Develop Reference

Is there a way to use the Scanner class inputFile.nextLine()); but have the selection be random? [duplicate]

Say there is a file too big to be put to memory. How can I get a random line from it? Thanks.
Update:
I want to the probabilities of getting each line to be equal.

Reading the entire file if you want only one line seems a bit excessive. The following should be more efficient:
Use RandomAccessFile to seek to a random byte position in the file.
Seek left and right to the next line terminator. Let L the line between them.
With probability (MIN_LINE_LENGTH / L.length) return L. Otherwise, start over at step 1.
This is a variant of rejection sampling.
Line lengths include the line terminator character(s), hence MIN_LINE_LENGTH >= 1. (All the better if you know a tighter bound on line length).
It is worth noting that the runtime of this algorithm does not depend on file size, only on line length, i.e. it scales much better than reading the entire file.

Here's a solution. Take a look at the choose() method which does the real thing (the main() method repeatedly exercises choose(), to show that the distribution is indeed quite uniform).
The idea is simple: when you read the first line it has a 100% chance of being chosen as the result. When you read the 2nd line it has a 50% chance of replacing the first line as the result. When you read the 3rd line it has a 33% chance of becoming the result. The fourth line has a 25%, and so on....
import java.io.*;
import java.util.*;
public class B {
public static void main(String[] args) throws FileNotFoundException {
Map<String,Integer> map = new HashMap<String,Integer>();
for(int i = 0; i < 1000; ++i)
{
String s = choose(new File("g:/temp/a.txt"));
if(!map.containsKey(s))
map.put(s, 0);
map.put(s, map.get(s) + 1);
}
System.out.println(map);
}
public static String choose(File f) throws FileNotFoundException
{
String result = null;
Random rand = new Random();
int n = 0;
for(Scanner sc = new Scanner(f); sc.hasNext(); )
{
++n;
String line = sc.nextLine();
if(rand.nextInt(n) == 0)
result = line;
}
return result;
}
}

Either you
read the file twice - once to count the number of lines, the second time to extract a random line, or
use reservoir sampling

Looking over Itay's answer, it looks as though it reads the file a thousand times over after sampling one line of the code, whereas true reservoir sampling should only go over the 'tape' once. I've devised some code to go over code once with real reservoir sampling, based on this and the various descriptions on the web.
import java.io.FileNotFoundException;
import java.io.IOException;
import java.util.List;
public class reservoirSampling {
public static void main(String[] args) throws FileNotFoundException, IOException{
Sampler mySampler = new Sampler();
List<String> myList = mySampler.sampler(10);
for(int index = 0;index<myList.size();index++){
System.out.println(myList.get(index));
}
}
}
import java.io.File;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.util.ArrayList;
import java.util.List;
import java.util.Random;
import java.util.Scanner;
public class Sampler {
public Sampler(){}
public List<String> sampler (int reservoirSize) throws FileNotFoundException, IOException
{
String currentLine=null;
//reservoirList is where our selected lines stored
List <String> reservoirList= new ArrayList<String>(reservoirSize);
// we will use this counter to count the current line number while iterating
int count=0;
Random ra = new Random();
int randomNumber = 0;
Scanner sc = new Scanner(new File("Open_source.html")).useDelimiter("\n");
while (sc.hasNext())
{
currentLine = sc.next();
count ++;
if (count<=reservoirSize)
{
reservoirList.add(currentLine);
}
else if ((randomNumber = (int) ra.nextInt(count))<reservoirSize)
{
reservoirList.set(randomNumber, currentLine);
}
}
return reservoirList;
}
}
The basic premise is that you fill up the reservoir, and then go back to it and fill in random lines with a 1/ReservoirSize chance. I hope this provides more efficient code. Please let me know if this doesn't work for you, as I've literally knocked it up in half an hour.

Use RandomAccessFile:
Construct a RandomAccessFile, file
Get the length of that file, filelen, by calling file.length()
Generate a random number, pos, between 0 and filelen
Call file.seek(pos) to seek to the random position
Call file.readLine() to get to the end of the current line
Read the next line by calling file.readLine() again
Using this method, I've been sampling lines from the Brown Corpus at random, and can easily retrieve a 1000 random samples from randomly chosen files in a few seconds. If I tried to do the same by reading through each file line-by-line it would take me much longer.
The same principle can be used for selecting random elements from a list. Rather than reading through the list and stopping at a random place, if you generate a random number between 0 and the length of the list, then you can index directly into the list.

