling Language (UML) has attempted to define the “Activity Diagram” - a standard for how such diagrams should be drawn.

• In practice, people often use parts of the UML Activity Diagram style mixed with their own personal styles.

Example

A virtual card game program randomly picks two cards, from 1 to 11.

• If those two cards add up to 21, the program outputs the text “Blackjack!”
• Otherwise, the program outputs the text, “Maybe next time!”

Example

UML Activity Diagram Rules

UML Activity Diagrams follow a few basic rules:

• program control starts at the top of the diagram and ends at the bottom.

• the start point is indicated with just filled circle, the end with a filled circle with a line around it

• any process is indicated with an ellipse (or a rounded rectangle)

• if any process being diagrammed is so complex it makes the diagram complex and difficult to understand, make a separate activity diagram for that sub-process, and represent it as a single process in this diagram.

• any decision point leading to more than one branch is indicated with a diamond shape (also called a rhombus)

• branches should have simple annotations indicating in what decision context they are followed

If/Else if/Else

Isolated ifs

if statements on their own either execute a particular branch or not.

int card1 = (int) (Math.random() * 11) + 1; // number from 1 to 11
int card2 = (int) (Math.random() * 11) + 1; // number  from 1 to 11

if (card1 + card2 == 21) {
    // if the cards add up to 21...
    System.out.println("Blackjack!");
}

Binary choice

if statements followed by else blocks can have control flow into one of two branches, depending on a single condition being met or not.

int card1 = (int) (Math.random() * 11) + 1; // number from 1 to 11
int card2 = (int) (Math.random() * 11) + 1; // number  from 1 to 11

if (card1 + card2 == 21) {
    // if the cards add up to 21...
    System.out.println("Blackjack!");
}
else {
    // otherwise...
    System.out.println("Maybe next time!");
}

n-ary

if statements followed by one or more else if blocks can have control flow into one of several different branches, depending on multiple conditions.

int card1 = (int) (Math.random() * 11) + 1; // number from 1 to 11
int card2 = (int) (Math.random() * 11) + 1; // number  from 1 to 11

if (card1 + card2 == 21) {
    // if the cards add up to 21...
    System.out.println("Blackjack!");
}
else if (card1 == 1 && card2 == 1) {
    // otherwise if the cards are both ones...
    System.out.println("Snake eyes!");
}
else {
    // otherwise...
    System.out.println("Maybe next time!");
}

UML Activity Diagram

Switch/Case

Concept

In Java, an alternative to if/else if/else statements is switch/case statements.

• these can be easier to read and write in particular situations.

• such as when the branch followed depends upon the value of a single variable

• and when multiple conditions lead to the same branch

Example

Imagine a program that recommends how to dress.

Not using switch / case

This could be handled by if/else if/else statements with some boolean operators thrown in.

Scanner scn = new Scanner(System.in);
String weather = scn.nextLine();

if ( weather.equals("very sunny") || weather.equals("raining") ) {
    // if it's raining or very sunny...
	System.out.println("Take an umbrella");
}
else if (weather.equals("snowing") ) {
    // otherwise, if it's snowing...
	System.out.println("Wear a hat!");
}
else {
    // otherwise...
	System.out.println("Good luck!");
}

Using switch / case

Using switch/case would remove the need for boolean operations and provide more readable code.

Scanner scn = new Scanner(System.in);
String weather = scn.nextLine();

switch (weather) {
    case "raining":
    case "very sunny":
        // if it's raining or very sunny...
        System.out.println("Take an umbrella!");
        break;
    case "snowing":
        // otherwise, if it's snowing...
        System.out.println("Wear a hat!");
        break;
    default:
        // otherwise...
        System.out.println("Good luck!");
}

The Ternary Operator

Concept

It is often the case that we use if/else statements to control setting the value of a variable.

if (breakfastWasServed && breakfastWasEaten) {
  satiationLevel = "full"
} else {
  satiationLevel = "hungry"
}

The ternary operator, ?, thankfully allows us to simplify such syntax:

satiationLevel = breakfastWasServed && breakfastWasEaten ? "full" : "hungry"

Conclusions

You now have a basic understanding of UML activity diagrams, if/else if/else statements, and switch/case statements.

• Thank you. Bye.