How Failures Come to be Andreas Zeller 1 An F-16 fighter plane An - - PDF document
How Failures Come to be Andreas Zeller 1 An F-16 fighter plane An F-16 on the northern (northern hemisphere) hemisphere. Why the northern hemisphere, you ask? 2 2 Because this is what An F-16 an F-16 on the (southern hemisphere)
2
(northern hemisphere)
3
(southern hemisphere)
4
5
September 9, 1947
6
7
8
9
10
(Sommerville 2004)
11
12
13
✘
defect – an error in the code.
creates an infection – an error in the state.
✘
Variables
This infection chain must be traced back – and broken. t
14
✘
a failure!
presence of errors – not their absence.
(Dijkstra 1972)
✘
Variables
15
✘
traced back to some infection, and every infection is caused by some defect.
relate a given failure to the defect – and to remove the defect.
✘
Variables
t variables
16
t variables
17
t variables
18
t variables
19
t variables
20
t variables
21
t variables
22
t variables
23
24
25
26
int main(int argc, char *argv[]) { int *a; int i; a = (int *)malloc((argc - 1) * sizeof(int)); for (i = 0; i < argc - 1; i++) a[i] = atoi(argv[i + 1]); shell_sort(a, argc); printf("Output: "); for (i = 0; i < argc - 1; i++) printf("%d ", a[i]); printf("\n"); free(a); return 0; }
27
value of a[0]. Where does it come from?
deduce value origins
irrelevant values
to shell_sort variables time !
28
static void shell_sort(int a[], int size) { int i, j; int h = 1; do { h = h * 3 + 1; } while (h <= size); do { h /= 3; for (i = h; i < size; i++) { int v = a[i]; for (j = i; j >= h && a[j - h] > v; j -= h) a[j] = a[j - h]; if (i != j) a[j] = v; } } while (h != 1); }
29
i = 0 a[i] = atoi(argv[i + 1]) i++ a[i] = atoi(argv[i + 1]) i++ shell_sort(a, argc) a = malloc(...) return 0 3 ? ? ? ? "11""14" 11 14 1 2 3 11 3 "11""14" ? 2 ?
argc argv [0] argv [1] a [0] a [1] i size h a [2] 30
static void shell_sort(int a[], int size) { int i, j; int h = 1; ... } sample 11 14 $ a[0] = 11 a[1] = 14 a[2] = 0 size = 3 Output: 0 11 fprintf(stderr, “At shell_sort”); for (i = 0; i < size; i++) fprintf(stderr, “a[%d] = %d\n”, i, a[i]); fprintf(stderr, “size = %d\n”, size);
31
shell_sort(a, argc); shell_sort(a, argc - 1); shell_sort(a, argc); int main(int argc, char *argv[]) { int *a; int i; a = (int *)malloc((argc - 1) * sizeof(int)); for (i = 0; i < argc - 1; i++) a[i] = atoi(argv[i + 1]); ... }
sample 11 14 $ Output: 11 14
32
33
34
t
int main(int argc, char *argv[]) { int *a; // Input array a = (int *)malloc((argc - 1) * sizeof(int)); for (int i = 0; i < argc - 1; i++) a[i] = atoi(argv[i + 1]); // Sort array shell_sort(a, argc); // Output array printf("Output: "); for (int i = 0; i < argc - 1; i++) printf("%d ", a[i]); printf("\n"); free(a); return 0; }
35
36
37
38
39
40 This work is licensed under the Creative Commons Attribution License. To view a copy of this license, visit http://creativecommons.org/licenses/by/1.0