问题描述
我正在尝试为作业编写程序.要求是递归创建Pascal的Triangle，然后打印给定的行.但是，在编译程序后，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)
I'm trying to write a program for an assignment. The requirements are to create Pascal's Triangle recursively and then print a given line. However, after compiling my program, I get several ArrayIndexOutOfBoundsExceptions. Here is the stack trace: 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)
有人知道我在做什么错吗?我已经尝试了所有方法，尤其是更换调味料，但是没有任何效果.这是我的代码:
Does anyone know what I'm doing wrong? I've tried everything, esp changing condiionals but nothing works. Here is my code:
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();
}
}
}
推荐答案
ArrayIndexOutOfoundsException似乎即将到来，因为此行允许递归以无效的row索引继续.
The ArrayIndexOutOfoundsException appears to be coming because this line allows the recursion to continue with an invalid row index.
else if (col >= pTriangle[row].length && row < pTriangle.length)
row变量可能是pTriangle.length - 1，然后您调用populateT(row+1, col)，并将pTriangle.length传递到递归调用中.然后，该递归调用最终尝试访问无效的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.
The row variable could be pTriangle.length - 1, and then you call populateT(row+1, col), passing pTriangle.length into the recursive call. Then that recursive call eventually attempts to access an invalid row index, causing the exception. Changing it to row < pTriangle.length - 1 will solve the immediate exception problem.
直接在其上方的条件row < pTriangle.length控制何时移至下一列，但此处不需要.您没有在这里修改row.
The condition directly above it, row < pTriangle.length, controls when to move to the next column, but you don't need it here; you're not modifying row here.
此外，您将希望在匹配row时停止列递归，而不是在与row的物理端匹配时停止，因此需要同时更改两个条件.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).
Also, you will want to stop the column recursion when you match row, not when you match the physical end of the row, so both conditions need to change. Change if (col < pTriangle[row].length && row < pTriangle.length) to if (col < row), and else if (col >= pTriangle[row].length && row < pTriangle.length) to else if (col >= row && row < pTriangle.length - 1).
在这些情况下，需要对代码进行类似的更改才能确定是写1还是添加上一行的相关数字.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).
Above those conditions, similar changes are needed in the code to determine whether to write a 1 or add the relevant numbers from the row above. Change else if (row != 0 && col == 0 || col == pTriangle[row].length-1) to else if (row != 0 && col == 0 || col == row) and else if(row > 1 && col != 0 && col != pTriangle[row].length-1) to else if(row > 1 && col != 0 && col != row).
当您添加前一行的元素以写入非1值时，似乎您添加的是错误的元素.假设一个有效的数据结构看起来像这样……
When you add the previous row's elements to write a non-1 value, it appears you're adding the wrong elements. Assuming that a valid data structure would look like this...
[1][0][0][0]
[1][1][0][0]
[1][2][1][0]
[1][3][3][1]
您需要在上方和左侧添加元素，在上方直接添加元素.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.)
You need to add the element above and to the left to the element directly above. Change (pTriangle[row-1][col-1]) + (pTriangle[row-1][col+1]); to (pTriangle[row-1][col-1]) + (pTriangle[row-1][col]);. (The col+1 changes to col.)
如果您在getLine中更改输出代码以添加空格，则可以更好地验证数字.另外，println已经在参数后打印了新行，因此您无需附加换行符(用\n代替/n).
If you change your output code in getLine to add a space, you'll be able to verify your numbers better. Also, println already prints a new line after your parameter, so you don't need to append a newline character (which is \n instead of /n).
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