Can someone please explain in more detail why we only need to change the inner loop in the nested enhanced for loops in order to change the elements in the 2D Array? I thought that with the enhanced for loop it was only a copy of the value. Why wouldn’t we need to make both the row and column for loops regular for loops?
This is from Unit 5 Lesson 5 exercise #4. The teacher explanation says " Since we changed the inner for loop from an enhanced for loop to a regular one, we are now able to update the values within the array. In this case, the function will take the value at each element and multiply it by two."
public static void changeValues(int numbers) {
for (int row : numbers) {
for (int value : row) {
value *= 2;
}
}
}
is changed to:
public static void changeValues(int numbers) {
for (int row : numbers) {
for (int col = 0; col < row.length; col++) {
row[col] *= 2;
}
}
}
when having a 2d array it’s important to know where you currently are in the array the for instance a row represents a shelf and the column represents the indexed positions of those items
the step through goes like this
→ i look on a shelf
→ pick a row of the shelf
→ check how many items exist within that row
→ reference all ids within that row and double each value when iterating
end result
tldr
without having an index you cannot change specific values within the nested array you’ll only be changing the local variable in the loop not the array
@cicaneser thank you for reaching out to the forum. Here’s an example on how to use enhanced for loops with 2D arrays. Let us know if this helps.
public class EnhancedFor2DArray {
public static void main(String args) {
int matrix = {
{1, 2, 3},
{4, 5, 6},
{7, 8, 9}
};
// Enhanced for loop to iterate through the 2D array
for (int[] row : matrix) {
for (int element : row) {
System.out.print(element + " ");
}
System.out.println(); // New line after each row
}
}
}
Thank you for your responses. I do know how 2D arrays work and that if we want to change the change the values we need to use regular for loops referring to the index; however, I am confused as to why we only need to do that on the inner loop and not the outer loop.
well your not changing the value of the first dimension of the array your changing the second one. I.E. when i move an item i don’t take the entire shelf with me. while you can index through both arrays if you really want to but it will look worse and require a bunch more unnecessary work as programmers it’s good to be lazily efficient which is why the solution does not do that
Hi @cicaneser,
Thanks for your question. I think I am understanding the spot where this gets confusing, in double loops that are set up like this:
for (int row : matrix) {// don’t need index here
for (int index = 0; index < row.length ; index ++) {//need index here
row[index] += 2 ;//this will change what is stored
}
}
The outer loop is an enhanced for loop that takes each inner array out of the 2D array. Since what it is getting is a 1D array, that row is stored using a reference address, and row copies the address that points to the “actual data”.
The inner loop now is going to get each primitive int value from the row. If we used an enhanced for loop there it would make a copy of that value and we wouldn’t be able to change what is stored. So having the inner loop set up with an index allows this to get to the ‘actual data’ not just a copy, and change the values stored.
This concept can feel confusing because the int row holds primitives, but since it is a 1D array itself, it is an object and the outer enhanced for loop copies the reference address.
I hope that helps,
Lindsay
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@cicaneser - Rosanne, I apologize for not reading your inquiry correctly. Lindsay has explained the process well. The outer loop moves from one inner array to the next. The inner loop access the individual elements in each inner array.
Again, sorry for not answering your question completely.
Sylvia
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