For Loops

64. Counting with a For Loop - Link

Task
Answer
Question

As you saw in Counting with a While Loop, a while loop can be used to to make something happen an exact number of times.However, this isn’t our best choice. while loops are designed to keep going as long as something is true. But if we know in advance how many times we want to do something, Java has a special kind of loop designed just for making a variable change values: the for loop.for loops are best when we know in advance how many times we want to do something.

Do this ten times.
Do this five times.
Pick a random number, and do it that many times.
Take this list of items, and do it one time for each item in the list.

On the other hand, while loops are best for repeating as long as something is true:

Keep going as long as they haven’t guessed it.
Keep going as long as you haven’t got doubles.
Keep going as long as they keep typing in a negative number.
Keep going as long as they haven’t typed in a zero.

Sample Output

Type in a message, and I'll display it five times.
Message: Hey, hey.
Hey, hey.
Hey, hey.
Hey, hey.
Hey, hey.
Hey, hey.

Type in the following code, and get it to compile. Then answer the questions down below.

CountingFor.java

import java.util.Scanner;

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

        System.out.println( "Type in a message, and I'll display it five times." );
        System.out.print( "Message: " );
        String message = keyboard.nextLine();

        for ( int n = 1 ; n <= 5 ; n = n+1 )
        {
            System.out.println( n + ". " + message );
        }
    }
}

Assignments turned in without these things will not receive any points.

What does n = n+1 do? Remove it and see what happens. (Then put it back.)
Answer: It increments the value of n by 1.
What does int n = 1 do? Remove it and see what happens. (Then put it back.)
Answer: It initializes the value of n to 1.
Change the code so that the loop repeats ten times instead of five.
Answer: Change n <= 5 to n <= 10.
See if you can change the for loop so that the message starts at 2 and counts by twos, like so:
```
Type in a message, and I'll display it ten times.
Message: qwerty
2. qwerty
4. qwerty
6. qwerty
8. qwerty
10. qwerty
```
Answer: Change int n = 1 to int n = 2 and n = n+1 to n = n+2.

65. Ten Times - Link

Task
Answer

Write a program that prints the important phrase “Mr. Mitchell is cool.” on the screen ten times. Use a for loop to do it.Sample Output

Mr. Mitchell is cool.
Mr. Mitchell is cool.
Mr. Mitchell is cool.
Mr. Mitchell is cool.
Mr. Mitchell is cool.
Mr. Mitchell is cool.
Mr. Mitchell is cool.
Mr. Mitchell is cool.
Mr. Mitchell is cool.
Mr. Mitchell is cool.

TenTimes.java

public class TenTimes
{
    public static void main( String[] args )
    {
        for ( int n = 1 ; n <= 10 ; n++ )
        {
            System.out.printf( "%d. Mr. Mitchell is cool.\n", n );
        }
    }
}

66. Counting Machine - Link

Task
Answer

Write a program that gets an integer from the user. Count from 0 to that number. Use a for loop to do it.Sample Output

Count to: 19
0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

CountingMachine.java

import java.util.Scanner;

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

        System.out.print( "Count to: " );
        int number = keyboard.nextInt();

        for ( int n = 0 ; n <= number ; n++ )
        {
            System.out.printf( "%d ", n );
        }
    }
}

67. Counting Machine Revisited - Link

Task
Answer

Write a program that gets three integers from the user. Count from the first number to the second number in increments of the third number. Use a for loop to do it.Sample Output

Count from: 4
Count to  : 13
Count by  : 3

4 7 10 13

CountingMachineRevisited.java

import java.util.Scanner;

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

        System.out.print( "Count from: " );
        int from = keyboard.nextInt();
        System.out.print( "Count to: " );
        int to = keyboard.nextInt();
        System.out.print( "Count by: " );
        int by = keyboard.nextInt();

        for ( int n = from ; n <= to ; n = n+by )
        {
            System.out.printf( "%d ", n );
        }
    }
}