Reading a random line from a file in java:
public String getRandomLineFromTheFile(String filePathWithFileName) throws Exception {
File file = new File(filePathWithFileName);
final RandomAccessFile f = new RandomAccessFile(file, "r");
final long randomLocation = (long) (Math.random() * f.length());
f.seek(randomLocation);
f.readLine();
String randomLine = f.readLine();
f.close();
return randomLine;
}

Use a BufferedReader and read line wise. Use the java.util.Random object to stop randomly ;)

Java- Saving score in a game

I'm having problems tyring to keep score in my "guessing" game. I have to use a for loop or while loop. I have it so 10 random numbers are created in a text file called mystery.txt and a file reader reads these numbers from the text file.
Your score starts at 0. If the user guesses the correct number from the text file they get -10 points. If they get the number wrong they add the the absolute value difference of the number they guessed from a number in the file. The lower the score in the end the better.
When I only run my if else statement once, it works correctly. Once I loop it more than once it starts to act up.
I have to use an if else statement and a for or while loop. Thanks!
Edit- Turns out I have to use a for loop not a while loop, I'm completely lost now.
How it should work:
When you run the program a text file gets generated with 10 different numbers (I already have the code for that ready) The user gets asked to enter a number, the number the user enters gets compared to the first file on the text file. If it is the same never they get -10 points to their score. If they get it wrong they get the difference of the number the guessed and the number in the text file added to the score. This is suppose to repeat ten times.
import java.io.BufferedReader;
import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.IOException;
import java.io.PrintWriter;
import java.util.Scanner;
import java.util.Random;
import java.lang.Math;
public class lab4Fall15 {
public static void numberGuessingGame() throws IOException {
Scanner myscnr = new Scanner (System.in);
PrintWriter mywriter = new PrintWriter("mysteryNumber.txt");
int randomNumber;
int i = 0;
i=1;
while (i<=10) {
Random randomGen = new Random();
randomNumber= randomGen.nextInt(11);
// then store (write) it in the file
mywriter.println(randomNumber);
i = i + 1;
}
//Decided to use a loop to generate the numbers-------
mywriter.close();
FileReader fr = new FileReader("./mysteryNumber.txt");
BufferedReader textReader = new BufferedReader(fr);
int numberInFile;
// the number in your file is as follows:
numberInFile = Integer.valueOf(textReader.readLine());
int score= 0;
int a = 1;
while (a<=10) {
a=a+1;
System.out.print ("Please enter a number between 0 and 10: ");
int userNumber= myscnr.nextInt();
if (userNumber==numberInFile){
score = score-10;
}
else{
score = score + Math.abs(userNumber-numberInFile);
}
System.out.println ("current score is: "+score);
}
System.out.println ("your score is "+score);
textReader.close();
}
public static void main(String[] args) throws IOException {
// ...
numberGuessingGame();
}
}

if (userNumber==numberInFile){
score = score-10;
}
I don't understand what you going to mean. but I can guess this. your above code not show any error. normally , you check your , above part of code. you take variable 'numberInFile'. sometime , your file reader take this with 'whitespace or String or e.t.c' . first you check this out put .you put manual data to this variable and check out put. if it work fine , you correct that function.

OK, first, let's just go over for loops, since that's what your question was asking about. From the code you provided, it seems that you already understand while loops, and that's good, because in Java, for loops are (usually) just while loops in disguise. In general, if you have this while loop,
int a = 0;
while (a < 10) {
// do stuff with a
a = a + 1; // or ++a or a++
}
You can always rewrite it like this:
for (int a = 0; a < 10; a = a + 1) {
// do stuff with a
}
By convention (and this convention is useful when you study arrays and Collection types) you'll want to index your loops from 0 rather than 1. Since you're just learning, take my word for it for now. Loop from 0 to n-1, not from 1 to n.
With that out of the way, let's tackle why you're getting the wrong answer (which, incidentally, has nothing at all to do with loops). Rewritten as a for loop, the ask-and-score part of your program looks like this.
for (int a = 0; a < 10; ++a) {
System.out.print ("Please enter a number between 0 and 10: ");
int userNumber = myscnr.nextInt();
if (userNumber == numberInFile){
score = score - 10;
} else {
score = score + Math.abs(userNumber - numberInFile);
}
System.out.println ("current score is: "+score);
}
You will note that nowhere in this section do you update the value of numberInFile. That means that every run of this loop is still looking at whatever value that variable had at the beginning of the loop. That value came from this line:
// the number in your file is as follows:
numberInFile = Integer.valueOf(textReader.readLine());
That line is executed exactly once, before the loop runs. If you want to load the next number every time the user guesses a number, you'll need to move it inside the loop. I'll leave that as an exercise to the reader.

