Consider the following method intended to swap the first and second rows in a two-dimensional array:

public static void swapRow(int[][] a)
{
/* missing code */
}

Which of the following correctly replaces /* missing code */?

Question

Consider the following method intended to swap the first and second rows in a two-dimensional array:

public static void swapRow(int[][] a) 
{
  /* missing code */
}

Which of the following correctly replaces /* missing code */?

BlackTom AI · Accepted Answer

Swap corresponding elements of row 0 and row 1 across all columns using a loop. The relevant concepts are: 2D array indexing a[row][col], the number of columns given by a[0].length, and using a temporary variable to swap two values without losing data.

Step-by-step reasoning:
1. We must iterate over every column index k in the rows to exchange a[0][k] with a[1][k]. The loop bound should be k < a[0].length (the number of columns in the first row).
2. For each column k, store the value of a[0][k] in a temporary int temp so it is not lost when we overwrite a[0][k].
3. Assign a[0][k] = a[1][k] to move the second-row element into the first row.
4. Assign a[1][k] = temp to place the saved first-row element into the second row.

Intermediate steps with an example:
Let a = {{1, 2, 3}, {4, 5, 6}}.
- k = 0: temp = a[0][0] = 1; a[0][0] = a[1][0] = 4; a[1][0] = temp = 1 → rows become {{4,2,3},{1,5,6}}.
- k = 1: temp = a[0][1] = 2; a[0][1] = a[1][1] = 5; a[1][1] = temp = 2 → {{4,5,3},{1,2,6}}.
- k = 2: temp = a[0][2] = 3; a[0][2] = a[1][2] = 6; a[1][2] = temp = 3 → {{4,5,6},{1,2,3}}.
After the loop the first and second rows have been exchanged element-wise.

Why the correct answer is correct:
- It iterates over columns using a[0].length (correct dimension).
- It uses a temporary variable to perform a safe swap for each column.
- It targets row indices 0 and 1, which are the first and second rows.

Why other common alternatives can be wrong:
- Using a.length in the loop would iterate over rows rather than columns and cause index errors when rows ≠ columns.
- Swapping only references (int[] temp = a[0]; a[0] = a[1]; a[1] = temp;) would also swap entire row references and is valid, but the provided correct choice explicitly swaps elements; other incorrect choices often misuse indices or loop bounds.

Therefore the provided code snippet is correct and matches the authoritative answer.

Consider the following method intended to swap the first and second rows in a two-dimensional array: public static void swapRow(int[][] a) { /* missing code / } Which of the following correctly replaces / missing code */?单项选择题

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Consider the following method intended to swap the first and second rows in a two-dimensional array: public static void swapRow(int[][] a) { /* missing code */ } Which of the following correctly replaces /* missing code */?单项选择题