68. Counting By Halves - Link

Task
Answer

Write a program that uses a for loop. With the loop, make the variable x go from -10 to 10, counting by 0.5. (This means that x can’t be an int.)Sample Output

x
------
-10.0
-9.5
-9.0
-8.5
-8.0
...
9.0
9.5
10.0

CountingByHalves.java

public class CountingByHalves
{
    public static void main( String[] args )
    {
        System.out.println( "x" );
        System.out.println( "------" );

        for ( double x = -10.0 ; x <= 10.0 ; x = x+0.5 )
        {
            System.out.println( x );
        }
    }
}

69. Xs and Ys - Link

Task
Answer

Write another program that uses a for loop. With the loop, make the variable x go from -10 to 10, counting by 0.5. (This means that x can’t be an int.)Inside the body of the loop, make another variable y become the current value of x squared. Then display the current values of both x and y.To get your output to line up like mine, use a tab.Sample Output

x      y
-----------------
-10.0   100.00
-9.5    90.25
-9.0    81.00
-8.5
-8.0    64.00
...
9.0     81.00
9.5     90.25
10.0    100.00

XsAndYs.java

public class XsAndYs
{
    public static void main( String[] args )
    {
        System.out.println( "x\ty" );
        System.out.println( "-----------------" );

        for ( double x = -10.0 ; x <= 10.0 ; x += 0.5 )
        {
            double y = Math.pow(x, 2);
            System.out.printf( "%s\t%s\n", x, y );
        }
    }
}

70. Noticing Even Numbers - Link

Task
Answer

Write a program that uses a for loop to display all the numbers from 1 to 20, marking those which are even (divisible by two). It should use modulus by 2: if the remainder is zero, it’s divisible by 2.This means you’ll need an if statement inside the loop.

for ( <stuff> )
{
    if ( <something with modulus> )
    {
        System.out.println( <something> );
    }
    else
    {
        System.out.println( <something different> );
    }
}

Sample Output

NoticingEvenNumbers.java

public class NoticingEvenNumbers
{
    public static void main( String[] args )
    {
        for ( int n = 1 ; n <= 20 ; n++ )
        {
            if ( n % 2 == 0 )
            {
                System.out.println( n + " <" );
            }
            else
            {
                System.out.println( n );
            }
        }
    }
}

71. Fizz Buzz - Link

Task
Answer

Write a program that prints the numbers from 1 to 100. But for multiples of three print “Fizz” instead of the number and for the multiples of five print “Buzz”. For numbers which are multiples of both three and five print “FizzBuzz”.The output of your program will look something like this:

1
2
Fizz
4
Buzz
Fizz
7
8
Fizz
Buzz
11
Fizz
13
14
FizzBuzz
16
17
Fizz
19
Buzz
...
97
98
Fizz
Buzz

FizzBuzz.java

public class FizzBuzz
{
    public static void main( String[] args )
    {
        for ( int n = 1 ; n <= 100 ; n++ )
        {
            if ( n % 3 == 0 && n % 5 == 0 )
            {
                System.out.println( "FizzBuzz" );
            }
            else if ( n % 3 == 0 )
            {
                System.out.println( "Fizz" );
            }
            else if ( n % 5 == 0 )
            {
                System.out.println( "Buzz" );
            }
            else
            {
                System.out.println( n );
            }
        }
    }
}

72. Letter at a Time - Link

Task
Answer
Question

Did you know that using a loop, you can examine a String one letter at a time? The two key built-in String methods are length() and charAt().

length() returns an int representing the total number of characters in the String (including punctuation and whitespace). For example, if the variable str contains the String "hello", then str.length() will return 5.
charAt( int n ) returns the nth character (char) in the String. The character positions are zero-based, so if the variable str contains the String "ligature", then str.charAt(0) will return 'l', and str.charAt(4) will return 't'.

Files Needed

LetterAtATime.java

Sample Output

What is your message? A man, a plan, a canal: Panama!

Your message is 31 characters long.
The first character is at position 0 and is 'A'.
The last character is at position 30 and is '!'.

