I want to use a hashmap to count the number of occurrences of several strings in a file. How would I go about doing this? Also, would I be able to count the number of unique strings in a similar fashion? Examples would be much appreciated.
2 Answers
As an example, here's a program that will read words from a file and count how many times a Java keyword was encountered.
import java.io.BufferedReader;
import java.io.File;
import java.io.FileNotFoundException;
import java.io.FileReader;
import java.io.IOException;
import java.util.Map;
import java.util.HashMap;
public class CountKeywords {
public static void main(String args[]) {
String[] theKeywords = { "abstract", "assert", "boolean", "break", "byte", "case", "catch", "char", "class", "const", "continue", "default", "do", "double", "else", "enum", "extends", "false", "final", "finally", "float", "for", "goto", "if", "implements", "import", "instanceof", "int", "interface", "long", "native", "new", "null", "package", "private", "protected", "public", "return", "short", "static", "strictfp", "super", "switch", "synchronized", "this", "throw", "throws", "transient", "true", "try", "void", "volatile", "while" };
// put each keyword in the map with value 0
Map<String, Integer> theKeywordCount = new HashMap<String, Integer>();
for (String str : theKeywords) {
theKeywordCount.put(str, 0);
}
FileReader fr;
BufferedReader br;
File file = new File(args[0]); // the filename is passed in as a String
// attempt to open and read file
try {
fr = new FileReader(file);
br = new BufferedReader(fr);
String sLine;
// read lines until reaching the end of the file
while ((sLine = br.readLine()) != null) {
// if an empty line was read
if (sLine.length() != 0) {
// extract the words from the current line in the file
if (theKeywordCount.containsKey(sLine)) {
theKeywordCount.put(sLine, theKeywordCount.get(sLine) + 1);
}
}
}
} catch (FileNotFoundException exception) {
// Unable to find file.
exception.printStackTrace();
} catch (IOException exception) {
// Unable to read line.
exception.printStackTrace();
} finally {
br.close();
}
// count how many times each keyword was encontered
int occurrences = 0;
for (Integer i : theKeywordCount.values()) {
occurrences += i;
}
System.out.println("\n\nTotal occurences in file: " + occurrences);
}
}
To answer your question about unique strings, you can adapt the way I use the HashMap in a similar fashion.
- create a new HashMap, call it
uniqueStrings - when reading strings from the file, check if the HashMap that keeps track of the count contains the current string
- if it doesn't, then add it to
uniqueStrings - if it does, then remove it from
uniqueStrings
- if it doesn't, then add it to
- after you're done reading the file, you will have only unique strings in
uniqueStrings
Let me know if you have questions.
I hope this helps.
Hristo
1 Comment
Terezi
Thank you, that is very helpful. They keywords are also from the file, would using StringTokenizer and then adding them to the HashMap work? I used poor wording about the unique strings. What I need to do is count the number of unique IP adresses in a log file - or rather, check if the if it already exists in the HashMap, if it does not then add it and if it does do not add it again, and in the end count the number of IP addresses in the HashMap.
For tracking unique strings, you don't need to keep track of the number of occurrences in the file. Rather, you can use a HashSet instead of a HashMap for code clarity.
Note: HashSet is internally backed by a HashMap with a final object used as the Value in the Key Value pair.
1 Comment
sharakan
Yeah, but a Set will only have all the unique strings, and OP wanted to count the occurrences of each unique string. How are you suggesting to do that with a Set?