Time estimate: 45 min.

3.2. if Statements and Control Flow¶

If you took an AP CSP course or used a block programming language like Scratch, you’ve probably seen if blocks or statements before. If statements are found in all programming languages as a way to make choices. Here’s a comparison of ifs in App Inventor blocks, AP CSP block and pseudocode and Java ifs.

../_images/BlocksIfComparison.png — Figure 1: Comparison of App Inventor if block, AP CSP ifs, and Java if statements¶

The statements in a Java main method normally run or execute one at a time in the order they are found from top to bottom. If statements (also called conditionals or selection) change the flow of control through the program so that some code is only run when something is true. In an if statement, if the condition is true then the next statement or a block of statements will execute. If the condition is false then the next statement or block of statements is skipped.

../_images/Condition.png — Figure 2: The order that statements execute in a conditional¶

A conditional uses the keyword if followed by Boolean expression inside of an open parenthesis ( and a close parenthesis ) and then followed by a single statement or block of statements. The single statement or block of statements are only executed if the condition is true. The open curly brace { and a close curly brace } are used to group a block of statements together. It is recommended to always put in the curly braces even if you have just one statement under the if statement. The questions you will see on the AP exam will use curly braces.

// A single if statement
if (boolean expression)
    Do statement;
// Or a single if with {}
if (boolean expression)
{
   Do statement;
}
// A block if statement: { } required
if (boolean expression)
{
   Do Statement1;
   Do Statement2;
   ...
   Do StatementN;
}

Note

Note that there is no semicolon (;) at the end of the boolean expression in an if statement even if it is the end of that line. The semicolon goes at the end of the whole if statement, often on the next line. Or { } are used to mark the beginning and end of the block of code under the if condition.

Imagine that your cell phone wanted to remind you to take an umbrella if it was currently raining in your area when it detected that you were leaving the house. This type of thing is going to become more common in the future and it is an area of research called Human Computer Interaction (HCI) or Ubiquitous Computing (computers are everywhere).

The variable isRaining is a boolean variable that is either true or false. If it is true then the message Take an umbrella! will be printed and then execution will continue with the next statement which will print Drive carefully. Run the code below to see this.

Check your understanding

3.2.1. Relational Operators in If Statements¶

Most if statements have a boolean condition that uses relational operators like ==, !=, <, >, <=, >=, as we saw in the last lesson.

Coding Exercise

Run the following active code a couple times until you see all the possible outputs. It prints out whether a random number is positive or equal to 0. Add another if statement that tests if it is a negative number.

public class TestNumbers
{
    public static void main(String[] args)
    {
        // Get a random number from -10 up to 10.
        int number = (int) (Math.random() * 21) - 10;
        System.out.println("The number is " + number);

// is it positive?
        if (number > 0)
        {
            System.out.println(number + " is positive!");
        }
        // is it 0?
        if (number == 0)
        {
            System.out.println(number + " is zero!");
        }
        // is it negative?
        // Add another if statement

}
}

====
// Test Code for Lesson 3.2.1 - Activity 1 - if-relational
import static org.junit.Assert.*;

import org.junit.Test;

import java.io.*;
import java.util.regex.MatchResult;
import java.util.regex.Pattern;

public class RunestoneTests extends CodeTestHelper
{

@Test
    public void testPositive()
    {
        String output = "";
        int num = -999, count = 0;

while (num <= 0 && count < 50)
        {
            output = getMethodOutput("main");
            num = getNumber(output);
            count++;

if (num == 9999999)
            {
                getResults(
                        "The number is ##\n## is positive!",
                        output,
                        "Did you forget to print the number?",
                        false);
                assertTrue(false);
                return;
            }
        }

String expect = "The number is " + num + "\n" + num + " is positive!";

boolean passed = output.contains("positive");
        getResults(expect, output, "Testing positive numbers", passed);
        assertTrue(passed);
    }

@Test
    public void testZero()
    {
        String output = "";
        int num = -999, count = 0;

while (num != 0 && count < 50)
        {
            output = getMethodOutput("main");
            num = getNumber(output);
            count++;

if (num == 9999999)
            {
                getResults(
                        "The number is ##\n## is zero!",
                        output,
                        "Did you forget to print the number?",
                        false);
                assertTrue(false);
                return;
            }
        }

String expect = "The number is " + num + "\n" + num + " is zero!";

boolean passed = output.contains("zero");
        getResults(expect, output, "Testing zero", passed);
        assertTrue(passed);
    }

@Test
    public void testNegative()
    {
        String output = "";
        int num = 999, count = 0;

while (num >= 0 && count < 50)
        {
            output = getMethodOutput("main");
            num = getNumber(output);
            count++;

if (num == 9999999)
            {
                getResults(
                        "The number is ##\n## is negative!",
                        output,
                        "Did you forget to print the number?",
                        false);
                assertTrue(false);
                return;
            }
        }

String expect = "The number is " + num + "\n" + num + " is negative!";

boolean passed = output.contains("negative");
        getResults(expect, output, "Testing negative numbers", passed);
        assertTrue(passed);
    }

private int getNumber(String output)
    {
        String regex = "[0-9]+";

String[] matches =
                Pattern.compile(regex)
                        .matcher(output)
                        .results()
                        .map(MatchResult::group)
                        .toArray(String[]::new);

int num = 9999999;

if (matches.length > 0)
        {
            num = Integer.parseInt(matches[0]);
        }

if (output.contains("-"))
        {
            num *= -1;
        }

return num;
    }
}

Note

A common mistake in if statements is using = instead of == in the condition by mistake. You should always use ==, not =, in the condition of an if statement to test a variable. One equal sign (=) assigns a value to a variable, and two equal signs (==) test if a variable has a certain value.

