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

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

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

while loop

• The programmer must do all three things separately in the while loop

– The control variable must be initialized before going into the loop

– The terminating condition is in parentheses

– The change to the control variable must be done inside the loop block

• while (condition)
{
statement block
}

• The statements inside the while loop block will be executed until the condition becomes false

• Note that if the condition is not true, none of the statements in the while loop will ever be executed

Writing a while loop

• Suppose we want to find the sum of the first 5 integers

• Pseudocode to write the while loop

– Declare and initialize two int variables: one to hold the total, the other to hold the control variable

– Decide on the terminating condition

– Write the block of the while statement

• Write the statement to sum the numbers

• Change the control variable

The while loop

public static void main(String[ ] args)
{
int sum = 0, num =1;

while (num < 11)

{ sum = sum + num;

num++;

}

System.out.println(sum);

}

Comparing the while loop and for loop

public static void main(String[ ] args)
{
int num = 5, sum=0;

for (int i=1; i<=num; i++)

{
sum = sum + i;

}

System.out.println(sum);

}

public static void main(String[ ] args)
{
int sum = 0, num =1;

while (num < 6)

{
sum = sum + num;

num++;

}

System.out.println(sum);

}

Note about the while loop

• while (condition)
{
statements
}

• The statements inside the while loop block will be executed until the condition becomes false

• Note that if the condition is not true, none of the statements in the while loop will ever be executed

• You may want the loop to execute at least once, regardless of the truth of the condition

• Then you use a do while loop

The do while loop

• do
{
statements
}
while (condition)

• Note that you always execute the statements in a do loop at least once

• The condition is checked at the end of the loop

Walk through this program

public static void main(String[ ] args)
{
final int LIMIT = 20;

int next=0, last=1, sum;

while (next < LIMIT)

{ System.out.print(last + " ");

sum = next + last;

next = last;
last = sum;

}

System.out.println("done");

}

• We looked at code to print out the Fibonacci series

– Each term is the sum of the previous two term

– 1, 1, 2, 3, 5, 8, 13, 21, 34, 55 ….

– You should do a walkthrough of this program

• In a walkthrough the person simulates the computer

• You keep a chart of the variables, and how they change as each statement executes

Sentinel values

• Example: You want to develop a class averaging program that will process an arbitrary number of grades

• Ask the user to enter the grades to process, then enter a sentinel value when all the grades have been entered

• The sentinel must be be confused with a grade

– Negative one would be good in this case

– Zz is sometimes used when entering strings

• You must keep in mind the sentinel must be of the same type as the other data that is being entered

Example using a sentinel value

public static void main(String[] args) throws Exception

{ int count = 0;

double avg, grade=0.0, sum = 0.0;

System.out.println("This program will find the average of the
grades you enter");

System.out.println("Please enter the grades separated by a
return. Enter -1 when finished");

grade = BasicIo.readInteger();

while (grade != -1)

{ sum = sum + grade;

count++;

grade = BasicIo.readInteger();

}

avg = sum/count;

System.out.println("The average of " + count + " grades is " +
avg);
}