if Statements

Every program you wrote in Unit 1 did the same thing every time it ran. Java started at the first statement in main, ran it, moved to the next one, and kept going until the last one. No statement was ever skipped. Running statements one after another, in the order they are written, is called sequential execution.

Sequential execution is a problem as soon as a program has to react to its data. Here is a weather app that is supposed to tell you to wear a jacket only when it is cold:

int temperature = 68;
System.out.println("Wear a jacket.");
System.out.println("Have a good day.");

Wear a jacket.
Have a good day.

It is 68 degrees, and the program still says to wear a jacket. Line 2 has no way to know it should be skipped: Java runs every line, in order, no matter what temperature holds.

In Topic 2.1 you described algorithms with selection: a step where what happens next depends on a true or false decision, such as “If it is below 50 degrees, wear a jacket.” In Topic 2.2 you learned to write that decision in Java as a Boolean expression, such as temperature < 50, which works out to true or false. This topic connects the two. You will make Java use the value of a Boolean expression to decide whether a piece of code runs.

The order in which a program’s statements actually run is called its flow of control. In a sequential program, the flow of control goes straight down the page. A selection statement is a statement that changes that flow: depending on a Boolean expression, it lets the program run some statements and skip others. Java’s main selection statement is the if statement.

Think of a sign at a fork in a hiking trail: “Scenic overlook, 200 steps. Skip it if you are short on time.” Every hiker reaches the sign. Some take the side trail and some walk past it, and then everyone continues on the same main trail. An if statement is that side trail. (The analogy stops at choice: a hiker decides on a whim, but Java decides only by the value of the Boolean expression.)

Here is the jacket check written correctly:

int temperature = 45;
if (temperature < 50)
{
    System.out.println("Wear a jacket.");
}
System.out.println("Have a good day.");

Take lines 2 to 5 apart:

An if statement with a single body that either runs or is skipped is called a one-way selection. There is only one special path: take the side trail, or do not.

This picture shows both paths through the jacket check. Each path joins back up at the last line.

     is temperature < 50 ?
       /             \
     true           false
      |               |
  print jacket     skip body
       \             /
   print "Have a good day."

Wear a jacket.
Have a good day.

Now change one thing: the temperature.

int temperature = 68;
if (temperature < 50)
{
    System.out.println("Wear a jacket.");
}
System.out.println("Have a good day.");

Have a good day.

int temperature = 68;
if (temperature < 50);
{
    System.out.println("Wear a jacket.");
}
System.out.println("Have a good day.");

Wear a jacket.
Have a good day.

int speed = 30;
int limit = 35;
if (speed > limit)
{
    System.out.println("Slow down!");
}
System.out.println("Speed: " + speed);

int temperature = 50;
if (temperature < 50)
{
    System.out.println("Wear a jacket.");
}
System.out.println("Have a good day.");

The jacket check had a one-line body and a simple comparison. Real decisions often need more: when an order qualifies for free shipping, the program must both change the shipping cost and tell the customer. This subsection looks closely at what can go in each part of an if statement.

The condition can be any expression that works out to a boolean value. From Topic 2.2 that includes comparisons such as total >= 50. It also includes a boolean variable on its own, because the variable already holds true or false, and a method call that returns a boolean, such as code.equals("SAVE5") from Topic 1.15. Whatever the condition is, it goes inside parentheses, and the parentheses are required: if total >= 50 does not compile.

The body can hold as many statements as you need. All of the statements between the braces form one group, called a block, and they run together: all of them when the condition is true, none of them when it is false. The body can do anything a normal statement can do, including changing a variable.

double total = 62.5;
double shipping = 5.0;
if (total >= 50)
{
    shipping = 0.0;
    System.out.println("Free shipping!");
}
total += shipping;
System.out.println("Total: " + total);

Free shipping!
Total: 62.5

Now vary the kind of condition. This store takes $2 off for members and $5 off with a coupon code. Each discount is its own one-way if statement.

double price = 20.0;
boolean isMember = true;
String code = "SAVE5";
if (isMember)
{
    price -= 2.0;
}
if (code.equals("SAVE5"))
{
    price -= 5.0;
}
System.out.println("Price: " + price);

Price: 13.0

int score = 100;
if (score = 100)
{
    System.out.println("Perfect score!");
}

error: incompatible types: int cannot be converted to boolean

int temperature = 68;
if (temperature < 50)
    System.out.println("Wear a jacket.");
    System.out.println("Bring gloves.");
System.out.println("Have a good day.");

