CODEWITHSUNDEEP
java
kakoli
kakoli

Reputation: 447

Best way to remove characters at certain positions in a Java string or stringbuilder

process_str(StringBuilder copy, List<Integer> idxList) {
        for(int i = 0; i < idxList.size(); ++i) {
            int pos  = idxList.get(i);
            copy.setCharAt(pos, '$');
        }
        StringBuilder output = new StringBuilder();
        for(int i = 0; i < copy.length(); ++i) {
            char ch = copy.charAt(i);
            if(ch != '$')
                output.append(ch);
        }
        return output.toString();
}

Constraint - $ does not appear in the input string. Is there a simpler way to do this?

Upvotes: 1

Views: 160

Answers (3)

dani-vta
dani-vta

Reputation: 7080

Since the title says: "best way to remove characters", it is not clear whether you mean most efficient or most concise. However, in the latter case, you could employ java streams.

String str = "Hello my new world!";
List<Integer> idxList = new ArrayList<>(List.of(3, 5, 6));
StringBuilder strBld = new StringBuilder();

IntStream.range(0, str.length()).boxed().forEach(i -> {
    if (!idxList.contains(i)) strBld.append(str.charAt(i));
});

Upvotes: 1

T.Lucas
T.Lucas

Reputation: 848

Solution similar to rikyeah but avoiding the complexity of contains. Here we first sort the input idxList in order to have the indices in ascendant order, then we create the output StringBuilder with a initial capacity computed from the length of the input StringBuilder and the number of index to remove. Finally we browse the input StringBuilder and avoid the copy when we reach the next index to remove.

public String process_str(StringBuilder copy, List<Integer> idxList) {
    List<Integer> sortedIndices = new ArrayList<>(idxList);
    Collections.sort(sortedIndices);
    Iterator<Integer> idxIterator = sortedIndices.iterator();
    int currIdx;
    if (idxIterator.hasNext()) {
        currIdx = idxIterator.next();
    } else {
        currIdx = copy.length();
    }
    StringBuilder output = new StringBuilder(copy.length() - idxList.size());
    for(int i = 0; i < copy.length(); ++i) {
        char ch = copy.charAt(i);
        if (i != currIdx) {
            output.append(ch);
        } else if (idxIterator.hasNext()) {
            currIdx = idxIterator.next();
        }
    }
    return output.toString();
}

Upvotes: 1

rikyeah
rikyeah

Reputation: 2013

You can remove the first loop like this:

process_str(StringBuilder copy, List<Integer> idxList) {
    StringBuilder output = new StringBuilder();
    for(int i = 0; i < copy.length(); ++i) {
        char ch = copy.charAt(i);
        if (idxList.contains(i))
            output.append(ch);
    }
    return output.toString();
}

The overall time complexity might be different depending on what type of List you use.

Upvotes: 2

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