String dna[] = {"ATCTA"};
 int i = 0;
 dna[i] = dna[i].replace('T', 'C');
 System.out.println(dna[i]);

This works as expected. Double check your code if you follow a similiar pattern.

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You may have expected, that dna[i].replace('T', 'C'); changes the content of the cell dna[i] directly. This is not the case, the String will not be changed, replace will return a new String where the char has been replaced. It's necessary to assign the result of the replace operation to a variable.

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To answer your last comment:

Strings are immutable - you can't change a single char inside a String object. All operations on Strings (substring, replace, '+', ...) always create new Strings.

A way to make more than one replace is like this:

dna[i] = dna[i].replace('T', 'C').replace('A', 'S');

Array values are mutable. Here's a quick solution: ```java import java.util.ArrayList; import java.util.List; import java.util.Random; public class ToDORNotTwoD { public static void main(String[] args) { int[][] array = new int[10][10]; List coordinateList = new ArrayList<>(); randomizeMatrix(array); printMatrix(array); manipulateMatrix(array, coordinateList); printCoordinateList(coordinateList); printMatrix(array); } private static void printMatrix(int[][] matrix) { for (int i = 0; i < matrix.length; i++) { for (int j = 0; j < matrix[0].length; j++) { System.out.printf("%3d", matrix[i][j]); } System.out.println(); } System.out.println(); } private static void printCoordinateList(List coordinateList) { System.out.println(coordinateList.toString()); System.out.println(); } private static void randomizeMatrix(int[][] matrix) { Random rand = new Random(); for (int i = 0; i < matrix.length; ++i) { for (int j = 0; j < matrix[0].length; ++j) { matrix[i][j] = rand.nextInt(100); } } } private static void manipulateMatrix(int[][] matrix, List coordinateList) { for (int i = 0; i < matrix.length; ++i) { for (int j = 0; j < matrix[0].length; ++j) { if (matrix[i][j] % 3 == 0) { matrix[i][j] = -1; String pair = "( " + String.valueOf(i) + " , " + String.valueOf(j) + ")"; coordinateList.add(pair); } } } } } ```

Not sure if I should give the answer directly: To access the variable in array you can use e.g. System.out.println(array[1] ) will print “1.0” from your example. To replace the value you can just assign the value to the index you want. array[index] = value to assign

Try using break.

for(int i = 0; i < array.length; i++) {
    if(fruit[i] == 0) {
        fruit[i] = X;
        break;
    }
}

This will ensure only one is changed, max.

First, I would give slow the same scope as fast - give them both loop scope:

for (int slow = 0, fast = 0; fast < nums.length; ++fast) {

Second, rather than creating a new variable i and iterating backwards, I would write my second loop like this:

for (; slow < nums.length; ++slow) {
    nums[slow] = 0;
}

Instead of creating a new variable int i = fast and iterating backward until it is equal to slow to set the remaining elements of the array to 0, I just start with the variable in slow and iterate until the end of the array is reached. This will work because the outer loop has finished iterating anyway because once fast == nums.length - 1, we exit the outer loop.

Third, put spaces around your mathematical operator(s) to make it easy to read:

if (fast == nums.length - 1) {

Otherwise, I see nothing wrong with your code, you use reasonable variable names, you use braces around your ifs and loops, and you indent your code nicely.