homework2

Questions:

 1)Identify the fault. If possible, identify a test case that does not execute the fault. (Reachability)

 2)If possible, identify a test case that executes the fault, but does not result in an error state. 

 3)If possible identify a test case that results in an error, but not a failure.

public int findLast (int[] x, int y) {
//Effects: If x==null throw
NullPointerException
// else return the index of the last element
// in x that equals y.
// If no such element exists, return -1
for (int i=x.length-1; i > 0; i--)
{
if (x[i] == y)
{
return i;
}
}
return -1;
}
// test: x=[2, 3, 5]; y = 2
// Expected = 0

public static int lastZero (int[] x) {
//Effects: if x==null throw
NullPointerException
// else return the index of the LAST 0 in x.
// Return -1 if 0 does not occur in x
for (int i = 0; i < x.length; i++)
{
if (x[i] == 0)
{
return i;
}
} return -1;
}
// test: x=[0, 1, 0]
// Expected = 2

1、（1）fault：由i>0知，i不可能为0，所以不可能得到期望值0，应该为 i >= 0

      (2) 不执行故障测试用例  ：x=[]; y=2  期望值=-1    

      (3)  执行故障不导致错误用例  ： x=[3,2,5]; y=2 期望值=1

      (4)导致错误但不是失败结果的用例： x=[1,3,5]; y=4 期望值=-1

2、 (1)fault: 从前到后遍历，数组第一个即为0，所以到第一个时已经返回了0，不可能得到期望2

      (2) 不执行故障测试用例 ：x=[]  期望值=-1

      (3) 执行故障不导致错误用例  ： x=[0] 期望值=0，实际值=0

      (4)导致错误但不是失败结果的用例： x=[1,3,0] 期望值=2，实际值=2