MERGE METHOD; public class ExternalSort { static Scanner openIn (File file) { tr

Question

MERGE METHOD;

public class ExternalSort {

static Scanner openIn (File file) {

try {

return new Scanner(file);

} catch (Exception e) {

System.out.println(e);

}

return null;

}

  

static PrintWriter openOut (File file) {

try {

return new PrintWriter(file);

} catch (Exception e) {

System.out.println(e);

}

return null;

}

// Read this to make sure you understand file I/O.

static void copy (String from, String to) {

File inFile = new File(from);

Scanner in = openIn(inFile);

File outFile = new File(to);

PrintWriter out = openOut(outFile);

while (in.hasNextLine()) {

String line = in.nextLine();

out.println(line);

}

in.close();

out.close();

}

// Merge the first n lines of in0 with the first n lines of in1 and

// write the output to out. in0 or in1 may have less than n lines.

static void merge (int n, Scanner in0, Scanner in1, PrintWriter out) {

// EXERCISE

}

static void sort (String from, String to) {

File inFile = new File(from);

File outFile = new File(to);

  

File[] tmpFile0 = { new File("tmp00"), new File("tmp01") };

File[] tmpFile1 = { new File("tmp10"), new File("tmp11") };

File[][] tmpFiles = { tmpFile0, tmpFile1 };

  

int size = 0;

int i = 0;

int j = 0;

  

// Split the input into two temporary file.

{

Scanner in = openIn(inFile);

PrintWriter[] outTmp = { openOut(tmpFiles[i][0]),

   openOut(tmpFiles[i][1]) };

while (in.hasNextLine()) {

String line = in.nextLine();

outTmp[j].println(line);

j = 1 - j;

size++;

}

in.close();

outTmp[0].close();

outTmp[1].close();

}

// Merge groups of 1, 2, 4, 8, ...

int n = 1;

while (2 * n < size) {

Scanner[] inTmp = { openIn(tmpFiles[i][0]),

openIn(tmpFiles[i][1]) };

PrintWriter[] outTmp = { openOut(tmpFiles[1-i][0]),

   openOut(tmpFiles[1-i][1]) };

while (inTmp[0].hasNextLine() || inTmp[1].hasNextLine()) {

merge(n, inTmp[0], inTmp[1], outTmp[j]);

j = 1 - j;

}

inTmp[0].close();

inTmp[1].close();

outTmp[0].close();

outTmp[1].close();

  

i = 1 - i;

n *= 2;

}

// Merge the two (sorted) temporary files.

{

Scanner[] inTmp = { openIn(tmpFiles[i][0]),

openIn(tmpFiles[i][1]) };

PrintWriter out = openOut(outFile);

merge(n, inTmp[0], inTmp[1], out);

inTmp[0].close();

inTmp[1].close();

out.close();

}

}

  

public static void main (String[] args) {

sort("numbers.txt", "sorted.txt");

}

}

6. Write the merge method in External Sort Test on number.txt and then make your own file with different lengths to make sure you have it right.

Explanation / Answer

import java.io.File;
import java.io.PrintWriter;
import java.util.Scanner;

public class ExternalSort {
static Scanner openIn (File file) {
try {
return new Scanner(file);
} catch (Exception e) {
System.out.println(e);
}
return null;
}
  
static PrintWriter openOut (File file) {
try {
return new PrintWriter(file);
} catch (Exception e) {
System.out.println(e);
}
return null;
}

// Read this to make sure you understand file I/O.
static void copy (String from, String to) {
File inFile = new File(from);
Scanner in = openIn(inFile);
File outFile = new File(to);
PrintWriter out = openOut(outFile);

while (in.hasNextLine()) {
String line = in.nextLine();
out.println(line);
}
in.close();
out.close();
}

// Merge the first n lines of in0 with the first n lines of in1 and
// write the output to out. in0 or in1 may have less than n lines.
static void merge (int n, Scanner in0, Scanner in1, PrintWriter out) {
   int a = 0, b = 0;
   boolean isA = true, isB = true;
   while(in0.hasNext() && in1.hasNext())
   {
       if (isA)
           a = in0.nextInt();
       if (isB)
           b = in1.nextInt();
         
       if (a < b)
       {
           out.write(a + " ");
           isB = false;
           isA = true;
       }
       else
       {
           out.write(b + " ");
           isB = true;
           isA = false;
       }
   }
     
   while(in0.hasNextInt())
   {
       out.write(in0.nextInt() + " ");
   }
     
   while(in1.hasNextInt())
   {
       out.write(in1.nextInt() + " ");
   }
// EXERCISE
}

static void sort (String from, String to) {
File inFile = new File(from);
File outFile = new File(to);
  
File[] tmpFile0 = { new File("tmp00"), new File("tmp01") };
File[] tmpFile1 = { new File("tmp10"), new File("tmp11") };
File[][] tmpFiles = { tmpFile0, tmpFile1 };
  
int size = 0;

int i = 0;
int j = 0;
  
// Split the input into two temporary file.
{
Scanner in = openIn(inFile);
PrintWriter[] outTmp = { openOut(tmpFiles[i][0]),
openOut(tmpFiles[i][1]) };
while (in.hasNextLine()) {
String line = in.nextLine();
outTmp[j].println(line);
j = 1 - j;
size++;
}
in.close();
outTmp[0].close();
outTmp[1].close();
}

// Merge groups of 1, 2, 4, 8, ...
int n = 1;
while (2 * n < size) {
Scanner[] inTmp = { openIn(tmpFiles[i][0]),
openIn(tmpFiles[i][1]) };
PrintWriter[] outTmp = { openOut(tmpFiles[1-i][0]),
openOut(tmpFiles[1-i][1]) };
while (inTmp[0].hasNextLine() || inTmp[1].hasNextLine()) {
merge(n, inTmp[0], inTmp[1], outTmp[j]);
j = 1 - j;
}
inTmp[0].close();
inTmp[1].close();
outTmp[0].close();
outTmp[1].close();
  
i = 1 - i;
n *= 2;
}

// Merge the two (sorted) temporary files.
{
Scanner[] inTmp = { openIn(tmpFiles[i][0]),
openIn(tmpFiles[i][1]) };
PrintWriter out = openOut(outFile);
merge(n, inTmp[0], inTmp[1], out);
inTmp[0].close();
inTmp[1].close();
out.close();
}
}
  
public static void main (String[] args) {
sort("numbers.txt", "sorted.txt");
}
}

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