Remove certain chars in-place

user_14459522 0
Viewed 173

As far as I know, in C we can modify a char-array in-place and then append the \0 to make the char-array shorter. I am wondering how that's done in Java.

Assume we want to remove the spaces from a char-array in Java.


char[] str = new String("cat love dogs").toCharArray();

for(int i = 0; i < str.length; i++) {
    if(str[i] == ' ')
        str[i] = '';
}

This does not seem to work.

EDIT: I already know about built-in functions to replace some chars with another in a string. I am not asking that. Curious to modify a char-array in-place with O(1) extra space.

java
4 Answers

well the best solution i can see is remove all spaces from your string first then get the chars from it :

 String myString = new String("cat love dogs").replaceAll("\\s+", ""); 
     // \\s+ is the regex for sapces 
    // myString becomes => catlovedogs
 char[] str = myString.toCharArray();
user_134704370

As String is immutable you may use a StringBuilder class:

StringBuilder sb = new StringBuilder("cat love dogs");

for(int i = 0; i < sb.length(); i++) {
        if(sb.charAt(i) == ' ') {
            sb.deleteCharAt(i);
            i--;
        }
    }

char[] str = sb.toString().toCharArray();
user_134151640

Now as far as space is concerned String class in Java is immutable!

Thus you cannot do anything in place there.

But you can use StringBuilder which is immutable.

Code:

StringBuilder stringBuilder = new StringBuilder("cat love dogs");
for(int i = 0; i < stringBuilder.length(); i++) {
    if (sb.charAt(i) == ' ')
        sb.deleteCharAt(i);
}
char[] str = stringBuilder.toString().toCharArray();

Or if you want to do it just using the char array itself its not possible as char arrays are not dynamic by themselves.

But here is what you could do:

char[] str = new String("cat love dogs").toCharArray();
int last = 0;
for(int i = 0; i < str.length; i++) {
    if(str[i] == ' '){
        str = ArrayUtils.remove(str, i);
    }
}
System.out.println(new String(str));
System.out.println(new String(str).length());

Output:

catlovedogs
11

For for this you need to import org.apache.commons.lang3.ArrayUtils

If you are not concerned about space:

In Java we can just set the character value to 0!

Here is the modified code:

char[] str = new String("cat love dogs").toCharArray();
for(int i = 0; i < str.length; i++) {
    if(str[i] == ' ')
        str[i] = 0;
}
System.out.println(new String(str));

Here is the output:

catlovedogs
user_149110940

I think the best way to do this is by creating a duplicate array of chars, where you copy every character besides the spaces and then return the new array. Here is how the code might look like:

        char[] str = new String("cat love dogs").toCharArray();
        List<Character> duplicate = new ArrayList<>();
        
        for(int i = 0; i < str.length; i++) {
            if(str[i] != ' ') {
                duplicate.add(str[i]);
            }
                
        }

it's better to use ArrayList because you don't really know the final size of the array and this way you can go only as big as needed.

user_151992820
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