Merging values of 2 string arrays in one concatenated strings array

Question

I have 2 string arrays.

string [] first = {"ajunkbc","ajunkHello","adedbc","abcjunk","add","ad","a","","junk","ajunk","aajunkbb"};
String  [] second = {"abc","aHello","adedbc","abcjunk","add","ad","a","","junk","a","aajunkbb"};

I'd like the result of my merge() method to concatenate each element from the first array with the respective element of the second array separated by a comma.

Below is my code

private static String[] merge(String [] tests, String [] expectations){

    List<String> testList = Arrays.asList(tests);
    List<String> expectationsList = Arrays.asList(expectations);

    List<String> retList = new ArrayList<String>();
    for(String test : testList){
        for(String val : expectationsList){
            retList.add(test+","+val);
            break;
        }
    }

This does not work. What's wrong with my code?

Answer 1

What's wrong is that you are looping over expectationsList and breaking out of the loop after the first iteration:

for(String val : expectationsList){
    retList.add(test+","+val);
    break; //<--- breaking out of loop after first iteration each time
}

So the result is that you are always retrieving the first element of expectationsList.

Since what you want is to loop over two arrays, you should use an index:

for (int i = 0; i < testList.size(); i++) {
    retList.add(testList.get(i)+","+expectationsList.get(i));
}

Also, note that this implies that the size of testList is the same as the size of expectationsList. Your method should probably throw an exception if this is not the case.

Note that you do not need to convert the input arrays into lists. You can use them as-is.

private static String[] merge(String[] tests, String[] expectations) {
    if (tests.length != expectations.length) {
        throw new IllegalArgumentException("input not of same length");
    }
    String[] result = new String[tests.length];
    for (int i = 0; i < tests.length; i++) {
        result[i] = tests[i] + "," + expectations[i]);
    }
    return result;
}

Java 8 solution:

private static String[] merge(String[] tests, String[] expectations) {
    if (tests.length != expectations.length) {
        throw new IllegalArgumentException("input not of same length");
    }
    return IntStream.range(0, tests.length).mapToObj(i -> tests[i] + "," + expectations[i]).toArray(String[]::new);
}

Answer 2

You're iterating through each member of testList and then for each one, iterating through each member of expectationsList. You want to iterate through each of both of them together.

What you want to do is something like this:

private static String[] merge(String[] tests, String[] expectations) {
    String[] result = new String[tests.length];
    for(int i = 0; i < tests.length; i++) {
        result[i] = tests[i] + "," + expectations[i];
    }
    return result;
}

This code makes the assumption that tests and expectations have the same length. You might want to do a check for that at the beginning:

if (tests.length != expectations.length) {
    throw new IllegalArgumentException("tests and expectations are of different lengths")
}

Notice how now you're getting the element at the same index from both arrays.

---

Sidenote: You can iterate over arrays with the for each format. This works just fine:

String[] myStringArray = getStringArray();
for (String myString : myStringArray) {
    // Do something
}

You don't need to convert to a List in order to iterate :)