Control Structures

Chapter 4

Control structures--statements that control the flow of execution

• All programs can be written in terms of three control structures

– Sequence structure

• Executing statements sequentially

– if (condition)-- selection structure

• Condition is either true or false

• Select between paths of execution

– Loops -- repetition structures

• Execute a block of statements over and over

Selection statements in Java

• Two forms of the if statement

• if (condition) do something;

– The condition must return either true or false

– if the condition is false, do nothing

• OR select between two different actions

if (condition) do something;
else do something different;

• The switch statement selects between multiple options

Pseudocode

• Thinking in English is easier than thinking in Java

• Suppose we want to write a program to ask the user to enter a student grade, then have the program respond passed or failed

– If the grade is greater than 60 the student passes, otherwise they fail

• Think about the different steps that are necessary to write this program

Translate pseudocode to Java

public static void main(String[ ] args
{ Scanner in = new Scanner(System.in);

System.out.println(“Please enter the grade”);

double grade = in.nextDouble();

if(grade > 60)
System.out.println(“Pass”);

else System.out.println(“Fail”);

}

Nested if statements

• If there is more than one decision to be made, the if statements can be nested.

if (grade > 90)
{ System.out.println("you get an A"); }

else if (grade > 80)
{ System.out.println("you get a B"); }

else if (grade > 70)
{ System.out.println("you get a C"); }

else { System.out.println("you get an F"); }

• One or the other of an if/else statement is executed, never both.

Formatting nested if statements

• Often the nested if is not further indented

if (grade > 90)
{ System.out.println("you get an A"); }

else if (grade > 80)
{ System.out.println("you get a B"); }

else if (grade > 70)
{ System.out.println("you get a C"); }

else { System.out.println("you get an F"); }

Dangling else problem

• The compiler always associates an else with the immediately preceding if

if (x > 90)
if (y > 90)
System.out.println(“x and y are > 90");

else
System.out.println(“x is <= 90");

• The compiler pays no attention to white space, so it thinks the else goes with the second if; braces will tell it otherwise

if (x > 90)
{ if (y > 90)
System.out.println(“x and y are > 90");

}

else
System.out.println(“x is <= 90");

When to use braces

• Often only one statement follows an if condition

– In this case no braces are necessary

• If several statements follow either an if or else statement, you must use braces to indicate that the belong to one block

if (grade > 90)
{ System.out.println(“Grade is an A");
System.out.println(“Good Job!");

}

else
{ System.out.println(“Not an A this time.");
System.out.println(“Maybe next time.");
}

Second control Structure: Loops

• Loops are also controlled by a conditional statement

• The statements within the loop are repeated until the condition becomes false

• Types of loops

– for loops

– while loops

– do while loops

for loops

• Executes a section of code a fixed number of times

• for(count=0;count<10;count++)
{
statements
}

• Note the three parts to the control statement

– Initial value of the counter

– Test for the final value; this is the terminating condition

– The modify the counter; here we use the increment operator

For loops

• For loops are used when you know ahead of time how many times you want to repeat the statements in the loop body

• Suppose we want to write Java code to find the sum of the first ten integers

• We put it inside a main method, since it is not really part of a class

Example: sum the first ten integers

public static void main(String[ ] args)
{ final int LIMIT = 10;

int i, sum=0;

for (i=1; i<=LIMIT; i++)

{ sum = sum + i;

}

System.out.println(sum);

}

Note the three parts to the control statement

– Initial value of the counter

– Test for the final value

– Modify the counter

Loops

• ALL loops must have three things that control how many times to go through the loop

– Initialize the control variable

– Tell what the terminating condition is

– Change the control variable so that at some point the terminating condition is reached.

• The while loop does these three things differently than the for loop

for loops

• All three things are done within the for parentheses

• for(count=0;count<10;count++)
{
statements
}

• Note that there are really three statement within the parentheses

– A semicolon terminates a statement