You are not actually capturing the number the user is entering. Try this:
int userNumber = Integer.parseInt(KeyIn.readLine());

Java: removing quotation marks from string

My thoughts/Questions:
I'm working on a Java challenge(Directions bellow). I have Part 1 finished(shown in the code bellow). I'm very close to having Part 2/3 finished.
As you'll see in my code I have 2 for-loops. The first, to iterate through my array of sorted names. The second, to iterate through the characters in each name.
As stated in the directions, an int values is to be generated for each character, and these values are then added. So, A is 1, B is 2, C is 3...and ABC is 6. This int values is then multiplied by the index number of the given String/name. So, if ABC(with a value of 6) was at index 2, it's score would be 12.
After the above step is complete, I am to total all of the all of the scores(each names score).
The above is my understanding of the directions.
The problem is my output looks like this:
"AARON"
0
"ABBEY"
-25
"ABBIE"
-82
"ABBY"
-90
"ABDUL"
-80
"ABE"
-260
"ABEL"
-240
"ABIGAIL"
-133
"ABRAHAM"
-128
"ABRAM"
-225
"ADA"
-540
"ADAH"
-506
"ADALBERTO"
216
"ADALINE"
-182
"ADAM"
-574
"ADAN"
-600
"ADDIE"
-592
"ADELA"
-629
I've ran through my logic a few times and it seems correct to me, but I don't know how I'm generating these numbers. The only thought I have is that the quotation marks(") are throwing off my calculations. They have an ASCII value of 34. I have attempted to remove them at multiple places in my code with both replace() & replaceAll(), but I have not been able too.
What am I doing wrong/how can I fix it/what do I need to do to complete this assignment/how can I improve my code?
Challenge Directions:
Use the names.txt file, a 46K text file containing over five-thousand first names found in the resources directory.
Part 1: Begin by sorting the list into alphabetical order. Save this new file as p4aNames.txt in the answers directory.
Part 2: Using p4aNames.txt, take the alphabetical value for each name, and multiply this value by its alphabetical position in the list to obtain a name score. For example, when the list is sorted into alphabetical order, COLIN, which is worth 3 + 15 + 12 + 9 + 14 = 53, is the 938th name in the list. So, COLIN would obtain a score of 938 × 53 = 49714. Save the list of all name scores as p4bNames.txt.
Part 3: What is the total of all the name scores in the file?
Pic Link Showing Output & Directory:
http://screencast.com/t/tiiBoyOpR
My Current Code:
package app;
import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.IOException;
import java.io.PrintWriter;
import java.util.Arrays;
public class AlphabetizedList {
public static void main() throws IOException {
new AlphabetizedList().sortingList();
}
public void sortingList() throws IOException {
FileReader fb = new FileReader("resources/names.txt");
BufferedReader bf = new BufferedReader(fb);
String out = bf.readLine();
out = out.substring(out.indexOf("\"")); //get rid of strange characters appearingbeforefirstname
// System.out.println(out); // output:
// "MARY","PATRICIA","LINDA","BARBARA","ELIZABETH","JENNIFER","MARIA"...
String[] sortedStr = out.split(",");
Arrays.sort(sortedStr);
PrintWriter pw = new PrintWriter(new BufferedWriter(new FileWriter("answers/p4aNames.txt")));
for (int i = 0; i < sortedStr.length; i++) {
pw.println(sortedStr[i]);
System.out.println(sortedStr[i]);// print to console just to see output
int score = 0;
// sortedStr[i].replaceAll("\"", ""); // I used this to try to remove the "s from my Strings
for (char ch: sortedStr[i].toUpperCase().toCharArray()) {
score += ((int)ch - 64); /* A is decimal 65 */
}
score = score * i; /* multiply by position in the list */
pw.println(score);
System.out.println(score);
}
bf.close();
fb.close();
pw.close();
}
}

