我正在尝试为作业编写程序。要求是递归地创建Pascal的三角形，然后打印给定的行。但是，在编译我的程序后，我得到了几个ArrayIndexOutOfBoundsExceptions。这是堆栈跟踪：java.lang.ArrayIndexOutOfBoundsException: 10 at pasTriangle.populateT(pasTriangle.java:79) at pasTriangle.populateT(pasTriangle.java:86) at pasTriangle.populateT(pasTriangle.java:86) at pasTriangle.populateT(pasTriangle.java:86) at pasTriangle.populateT(pasTriangle.java:86) at pasTriangle.populateT(pasTriangle.java:86) at pasTriangle.populateT(pasTriangle.java:86) at pasTriangle.populateT(pasTriangle.java:86) at pasTriangle.populateT(pasTriangle.java:86) at pasTriangle.populateT(pasTriangle.java:93) at pasTriangle.populateT(pasTriangle.java:86) at pasTriangle.populateT(pasTriangle.java:86) at pasTriangle.populateT(pasTriangle.java:86) at pasTriangle.populateT(pasTriangle.java:86) at pasTriangle.populateT(pasTriangle.java:86) at pasTriangle.populateT(pasTriangle.java:86) at pasTriangle.populateT(pasTriangle.java: 93) at pasTriangle.populateT(pasTriangle.java:86) at pasTriangle.populateT(pasTriangle.java:86)at pasTriangle.populateT(pasTriangle.java:86)at pasTriangle.populateT(pasTriangle.java:86)at pasTriangle.populateT(pasTriangle.java:86)
有谁知道我做错了什么？我已经尝试了所有的东西，特别是改变了condiionals，但没有任何作用。这是我的代码：
public class pasTriangle
{
private int size, row, col; //Represents the number of lines the triangle has.
private int [][] pTriangle; //2-D array to hold int values of triangle
/* ****************************************************************************************************************************************************
Constructor creates a 2D array to hold the Pascales triangle. Note the number of numbers on each line is the same as the number of lines in
the triangle, so size can be used for both values. Calls populateT method to populate the triangle.
***************************************************************************************************************************************************/
public pasTriangle(int size)
{
this.size = size;
pTriangle = new int[size][size];
}
/* ****************************************************************************************************************************************************
Method which populates the Pascal's Triangle recursively. Note case where size = 1, recursion does not occur since only 1 integer can be added
to array.
Also note base case where base of triangle is reached and recursion stops.
Also note cases for first and last value of each line (row).
Appropriate values added to each index according to conditions.
*********************************************************************************************************************************************************/
public void populateT(int row, int col)
{
if(size > 0 && size == 1 && row < size)
{
pTriangle[0][0] = 1;
}
else if(size > 1 && row < size)
{
if (col==0 && row == 0) //First value.
{
pTriangle[row][col] = 1;
}
else if (row != 0 && col == 0 || col == pTriangle[row].length-1)
{
pTriangle [row][col] = 1; //1 Set for first value in each line and last value in each line.
}
else if(row > 1 && col != 0 && col != pTriangle[row].length-1) //Values in between first and last calculated from the two above them, to left and right.
{
pTriangle[row][col] = (pTriangle[row-1][col-1]) + (pTriangle[row-1][col+1]);
//Line 79, exception here.
}
if (col < pTriangle[row].length && row < pTriangle.length) //Move over and give values to indexes recursively until end of row is reached
{ **//Line 87, exception here.**
populateT(row, col+1);
}
else if (col >= pTriangle[row].length && row < pTriangle.length) //If end of row is reached and number of rows is not exceeded.
{
col = 0; //Col reset.
**//Line 93 Exception here.**
populateT(row+1, col);
}
}
}
/* ***********************************************************************************************************************************************
Prints a string containing the values on a given line of the pasTriangle. Note 1 is subtracted from lineNumber to get correct index.
***********************************************************************************************************************************************/
public String getLine(int lineNumber)
{
lineNumber = lineNumber - 1;
String result = "";
for(int biz = 0; biz < pTriangle[lineNumber].length; biz++)
{
result += Integer.toString(pTriangle[lineNumber][biz]);
}
System.out.println(result+"/n");
return result;
}
}
{
public static void main (String [] args)
{
try{
pasTriangle T1 = new pasTriangle(1);
pasTriangle T2 = new pasTriangle(9);
pasTriangle T3 = new pasTriangle(3);
pasTriangle T4 = new pasTriangle(5); //Triangle with only one line created (so not a triangle); test for condition size == 1.
T1.populateT(0, 0);
T2.populateT(0, 0);
T3.populateT(0, 0);
T4.populateT(0, 0);
T1.getLine(1);
T2.getLine(4);
T2.getLine(9); //Test for last line.
T3.getLine(1); //Test for first line.
T3.getLine(2);
T4.getLine(1); //Test for first line.
}
catch(ArrayIndexOutOfBoundsException exception)
{
exception.printStackTrace();
}
}
}
答案 0 :(得分：0)
ArrayIndexOutOfoundsException似乎即将到来，因为此行允许递归继续使用无效的row索引。
else if (col >= pTriangle[row].length && row < pTriangle.length)
row变量可以是pTriangle.length - 1，然后您调用populateT(row+1, col)，将pTriangle.length传递给递归调用。然后，该递归调用最终会尝试访问无效的row索引，从而导致异常。将其更改为row < pTriangle.length - 1将解决即时异常问题。
正上方的条件row < pTriangle.length控制何时移动到下一列，但此处不需要它;你不是在这里修改row。
此外，您需要在匹配row时停止列递归，而不是在匹配row的物理结束时停止列递归，因此两个条件都需要更改。将if (col < pTriangle[row].length && row < pTriangle.length)更改为if (col < row)，将else if (col >= pTriangle[row].length && row < pTriangle.length)更改为else if (col >= row && row < pTriangle.length - 1)。
在这些条件之上，代码中需要进行类似的更改，以确定是编写1还是添加上述行中的相关数字。将else if (row != 0 && col == 0 || col == pTriangle[row].length-1)更改为else if (row != 0 && col == 0 || col == row)，将else if(row > 1 && col != 0 && col != pTriangle[row].length-1)更改为else if(row > 1 && col != 0 && col != row)。
当您添加上一行的元素以写入非1值时，您似乎添加了错误的元素。假设有效的数据结构看起来像这样......
[1][0][0][0]
[1][1][0][0]
[1][2][1][0]
[1][3][3][1]
您需要将上方和左侧的元素添加到上方的元素中。将(pTriangle[row-1][col-1]) + (pTriangle[row-1][col+1]);更改为(pTriangle[row-1][col-1]) + (pTriangle[row-1][col]);。 （col+1更改为col。）
如果您在getLine更改输出代码以添加空格，则可以更好地验证您的数字。另外，println已经在参数后面打印了一个新行，因此您无需附加换行符（\n而不是/n）。