Here are all the characters, one at a time:

- 'A'
- ' '
- 'm'
- 'a'
- 'n'
- ','
- ' '
- 'a'
- ' '
- 'p'
- 'l'
- 'a'
- 'n'
- ','
- ' '
- 'a'
- ' '
- 'c'
- 'a'
- 'n'
- 'a'
- 'l'
- ':'
- ' '
- 'P'
- 'a'
- 'n'
- 'a'
- 'm'
- 'a'
- '!'

Your message contains the letter 'a' 10 times. Isn't that interesting?

LetterAtATime.java

import java.util.Scanner;

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

        System.out.print("What is your message? ");
        String message = kb.nextLine();

        System.out.println("\nYour message is " + message.length() + " characters long.");
        System.out.println("The first character is at position 0 and is '" + message.charAt(0) + "'.");
        int lastpos = message.length() - 1;
        System.out.println("The last character is at position " + lastpos + " and is '" + message.charAt(lastpos) + "'.");
        System.out.println("\nHere are all the characters, one at a time:\n");

        for ( int i=0; i<message.length(); i++ )
        {
            System.out.println("\t" + i + " - '" + message.charAt(i) + "'");
        }

        int a_count = 0;

        for ( int i=0; i<message.length(); i++ )
        {
            char letter = message.charAt(i);
            if ( letter == 'a' || letter == 'A' )
            {
                a_count++;
            }
        }

        System.out.println("\nYour message contains the letter 'a' " + a_count + " times. Isn't that interesting?");
    }
}

Assignments turned in without these things will not receive any points.

The for loop is defined so that it repeats as long as i < message.length(). Try changing it to <=. What happens? Answer in a comment, then change it back.
Answer: It will throw an IndexOutOfBoundsException because the index is out of bounds.
If a string variable contains the value "box", what is its length()? What is the position of the last character (the 'x')?
Answer: The length is 3, and the position of the last character is 2.
So, why does the for loop repeat as long as i < message.length() instead of i <= message.length()?
Answer: Because the index is zero-based, and the last index is message.length() - 1.
Currently the code prints out the number of ‘a’s in the message. Change it so that it prints out the number of vowels (a A e E i I o O u U).
Answer: Change if ( letter == 'a' || letter == 'A' ) to if ( letter == 'a' || letter == 'A' || letter == 'e' || letter == 'E' || letter == 'i' || letter == 'I' || letter == 'o' || letter == 'O' || letter == 'u' || letter == 'U' ).