Bring gloves.
Have a good day.

int temperature = 68;
if (temperature < 50)
{
    System.out.println("Wear a jacket.");
    System.out.println("Bring gloves.");
}
System.out.println("Have a good day.");

Ouch!
Lives: 2

int lives = 2;
boolean hitByEnemy = false;
if (hitByEnemy)
    lives--;
    System.out.println("Ouch!");
System.out.println("Lives: " + lives);

int lives = 2;
boolean hitByEnemy = false;
if (hitByEnemy)
{
    lives--;
    System.out.println("Ouch!");
}
System.out.println("Lives: " + lives);

Lives: 2

A one-way if statement handles “do this, or do nothing.” Many decisions have two real outcomes instead. A quiz app should say “You passed.” for a score of 70 or more, and “Try again next time.” otherwise. Every student should see exactly one of those two messages.

You could try two one-way if statements, one with score >= 70 and one with score < 70. That works here, but it makes Java check two conditions when one answer settles everything, and it relies on you writing the second condition as the exact opposite of the first. Java has a statement built for this situation.

An if-else statement has two bodies. The first runs when the condition is true. The second, after the keyword else, runs when the condition is false. Because a condition is always either true or false, exactly one of the two bodies runs: never both, never neither. This is called two-way selection. In the trail analogy, the path splits into a left branch and a right branch, every hiker takes one of them, and the branches meet again further on.

int score = 64;
if (score >= 70)
{
    System.out.println("You passed.");
}
else
{
    System.out.println("Try again next time.");
}
System.out.println("Score: " + score);

The else has no condition because it does not need one. It covers every case the if did not, so the condition is checked once and decides between both bodies.

       is score >= 70 ?
        /           \
      true         false
       |             |
 "You passed."  "Try again..."
        \           /
     print the score

Try again next time.
Score: 64

Now vary the condition. Topic 1.3’s remainder operator gives a classic two-way decision: a whole number is even when dividing it by 2 leaves remainder 0, and odd otherwise.

int number = 17;
if (number % 2 == 0)
{
    System.out.println(number + " is even.");
}
else
{
    System.out.println(number + " is odd.");
}

17 is odd.

int score = 64;
if (score >= 70);
{
    System.out.println("You passed.");
}
else
{
    System.out.println("Try again next time.");
}

error: 'else' without 'if'

int age = 12;
double price;
if (age < 13)
{
    price = 8.5;
    System.out.println("Child ticket");
}
else
{
    price = 12.0;
    System.out.println("Adult ticket");
}
System.out.println("Price: " + price);

Child ticket
Price: 8.5

int age = 15;
if (age >= 16)
{
    System.out.println("You may drive.");
}
else (age < 16)
{
    System.out.println("Not yet.");
}

int age = 15;
if (age >= 16)
{
    System.out.println("You may drive.");
}
else
{
    System.out.println("Not yet.");
}

You now have two tools: a one-way if statement, which runs its body or skips it, and an if-else statement, which always runs exactly one of its two bodies. Picking the right one depends on the question the program is asking.

• Use a one-way if when there is something extra to do in one case and nothing special to do otherwise: add a bonus, print a warning, apply a discount.
• Use an if-else when there are two outcomes and exactly one of them must happen: pass or try again, even or odd, child ticket or adult ticket.

When a program has several one-way if statements in a row, each one is a separate statement with its own condition. Topic 2.1 noted that the order of the steps in an algorithm affects its result. That matters here: Java runs the if statements one after another, checks every condition, and each condition sees the variables as earlier statements left them. So any number of their bodies can run: all, some, or none.

int score = 93;
if (score >= 90)
{
    System.out.println("Excellent work!");
}
if (score >= 70)
{
    System.out.println("You passed.");
}

Excellent work!
You passed.

