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Chapter 4: Control Structures. Repetition-Counter control Loop. Why Is Repetition Needed?. How can you solve the following problem: What is the sum of all the numbers from 1 to 100. The answer will be 1 + 2 + 3 + 4 + 5 + 6 + … + 99 + 100. Why Is Repetition Needed?. - PowerPoint PPT Presentation
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Chapter 4: Control Structures
REPETITION-COUNTER CONTROL LOOP
Why Is Repetition Needed?How can you solve the following problem: What is the sum of all the numbers from 1
to 100. The answer will be
1 + 2 + 3 + 4 + 5 + 6 + … + 99 + 100.
Why Is Repetition Needed?Here’s some sample Java code:
int sum=0;
sum = sum+1;
sum = sum +2;
sum = sum +3;
sum = sum +4;
…
sum = sum +99;
sum = sum +100;
System.out.println(“The sum from 1 to 100 = “ +sum);
Why Is Repetition Needed?This solution has problems: It would take a long time to type in. There is a high risk of making an error
while typing it in. It doesn’t easily scale. This may work for
100 numbers but how would you handle having to add from 1 to a 1000? Or to 1000000?Or to 1000000000?
Why Is Repetition Needed?
The Algorithm
1. Create a variable to hold the sum.2. Initialize the sum to zero.3. Create a variable to hold a counter from 1 to 100.4. Initialize the counter to 1.5. While the counter is less-than-or-equal to 1006. add the counter to the sum7. add one to the counter8. Now repeat9. Print the sum
Why Is Repetition Needed?We can use pseudo-code:sum = 0count = 1loop while count <= 100 sum = sum + count count++endloopprint sum
Why Is Repetition Needed?loop while condition <body>endloop
This pseudo-code means: before executing the statements in the body, evaluate the condition. If the condition is true then execute the body once.
Once you have executed the body statements once, go back to the loop condition and re-evaluate it. If it is true, execute the body code again. If the condition is not true then the body will not be executed!
The while Looping (Repetition) Structure
• Infinite loop: is a loop that continues to execute endlessly.
• So, expression is always true in an infinite loop.• Statements must change value of expression to
false. 8
Examplei = 0loop while (i <= 20)print(i)i = i + 5
end loop
start
i = 0
i <= 20
end
No
i = i + 5
Print i
Yes0
i
Output
05101520
5
10
15
20
25 What will happen if you omit (i= i + 5) ?
While Loop Types
Counter-Controlled Loop Sentinel-Controlled Loop Flag-Controlled Loop
Counter-Controlled Loop Used when exact number of data or entry pieces is
known. General form:
N = …counter = 0Loop while (counter < N) . . . counter = counter + 1 . . .End loop
N = …
counter = 0
Read Nor
counter < N
do operation
counter = counter + 1
No
Yes
(1) Initialization
stmt.
(3) Update stmt.
(2) Loop condition
Example Write a program to allow the user to enter a
set of integer numbers, then print the sum of these numbers.
Start ProgramRead setSizecounter = 0sum = 0Loop while (counter < setSize)
Read numbersum = sum + numbercounter = counter + 1
End loopPrint sum
End Program
startStart Program
Read setSize
counter = 0
sum = 0
Loop while (counter < setSize)
Read number
sum = sum + number
counter = counter + 1
End loop
Print Sum
End Program
counter = counter + 1
Read setSize
counter = 0
sum = 0
counter < setSize
YesRead number
sum = sum + number
Output
3110415
1
2
3
0counter
3setSize
0sum
No
end
Print sum
1
11
15
1number
10
4
The while Looping (Repetition) Structure
Syntax:while (expression) statement
Statements must change value of expression to false. A loop that continues to execute endlessly is called an
infinite loop (expression is always true).
14
The while Looping (Repetition) Structure
Example 5-1
i = 0; while (i <= 20){
System.out.print(i + " "); i = i + 5;
}System.out.println();
Output
0 5 10 15 20
15
The while Looping (Repetition) Structure
Typically, while loops are written in the following form:
//initialize the loop control variable(s)
while (expression) //expression tests the LCV
{
.
.
.
//update the loop control variable(s)
.
.
}
16
Counter-Controlled while Loop17
Used when exact number of data or entry pieces is known.