You wrote
// sortedStr[i].replaceAll("\"", ""); // I used this to try to remove the "s from my Strings
Java String is immutable. That function returns a new string with the quotes removed. You can use
sortedStr[i] = sortedStr[i].replaceAll("\"", "");
and it should work fine.

counting elements in an array imported from a data file

I am writing a program that will import values from a txt file in to an array, I then need to count how many of those elements are greater than or equal to 36. The data imports fine, and the total amount of values it displays is correct, but I can not get it display the amount of times the number 36 is found in the file. Thanks for any help!
public static void main(String[] args) throws Exception {
int[] enrollments = new int [100];
int count;
int FullClass;
double ClassPercentage;
return count (number of data items)
count = CreateArray(enrollments);
System.out.println (count );
FullClass = AddValues (enrollments);
System.out.println (FullClass)
ClassPercentage= FullClass/count;
System.out.print(ClassPercentage +"% of classes are full");
}//end main
/**
*
* #param classSizes
*/
public static int CreateArray(int[] classSizes) throws Exception{
int count = 0;
File enrollments = new File("enrollments.txt");
Scanner infile = new Scanner (enrollments);
while (infile.hasNextInt()){
classSizes[count] = infile.nextInt();
count++}//end while
return count; //number of items in an array
} // end CreateArray
/**************************************************************************/
/**
*
* #throws java.lang.Exception
*/
public static int AddValues (int[] enrollments) throws Exception{
{
int number = 0;
int countOf36s = 0;
while (infile.hasNextInt()) {
number = infile.next();
classSizes[count] = number;
if(number>=36) {
countOf36s++;
}
count++;
}
return countOf36s;
}// end AddValues
}//end main

Try this code to count the numbers that are greater than or equal to 36 while you are reading the file only. Change the code in your createArray method or write the below logic where ever you want to.
I tried executing this program. It works as expected. See below code
import java.util.*;
import java.io.*;
public class Test { //Name this to your actual class name
public static void main(String[] args) throws Exception {
int[] enrollments = new int [100]; //assuming not more than 100 numbers in the text file
int count; //count of all the numbers in text file
int FullClass; //count of numbers whose value is >=36
double ClassPercentage;
count = CreateArray(enrollments);
System.out.println (count);
FullClass = AddValues (enrollments);
System.out.println (FullClass);
ClassPercentage= FullClass/count;
System.out.print(ClassPercentage +"% of classes are full");
}
//Method to read all the numbers from the text file and store them in the array
public static int CreateArray(int[] classSizes) throws Exception {
int count = 0;
File enrollments = new File("enrollments.txt"); //path should be correct or else you get an exception.
Scanner infile = new Scanner (enrollments);
while (infile.hasNextInt()) {
classSizes[count] = infile.nextInt();
count++;
}
return count; //number of items in an array
}
//Method to read numbers from the array and store the count of numbers >=36
public static int AddValues (int[] enrollments) throws Exception{
int number = 0;
int countOf36s = 0;
for(int i=0; i<enrollments.length; i++) {
number = enrollments[i];
if(number>=36) {
countOf36s++;
}
}
return countOf36s;
}
}