Check your understanding

3-2-4: Consider the following code segment. What is printed as a result of executing the code segment?

int x = 3;
if (x > 2)
{
    x = x * 2;
}
if (x > 4)
{
   x = 0;
}
System.out.print(x);

3
x is changed by the if statements.
6
What happens when x is greater than 2 and then greater than 4? Do both if statements.
0
If x is greater than 2, it's always doubled, and then that result is always greater than 4, so it's set to 0 in the second if statement.
4
x is changed by the if statements.
The code will not compile
This code will compile.

3.2.2. Common Errors with If Statements¶

Here are some rules to follow with if statements to avoid some common errors:

Always use curly braces ({ and }) to enclose the block of statements under the if condition. Java doesn’t care if you indent the code—it goes by the { }.

Don’t put in a semicolon ; after the first line of the if statement, if (test);. The if statement is a multiline block of code that starts with the if condition and then { the body of the if statement }.

Always use ==, not =, in the condition of an if statement to test a variable. One = assigns, two == tests!

Coding Exercise

The code below doesn’t work as expected. Fix it to only print Wear a coat and Wear gloves when isCold is true.

3.2.3. Programming Challenge : Magic 8 Ball¶

Have you ever seen a Magic 8 ball? You ask it a yes-no question and then shake it to get a random response like Signs point to yes!, Very doubtful, etc. If you’ve never seen a Magic 8 ball, check out this simulator.

We encourage you to work in pairs for this challenge. Come up with 8 responses to yes-no questions. Write a program below that chooses a random number from 1 to 8 and then uses if statements to test the number and print out the associated random response from 1-8. If you need help with random numbers, see lesson 2.9.

public class Magic8Ball
{
    public static void main(String[] args)
    {
        // Get a random number from 1 to 8

// Use if statements to test the random number
        // and print out 1 of 8 random responses

}
}

====
import static org.junit.Assert.*;

import org.junit.*;

import java.io.*;
import java.util.ArrayList;

public class RunestoneTests extends CodeTestHelper
{
    public RunestoneTests()
    {
        super("Magic8Ball");
    }

@Test
    public void test1()
    {
        String output = getMethodOutput("main");

boolean passed = output.length() > 0;

passed =
                getResults(
                        "Output length > 0",
                        "Output length of " + output.length(),
                        "Prints a statement",
                        passed);
        assertTrue(passed);
    }

@Test
    public void test2()
    {
        String[] output = new String[200];

for (int i = 0; i < output.length; i++)
        {
            output[i] = getMethodOutput("main");
        }

ArrayList<String> lines = new ArrayList<String>();

for (int i = 0; i < output.length; i++)
        {
            if (!lines.contains(output[i]))
            {
                lines.add(output[i]);
            }
        }

int responses = lines.size();
        boolean passed = lines.size() >= 8;

passed = getResults("8", "" + responses, "Unique responses", passed);
        assertTrue(passed);
    }

@Test
    public void test3()
    {
        String code = getCodeWithoutComments();

int numIfs = countOccurences(code, "if");

boolean passed = numIfs >= 7;

passed = getResults("7 or more", "" + numIfs, "Code has at least 7 if statements", passed);
        assertTrue(passed);
    }
}

You can make this code more interactive by using the Scanner class to have the user ask a question first; you can try your code with input in JuiceMind or replit or a local IDE.

3.2.4. Summary¶

if statements test a boolean expression and if it is true, go on to execute the following statement or block of statements surrounded by curly braces ({}) like below.

// A single if statement
if (boolean expression)
    Do statement;
// A block if statement
if (boolean expression)
{
   Do Statement1;
   Do Statement2;
   ...
   Do StatementN;
}

Relational operators (==, !=, <, >, <=, >=) are used in boolean expressions to compare values and arithmetic expressions.
Conditional (if) statements affect the flow of control by executing different statements based on the value of a Boolean expression.

3.2.5. AP Practice¶

3-2-7: Consider the following code segment.

int speed = 35;
boolean rain = false;

if (rain)
{
   speed -= 10;
}

if (rain == false)
{
  speed += 5;
}

if (speed > 35)
{
   speed = speed - 2;
}

System.out.println(speed);

What is printed as a result of executing the code segment?

28
Some of the if statement conditions are false so they will not run.
35
Take a look at the changes to speed in the if statements.
38
Correct! The first if statement condition is false, and the second and third if conditions are true.
25
The first if statement would only run if rain is true.
33
The second if statement would run since rain is false.

3-2-8: Consider the following code segment.

int x = 5;

if (x < 5)
{
   x = 3 * x;
}

if (x % 2 == 1)
{
   x = x / 2;
}

System.out.print(2*x + 1);