General form:int N = //value input by user or specified //in programint counter = 0;while (counter < N){ . . . counter++; . .}
Counter-Controlled while Loop-Example 5-3
18//Counter-controlled while loop
import java.util.*;public class CounterControlledWhileLoop{ static Scanner console = new Scanner(System.in);
public static void main(String[] args) { int limit; //store the number of items //in the list int number; //variable to store the number int sum; //variable to store the sum int counter; //loop control variable
System.out.print("Enter the number of " + "integers in the list: "); limit = console.nextInt();
System.out.println();
sum = 0;
counter = 0; System.out.println("Enter " + limit+ " integers.");
Counter-Controlled while Loop-Example 5-3 (continued)
19
while (counter < limit) { number = console.nextInt();
sum = sum + number; counter++; }
System.out.printf("The sum of the %d " +"numbers = %d%n", limit, sum);
if (counter != 0)
System.out.printf("The average = %d%n",(sum / counter)); else System.out.println("No input.");
}
}
Sample Run:Enter the number of integers I the list: 4
Enter 4 Integers2 1 5 8The sum of the 4 numbers = 16
The average = 4
Counter-Controlled Loop: Another way for expressing it
N = …counter = 1Loop while (counter<=N) .
.
counter = counter + 1End loop
N = …For(counter = 1, counter <= N, counter = counter + 1) .
.
End For
While loop
For loop
Initialization Condition Increment \ Decrement
Step 1counter = 1 to N
Counter-Controlled Loop –For Loop
N = …
counter = 1
Read Nor
counter <= N
do operation
counter = counter + 1
No
Yes
N = …Read
Nor
do operation
Next counter
Can be simplifie
d to:For counter = 1
to N, step 1
The for loop does not have a standard flowcharting method and you will find it done in different ways.
Example
1. Write down an algorithm and draw a flowchart to find and print the largest of N (N can be any number) positive numbers. Read numbers one by one.
2. Verify your result by tracing the developed algorithm. (Assume N to be 4 and the following set to be the numbers {5 2 6 1})
start
Solution
Start Program
Read N
max = 0
For(counter = 1 to N, step 1)
Read number
if (number > max)
max = number
End For
Print max
End Program
max = 0
Read N
YesRead number
number > max
end
No
max = number
Yes
(1) For counter = 1 to N,
step 1
Next counter
(2) • Trace the developed algorithm. • Assume N to be 4 and the following
set to be the numbers {5 2 6 1}
4N
0max 1
counter 5
number
5 2 2
3 66
4 1
5
Print max 6
startstart
max = 0
Read N
YesRead number
number > max
end
No
max = number
Yes
For counter = 1 to N,
step 1
Next counter
The for Looping (Repetition) Structure
25
Specialized form of while loop. Its primary purpose is to simplify the writing of counter-controlled
loops. For this reason, the for loop is typically called a counted or indexed for loop. .
Syntax:
for (initial statement; loop condition; update statement)
statement
The for Looping (Repetition) Structure
Execution:1. Initial statement executes. 2. Loop condition is evaluated.3. If loop condition evaluates to true,
1. execute for loop statement and 2. execute update statement.
4. Repeat step 2 until loop condition is false.
26
The for Looping (Repetition) StructureExample 5-9
The following for loop prints the first 10 nonnegative integers:
for (i = 0; i < 10; i++)
System.out.print(i + " ");
27
The for Looping (Repetition) StructureExample 5-10
1. The following for loop outputs the word Hello and a star (on separate lines) five times:
for (i = 1; i <= 5; i++)
{
System.out.println("Hello");
System.out.println("*");
}
2. The following for loop outputs the word Hello five times and the star only once:
for (i = 1; i <= 5; i++)
System.out.println("Hello");
System.out.println("*");
28
The for Looping (Repetition) Structure29
Does not execute if loop condition is initially false. Update expression changes value of loop control
variable, eventually making it false. If loop condition is always true, result is an infinite
loop. Infinite loop can be specified by omitting all three
control statements. If loop condition is omitted, it is assumed to be true.
Action of for loop ending in semicolon is empty.
For Loop Programming Example: Classify Numbers
30
Input: N integers (positive, negative, and zeros).
int N = 20; //N easily modified
Output: Number of 0s, number of even integers, number of odd integers.
For Loop Programming Example: Classify Numbers (solution)
31
for (counter = 1; counter <= N; counter++){ number = console.nextInt(); System.out.print(number + " "); switch (number % 2) { case 0: evens++; if (number == 0) zeros++; break; case 1: case -1: odds++; } //end switch} //end for loop
While Loop Programming Example: Fibonacci Number
32
Fibonacci formula for any Fibonacci sequence:
an = an-1 + an-2
Input: First two Fibonacci numbers in sequence, position in sequence of desired Fibonacci number (n).
int previous1 = Fibonacci number 1 int previous2 = Fibonacci number 2 int nthFibonacci = Position of nth Fibonacci number
Output: nth Fibonacci number.
While Loop Programming Example: Fibonacci Number
33 if (nthFibonacci == 1) current = previous1; else if (nthFibonacci == 2) current = previous2; else { counter = 3; while (counter <= nthFibonacci) { current = previous2 + previous1; previous1 = previous2; previous2 = current; counter++; } }
Final result found in last value of current.