Now vary the code so the first body changes the variable that the second condition checks. A student with $12 on a transit card buys a $10 ticket if they can afford it, and the program warns when the balance drops below $10.

int balance = 12;
if (balance >= 10)
{
    balance -= 10;
    System.out.println("Bought a ticket.");
}
if (balance < 10)
{
    System.out.println("Balance is low.");
}
System.out.println("Balance: " + balance);

Bought a ticket.
Balance is low.
Balance: 2

int balance = 12;
if (balance >= 10)
{
    balance -= 10;
    System.out.println("Bought a ticket.");
}
else
{
    System.out.println("Balance is low.");
}
System.out.println("Balance: " + balance);

Bought a ticket.
Balance: 2

int score = 85;
String message;
if (score >= 70)
{
    message = "Pass";
}
System.out.println(message);

error: variable message might not have been initialized

int score = 85;
String message;
if (score >= 70)
{
    message = "Pass";
}
else
{
    message = "Not yet";
}
System.out.println(message);

int stock = 4;
if (stock < 5)
{
    stock += 10;
    System.out.println("Restocked.");
}
if (stock >= 5)
{
    System.out.println("In stock.");
}
System.out.println("Stock: " + stock);

Restocked.
In stock.
Stock: 14

A library charges 25 cents per day for a late book, up to a maximum of $2.00. This program reads the number of days late, then uses an if-else statement and a one-way if statement to work out the fee. As in Unit 1, the Scanner reads from a string so the output is always the same; reading from the keyboard works the same way.

import java.util.Scanner;

public class LibraryFee
{
    public static void main(String[] args)
    {
        Scanner input = new Scanner("11");
        int daysLate = input.nextInt();
        double fee = 0.0;

        if (daysLate > 0)
        {
            fee = daysLate * 0.25;
            System.out.println("Overdue by " + daysLate + " days.");
        }
        else
        {
            System.out.println("Returned on time. Thank you!");
        }

        if (fee > 2.0)
        {
            fee = 2.0;
            System.out.println("Fee capped at the maximum.");
        }

        System.out.println("Fee: $" + fee);
    }
}

Overdue by 11 days.
Fee capped at the maximum.
Fee: $2.0

• Without selection, Java runs every statement in order. A selection statement changes this flow of control, so some statements run and others are skipped depending on a Boolean expression.
• In a one-way if statement, the body runs only when the condition is true; when it is false, Java skips to the statement after the closing brace. Statements after the if statement run either way.
• The condition must be a boolean expression in parentheses: a comparison, a boolean variable, or a method call such as equals. Braces group a body of several statements; without them, only the next statement belongs to the body, whatever the indentation.
• In an if-else statement, the condition is checked once: the if body runs when it is true and the else body when it is false, so exactly one body runs. else has no condition.
• Separate if statements are each checked in turn, so several bodies or none may run, and a later condition sees any changes made by an earlier body.
• A semicolon right after if (...) ends the if statement: on its own it silently makes the body always run, and before an else it causes a compile error.

String day = "sat";
if (day.equals("Sat"))
{
    System.out.println("Weekend");
}
else
{
    System.out.println("Weekday");
}

int minutes = 135;
int hours = minutes / 60;
int extra = minutes % 60;
if (extra >= 30)
{
    hours++;
}
if (hours > 2)
{
    System.out.println("Long movie");
}
else
{
    System.out.println("Short movie");
}
System.out.println("About " + hours + " hours");

Short movie
About 2 hours

int score = 70;
if (score > 70)
{
    System.out.println("You passed.");
}
else
{
    System.out.println("Try again next time.");
}

int score = 70;
if (score >= 70)
{
    System.out.println("You passed.");
}
else
{
    System.out.println("Try again next time.");
}

double average = 89.6;
if ((int) average >= 90)
{
    System.out.println("A");
}
else
{
    System.out.println("B");
}

double total = 18.0;
if (total >= 25.0)
{
    System.out.println("You earned a coupon.");
    System.out.println("Thanks for shopping!");
}
else
{
    System.out.println("Spend $25 for a coupon.");
    System.out.println("Thanks for shopping!");
}

double total = 18.0;
if (total >= 25.0)
{
    System.out.println("You earned a coupon.");
}
else
{
    System.out.println("Spend $25 for a coupon.");
}
System.out.println("Thanks for shopping!");

int x = 7;
if (x > 5)
{
    x -= 3;
}
if (x <= 5)
{
    System.out.println("small");
}

int x = 7;
if (x > 5)
{
    x -= 3;
}
else
{
    System.out.println("small");
}

Check each statement. If the answer is “not yet,” reread the linked subsection.

• I can explain how an if statement changes the usual top-to-bottom order of a program. Reread
• I can trace a one-way if statement and say whether its body runs. Reread
• I can write a condition that is a boolean expression and use braces around a body of several statements. Reread
• I can trace an if-else statement and explain why exactly one body runs. Reread
• I can choose between a one-way if and an if-else, and trace separate if statements that affect each other. Reread