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very new to java and oop in general. so be kind.

I have one text file which contains 10 integers, searchkeysArray.txt. The program creates an array named keysArr. I have another text file of 500 random integers, array1.txt. The program creates another array named array1.

I want to use the linearSearch method I created to search for the elements of keysArr within array1 and output the index which it exist. 

public static int linearSearch(int arr[], int x) { int size = arr.length; for(int i = 0; i < size; i++) { if(arr[i] == x) return i; } return -1; }

readFile method

public static int[] readFile(String file) { try { File f = new File(file); Scanner s = new Scanner(f); int ctr = 0; while (s.hasNextInt()) { ctr++; s.nextInt(); } int[] arr = new int[ctr]; //create array of that size Scanner scanner2 = new Scanner(f); for (int i = 0; i < arr.length; i++) arr[i] = scanner2.nextInt(); return arr; } catch(Exception e) { return null; }

the program.

 public static void main(String[] args) { int[] keysArr = readFile("searchkeysArray"); int[] array1 = readFile("array17"); int key = 34; int result = linearSearch(array1, key); if (result != -1) System.out.print("The element " +key+" is present in the array, at index " + result + " "); else System.out.print("The element " +key+" is not present in the array "); }

and it outputs

The element 34 is present in the array, at index 359

which makes sense. I've manually tested numbers and (apparently) everything works fine. But I do not quite understand how I'm supposed to use keysArr as my key rather than int x = some number.

Want to output something like

The element [keysArr[0]] is present in the array, at index 359 The element [keysArr[1]] is present in the array, at index 547 ... The element [keysArr[4]] is not present in the array

and so on. Right now keysArr just an array of 10 integers but I will eventually use hundreds..

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2 Answers 2

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Rather than using a specific hard-coded key, such as int key = 34, you wish to loop over your array of keys keysArr. You can achieve that by using code like:

for (int key : keysArr) { int result = linearSearch(array1, key); if (result != -1) System.out.print("The element " +key+" is present in the array, at index " + result + " "); else System.out.print("The element " +key+" is not present in the array "); }
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Your linear algorithm worth O(n^2) which is pretty bad if you are going to increase the number of integers.

I suggest you to load dictionary elements into a hash-map Value -> Index and iterate through the values array looking up for the position in the hashMap. This will give you O(2n) - the complexity will be linear. You'll require 2n of memory though. But in your case it is insignificant.

public static void main(String[] args) { printResults(new int[]{1, 2, 3, 4}, new int[]{3, 4, 7, 9}); } public static void printResults(int dictionary[], int valuesToSearch[]){ Map<Integer, Integer> positionsMap = new HashMap<>(); IntStream.range(0, dictionary.length).forEach(index -> positionsMap.put(dictionary[index], index)); Arrays.stream(valuesToSearch).mapToObj(value -> positionsMap.containsKey(value) ? format("Value %s is present in the array at index %s", value, positionsMap.get(value)) : format("Value %s is not present in the array", value) ).forEach(System.out::println); }
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