Assignment 4: Get carried array.

Due Thursday, October 6, before midnight

The goals of this assignment are:

  • Work with loops, conditionals, and variables

  • Work with arrays and strings

  • Use the draw API included with your textbook

Coding guidelines

In addition to producing the correct output, in this assignment you are also asked to write code that is easy to read and understand. In particular, part of your score on this assignment will be determined by:

  • Variable naming: Variables should have meaningful names that indicate what they represent, using full English words or common abbreviations, e.g. “wins” or “votes” instead of “w” or “vot”.

  • Appearance:The code should be formatted so that indentation and spacing make it easy to understand which parts of the code are within the bodies of if-statements and loops.

Additionally, there should be spacing between variables and operators to make it easy to read each individual line of code. This is called style and is an essential requirement for producing clean, readable code. For example, a piece of code that determines the maximum value in an array called values should look something like this:

int  max = values[0];
for (int i = 1; i < values.length; i++) {
    if (values[i] > max) {
        max = values[i];
System.out.println("Max is " + max);

Not like this:

int  a=values[0];
for (int i=1;i<values.length;i++) {
if (values[i]>a) {
System.out.println("Max is "+a);

Please speak with your TAs or instructor if you have any questions about style!

1. Fibonacci

Write a program,, that stores the first N values of the Fibonacci sequence in an array and prints it out. Your program should take the number of values as a command line argument. Each value \(F_n\) in the Fibonacci sequence is equal to the sum of the previous two values. In other words,

\[F_n = F_{n-1} + F_{n-2}, F_2 = 1, F_1 = 1\]
$ java Fibonacci 10
1 1 2 3 5 8 13 21 34 55

$ java Fibonacci 1

$ java Fibonacci 0


  • Your program should use comamnd line arguments

  • Your program can assume the user enters a non-negative integer

2. Crossword

In, implement a program that inputs text and outputs a word square whose letters are arranged like a crossword puzzle.

$ java Crossword
Enter a phrase: top

o o

$ java Crossword
Enter a phrase: otter

t   e
t   t
e   t


  • The first row should match the text that was entered

  • The last row is the reverse of the input text

  • The left side spells the input word from top to bottom

  • The right side spells the input word from bottom to top

If this problem looks daunting, solve it one step at a time! Let’s break this problem into simpler subproblems and work our way up to the final solution.

2.1. Subproblem 1: Top row

The top row prints the input string. Write a program that outputs the user’s input.

$ java Crossword
Enter a phrase: otter


2.2. Subproblem 2: Bottom row

Using string accumulation, compute the reverse of the input string. Then print both the original and reversed string.

$ java Crossword
Enter a phrase: otter


2.3. Subproblem 3: Left column

The left column prints each character on its own line, excluding the first and last character. To start, simply print all the characters on their own line.

$ java Crossword
Enter a phrase: otter


Now, exclude the first and last characters.

$ java Crossword
Enter a phrase: otter


2.4. Subproblem 4: Right colums

Excluding the first and last character, print each character of the reversed string on its own line. Don’t worry about spacing to start.

$ java Crossword
Enter a phrase: otter

t e
t t
e t

Now, add spaces between the left and right columns.

$ java Crossword
Enter a phrase: otter

t   e
t   t
e   t

3. Letter Frequency

credit: Deepak Kumar

Write a program,, that plots the frequency of different letters in a corpus of text using a histogram.

Download the text: tolstoy.txt

This question is based on this this writeup.

3.1. Before You Begin

Because this assignment focuses on keeping track of the number of occurrences of individual letters in the program input, we will be making heavy use of the Java char datatype, which was presented in Section 1.2 of your course textbook. Review this part of your textbook if you are not familiar with this datatype.

A Java char variable represents a single character, which can be a letter like ‘a’ or ‘M’; a digit like ‘2’ or ‘8’; or punctuation like ‘+’ or ‘?’.