Your code indicates that you might have misunderstood a couple of concepts and stylistic things. As you say in your comments you are new at this and would like some guidance as well as the answer to the question - here it is:
Style
Method names and variable names are by convention written starting with a lower case letter and then in camel case. This is in contrast to classes that are named starting with an upper case letter and camel case. Sticking to these conventions make code easier to read and maintain. A full list of conventions is published - this comment particularly refers to naming conventions.
Similarly, by convention, closing braces are put on a separate line when they close loops or if-else blocks.
throws Exception is very general - it's usual to limit as much as possible what Exceptions your code actually throws - in your case throws FileNotFoundException should be sufficient as this is what Scanner or File can throw at runtime. This specificity can be useful to any code that uses any of your code in the future.
Substance
You are creating the array up front with 100 members. You then call CreateArray which reads from a file while that file has more integers in it. Your code does not know how many that is - let's call it N. If N <= 100 (there are 100 integers or less), that's fine and your array will be populated from 0 to N-1. This approach is prone to confusion, though - the length of your array will be 100 no matter how many values it has read from the file - so you have to keep track of the count returned by CreateArray.
If N > 100 you have trouble - the file reading code will keep going, trying to add numbers to the array beyond its maximum index and you will get a runtime error (index out of bounds)
A better approach might be to have CreateArray return an ArrayList, which can have dynamic length and you can check how many there are using ArrayList.size()
Your original version of AddValues called CreateArray a second time, even though you pass in the array which already contains the values read from file. This is inefficient as it does all the file I/O again. Not a problem with this small example, but you should avoid duplication in general.
The main problem. As per prudhvi you are checking the number of integers in the file against 36, not each value. You can rectify this as suggested in that answer.
You do ClassPercentage= FullClass/count; Although ClassPercentage is a double, somewhat counter intuitively - because both the variables on the Right Hand Side (RHS) are int, you will have an int returned from the division which will always round down to zero. To make this work properly - you have to change (cast) one of the variables on the RHS to double before division e.g. ClassPercentage= ((double)FullClass)/count;.
If you do keep using arrays rather than ArrayList, be careful what happens when you pass them into methods. You are passing by reference, which means that if you change an element of an array in your method, it remains changed when you return from that method.
In your new version you do
...
classSizes[count] = number;
if(number>=36) {
...
You almost certainly mean
...
number = classSizes[count];
if(number>=36) {
...
which is to say in programing the order of the assignment equals is important, so a = b is not equivalent to b = a
Code
A cleaned up version of your code - observing all the above (I hope):
import java.io.File;
import java.io.FileNotFoundException;
import java.util.ArrayList;
import java.util.Scanner;
public class ClassCounter
{
public static void main(String[] args) throws FileNotFoundException
{
int count;
int fullClass;
double classPercentage;
ArrayList<Integer> enrollments = createArray();
count = enrollments.size();
System.out.println(count);
fullClass = addValues(enrollments);
System.out.println(fullClass);
classPercentage = fullClass / count;
System.out.print(classPercentage + "% of classes are full");
}
/**
* scans file "enrollments.txt", which must contain a list of integers, and
* returns an ArrayList populated with those integers.
*
* #throws FileNotFoundException
*/
public static ArrayList<Integer> createArray() throws FileNotFoundException
{
ArrayList<Integer> listToReturn = new ArrayList<Integer>();
File enrollments = new File("enrollments.txt");
Scanner infile = new Scanner(enrollments);
while (infile.hasNextInt())
{
listToReturn.add(infile.nextInt());
}
return listToReturn;
}
/**
* returns the number of cases where enrollments >= 36 from the list of
* all enrollments
*
* #param enrollments - the list of enrollments in each class
* #throws FileNotFoundException
*/
public static int addValues(ArrayList<Integer> enrollments)
{
int number = 0;
int countOf36s = 0;
int i = 0;
while (i < enrollments.size())
{
number = enrollments.get(i);
if (number >= 36)
{
countOf36s++;
}
}
return countOf36s;
}
}

generating int up to 100 million throws exception

I am trying to generate integers up to 100 million, then combine it with pre-defined integer/string.
Example: predefined = 1010 Generated: gen = 5020315 Combined =
10105020315
then save that number to .txt file, so text file should have 100 million lines.
Here is a code I wrote:
import java.io.FileNotFoundException;
import java.io.PrintWriter;
import java.io.UnsupportedEncodingException;
public class exec{
public static void main(String[] args) throws FileNotFoundException, UnsupportedEncodingException
{
int initial = 6618;
PrintWriter writer = new PrintWriter("variations.txt", "UTF-8");
for(int a = 0; a < 100000000; a++){
int a2 = Integer.parseInt(Integer.toString(initial) + Integer.toString(a));
writer.println(a2);
}
writer.close();
}
}
But it throws the following error:
Exception in thread "main" java.lang.NumberFormatException: For input
string: "6618100000"
Why does this happen? Where is the problem?

you need long , and you can use Long.parseLong();.
largest value for int is long 2^31-1 but for long is 2^63-1.

Combining 6618 which the value of a leads to a number too large to be held in an int variable (For example, 6618100000 is too large to be held in an int variable). The largest value for int is 2^31-1. You can use Long.parseLong() instead.

Whenever you are parsing, you need to make sure that the Integer you want to create from the string is smaller than Integer.MAX.
Integer.MAX is equal to 2147483647, and so any value bigger than this will cause an exception.