73. For Loop Challenge - Link

Task
Answer

Get a blank sheet of paper (scratch paper is fine) and something to write with. Tell Mr. Mitchell you’d like to take the For Loop Challenge. He will ask you to write an arbitrary for loop on the sheet of paper. If you do it without any mistakes, you will receive full points.You may take this challenge more than once, but you’ll receive 5 fewer points each time you do it. The for loop you must write will be different each time.Here’s an example of what you’ll be asked to do:

~~~~~

“Write a for loop that makes the variable j go from 15 to 30, counting by 3s.”

ForLoopChallenge.java

public class ForLoopChallenge
{
    public static void main( String[] args )
    {
        for ( int j = 15 ; j <= 30 ; j += 3 )
        {
            System.out.println( j );
        }
    }
}

74. Adding Values in a For Loop - Link

Task
Answer

Write a program that gets an integer from the user. Add up all the numbers from 1 to that number, and display the total. Use a for loop to do it.You have done something like this before.Sample Output

Number: 5

1 2 3 4 5
The sum is 15.

AddingValuesWithAForLoop.java

import java.util.Scanner;

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

        System.out.print( "Number: " );
        int number = keyboard.nextInt();

        int sum = 0;
        for ( int n = 1 ; n <= number ; n++ )
        {
            System.out.print( n + " " );
            sum += n;
        }

        System.out.printf( "\nThe sum is %s.\n", sum );
    }
}

116. A Refresher - Link

Task
Answer

Just a short program to refresh your memory about how to program. Write a program that prompts the user for a name. Then display that name ten times. You must use a loop. If the name given is “Mitchell”, display it only five times.Sample Output

What is your name: gump

gump
gump
gump
gump
gump
gump
gump
gump
gump
gump

Refresher.java

import java.util.Scanner;

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

        System.out.print( "What is your name: " );
        String name = keyboard.nextLine();

        if ( name.equals("Mitchell") ) {
            for ( int n = 1 ; n <= 5 ; n++ )
            {
                System.out.println( name );
            }
        } else {
            for ( int n = 1 ; n <= 10 ; n++ )
            {
                System.out.println( name );
            }
        }
    }
}

117. Refresher Challenge - Link

Task
Answer

This assignment is almost the same as A Refresher.Write a program that prompts the user for a name. Then display that name ten times using a loop. However, if the name given is “Mitchell”, display it only five times.So here’s the challenge: write the program using only one if statement (no else) and one for loop.Sample Output

What is your name: gump

gump
gump
gump
gump
gump
gump
gump
gump
gump
gump

Refresher2.java

import java.util.Scanner;

public class Refresher2
{
    public static void main( String[] args )
    {
        Scanner keyboard = new Scanner(System.in);

        System.out.print( "What is your name: " );
        String name = keyboard.nextLine();
        System.out.println();

        for ( int n = 1 ; n <= 10 ; n++ )
        {
            if ( name.equals("Mitchell") && n > 5 ) {
                break;
            }
            System.out.println( name );
        }
    }
}

118. Displaying Some Multiples - Link

Task
Answer

Write a program to calculate the multiples of a given number. Have the user enter a number, and then use a for loop to display all the multiples of that number from 1 to 12. It is not necessary to use a function.You must use a for loop.Sample Output

Choose a number: 7

7x1 = 7
7x2 = 14
7x3 = 21
7x4 = 28
7x5 = 35
7x6 = 42
7x7 = 49
7x8 = 56
7x9 = 63
7x10 = 70
7x11 = 77
7x12 = 84

DisplayingSomeMultiples.java

import java.util.Scanner;

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

        System.out.print( "Choose a number: " );
        int number = keyboard.nextInt();
        System.out.println();

        for ( int n = 1 ; n <= 12 ; n++ )
        {
            System.out.printf( "%s x %s = %s\n", number, n, number * n );
        }
    }
}

Persiapan Belajar

Materi

Task

Programming by Doing

Compiler

64. Counting with a For Loop - Link

65. Ten Times - Link

66. Counting Machine - Link

67. Counting Machine Revisited - Link

68. Counting By Halves - Link

69. Xs and Ys - Link

70. Noticing Even Numbers - Link

71. Fizz Buzz - Link

72. Letter at a Time - Link

73. For Loop Challenge - Link

74. Adding Values in a For Loop - Link

116. A Refresher - Link

117. Refresher Challenge - Link

118. Displaying Some Multiples - Link

Persiapan Belajar

Materi

Task

Programming by Doing

Compiler

​64. Counting with a For Loop - Link

​65. Ten Times - Link

​66. Counting Machine - Link

​67. Counting Machine Revisited - Link

​68. Counting By Halves - Link

​69. Xs and Ys - Link

​70. Noticing Even Numbers - Link

​71. Fizz Buzz - Link

​72. Letter at a Time - Link

​73. For Loop Challenge - Link

​74. Adding Values in a For Loop - Link

​116. A Refresher - Link

​117. Refresher Challenge - Link

​118. Displaying Some Multiples - Link

64. Counting with a For Loop - Link

65. Ten Times - Link

66. Counting Machine - Link

67. Counting Machine Revisited - Link

68. Counting By Halves - Link

69. Xs and Ys - Link

70. Noticing Even Numbers - Link

71. Fizz Buzz - Link

72. Letter at a Time - Link

73. For Loop Challenge - Link

74. Adding Values in a For Loop - Link

116. A Refresher - Link

117. Refresher Challenge - Link

118. Displaying Some Multiples - Link