When working with letters, as we will do in this assignment, it is even possible to do math with char variables, for instance:

char first = 'k';
char second = (char)(first + 3);
StdOut.printf("%c", second);

This code will print the letter ‘n’, since ‘n’ is three letters after ‘k’ in the alphabet, and the “%c” format string means to treat the value as a char. Similarly, you could have something like this:

char letter = 'g';
int value = letter - 'a';
StdOut.printf("%d", value);

This code will print 6, since ‘g’ is six letters after ‘a’. Be sure you understand this before starting, and ask your TAs or Instructor or post a question on slack if you are not sure how this works!

3.2. Count characters

The first step in building the histogram is to read the input text one character at a time, and to keep track of and then display (to stdout) the number of occurrences of each letter between ‘a’ and ‘z’.

For instance, if the input text were “java yeah!”, the program should indicate that the letter ‘a’ occurs three times and that the letters ‘e’, ‘h’, ‘j’, ‘v’, and ‘y’ each occur once. It should ignore the blank space between “java” and “yeah!”, as well as the ‘!’ character, since those are not letters.

Your program should display the number of occurrences for all 26 letters, even if a given letter does not appear. For instance, in the above example, the output should look like this:

a: 3
b: 0
c: 0
d: 0
e: 1
. . . output omitted but you get the idea...
x: 0
y: 1
z: 0

For simplicity, you may assume that your program only needs to keep track of lowercase letters, i.e. that any uppercase characters in the input have already been converted to lowercase, and your program only needs to track the 26 lowercase letters used in English.

The following hints should help you with this part of the assignment:

  • You can use StdIn.isEmpty() to determine whether or not there is more input text to read.

  • You can use StdIn.readChar() to read a single character from the input and store the character in a char variable.

  • Use a 26-element array of counts to keep track of number of occurrences of each letter. Do not define 26 different variables to store the number of occurrences of each letter!

  • You should store the number of occurrences of the letter ‘a’ in index #0, the number of occurrences of ‘b’ in index #1, and so on. You can use the “math with char variables” advice from above to figure out the corresponding array index for each letter.

  • To determine whether a char variable holds a lowercase letter, you can determine whether it is in the range [‘a’, ‘z’] in the same way in which you would determine whether an int variable is in a certain numerical range by using comparison operators.

To test your program with the default source for stdin, i.e. the Terminal, then follow these steps:

  1. Run the program with the command java-introcs LetterFrequency

  2. Type characters into the Terminal. Don’t worry about putting spaces between them; your program should be able to read each character one at a time. Your program should only be counting the occurrences of lowercase letters, i.e. ‘a’ through ‘z’.

  3. When you are done entering characters, you can indicate that there are no more characters to read (so that StdIn.isEmpty() will return true) by typing Ctrl-D (that is, pressing the Control and D keys at the same time) if you are using a Mac, or Enter then Ctrl-Z then Enter if you are using Windows.

  4. Your program should then display the number of times you entered each character from ‘a’ to ‘z’.

Once you have tested your program by entering characters into the Terminal, try it with the text of “War and Peace”, which is available in the file tolstoy.txt. Assuming that tolstoy.txt is in the same directory as your Java source code, all you need to do is run the program like this:

java-introcs LetterFrequency < tolstoy.txt

Then you should see the number of occurrences for each of the characters ‘a’ through ‘z’. Your program should produce the following output:

a: 204416
b: 34371
c: 60658
d: 117751
e: 313007
f: 54504
g: 50907
h: 166515
i: 172640
j: 2485
k: 20282
l: 96032
m: 61282
n: 183126
o: 191487
p: 44717
q: 2319
r: 146889
s: 162125
t: 224506
u: 64917
v: 26787
w: 58928
x: 4032
y: 45936
z: 2386

Be sure you complete this part before moving on to Part 2!

3.3. Scale values

At this point, we know the number of occurrences of each lowercase letter, but before we can draw the histogram, we need to scale the values so that the letter that occurs the most has a value of 1.0, and the other letters’ values are scaled accordingly.

For instance, using the example of “War and Peace,” the letter ‘e’ occurs 313,007 times, which is the most. So we scale the value for ‘e’ to 1.0.

Then we determine the value for each of the other letters by dividing by 313,007, such that ‘a’ gets a value of 204,416 / 313,007 = 0.653, since ‘a’ occurs 65.3% as much as ‘e’, and so on.

Modify yourLetterFrequency program so that it scales the values for each letter based on the number of occurrences of the letter that appears the most, and then prints out each letter ‘a’ through ‘z’ and its new, scaled value to three digits of precision, i.e. three digits after the decimal point.

For instance, when running the program with “War and Peace” as the input, the output for this part should be as follows:

a: 0.653
b: 0.110
c: 0.194
d: 0.376
e: 1.000
f: 0.174
g: 0.163
h: 0.532
i: 0.552
j: 0.008
k: 0.065
l: 0.307
m: 0.196
n: 0.585
o: 0.612
p: 0.143
q: 0.007
r: 0.469
s: 0.518
t: 0.717
u: 0.207
v: 0.086
w: 0.188
x: 0.013
y: 0.147
z: 0.008

Be sure you complete this part before moving on to Part 3!

3.4. Draw a histogram

Now that we know the relative/scaled values for the number of occurrences of each letter, we can use the StdDraw library to draw the histogram and represent these values visually.

Our histogram will consist of a rectangle for each letter, going left to right ‘a’ through ‘z’, with the height of each letter being equal to the relative/scaled value calculated in Part 2.

To draw a rectangle using this library, you can call

StdDraw.rectangle(x, y, r1, r2)


  • x is the x-axis coordinate of the center of the rectangle

  • y is the y-axis coordinate of the center of the rectangle

  • r1 the x-axis distance from the center of the rectangle to a vertical side, i.e. half the width

  • r2 is y-axis distance from the center of the rectangle to a horizontal side, i.e. half the height.

Recall that the default coordinate system in the StdDraw library is [0.0, 1.0] on both the x- and y-axes. For our purposes, this is fine for the y-axis because the relative/scaled values from Part 2 are all in [0.0, 1.0] anyway, and will represent the height of each rectangle. However, since we will have 26 rectangles, we can rescale the x-coordinates to [0.0, 26.0] using StdDraw.setXscale(0, 26) so that the width of each rectangle is 1 and not 1/26. This will make the math a little easier.Using the above, we can now describe how to draw the rectangle for the ith letter (starting from 0, which is ‘a’):

  • x is i + 0.5; this is so that the x-coordinate of the center of the ‘a’ rectangle is 0.5, the x-coordinate of the center of the ‘b’ rectangle is 1.5, and so on.

  • y is 0.5 times the letter’s scaled value (from Part 2)

  • r1 is 0.5, since this is half the width

  • r2 is 0.5 times the letter’s scaled value (from Part 2), since this is half the height.

Modify your LetterFrequency program so that it uses StdDraw to draw a rectangle for each lowercase letter as described above. Your program output should look something like the picture shown below

Submit an image of your histogram along with your code for this week’s assignment

A04 WarAndPeace Histogram

3.5. Extra challenge

Add labels and colors to your histogram. For example, below different colors are used for alternating bars. Each bar is labeled with its corresponding letter.

A04 WarAndPeace Histogram Pretty

4. What to hand-in

  1. The programs,,, and

  2. An image of your histogram, drawn for the last question.

  3. Make sure each program has a header containing your name, date, and purpose of the program

  4. A brief write-up containing your name, assignment number, and a few sentences about how long you spent on the assignment and any interesting bugs you solved. Be sure to highlight any customizations you make for your own histogram.

4.1. How to hand-in

  1. Copy your programs to your dropbox, into the folder called A04.