Presentación en el Segundo Encuentro de programadores C++ de Buenos Aires

1. ¿Fastware?
Fernando Pelliccioni
componentsprogramming.com/
@ferpelliccioni
fpelliccioni@gmail.com

3. Búsqueda Lineal

4. Range find(Range, E)(Range r, E e) {
for (; !r.empty; r.popFront) {
if (r.front == e) break;
}
return r;
}
Código en lenguaje de programación D.
¡Perdón!

5. template <typename I, typename T>
I find(I f, I l, T const& x) {
while (f != l && *f != x) ++f;
return f;
}
Versión C++98
(sin especificaciones)

6. Estructuras de Datos
Lineales

7. Estructuras de Datos
Lineales

8. Estructuras de Datos
Lineales

9. Estructuras de Datos
Lineales

10. Rangos

11. Rangos

12. Rangos

13. Rangos
half-open ranges
[first, last)

14. Rangos

15. Rangos

16. Rangos

17. Rangos
char const s[] = “Hello”;
print(begin(s), end(s));
print(s+1, s+4);
> Hello
> ell
>
s:

18. template <typename I, typename T>
I find(I f, I l, T const& x) {
while (f != l && *f != x) ++f;
return f;
}
Búsqueda lineal

19. H e l l o , _ W o r l d !
$
,element:
range
string s = “Hello, World!”;
auto c = find(begin(s), end(s), ','));
print(c, end(s));

20. template <typename I, typename T>
I find(I f, I l, T const& x) {
while (f != l && *f != x) ++f;
return f;
}

21. template <typename I, typename T>
I find(I f, I l, T const& x) {
while (f != l && *f != x) ++f;
return f;
}

22. template <typename I, typename T>
I find(I f, I l, T const& x) {
while (f != l && *f != x) ++f;
return f;
}

23. template <typename I, typename T>
I find(I f, I l, T const& x) {
while (f != l && *f != x) ++f;
return f;
}

24. template <typename I, typename T>
I find(I f, I l, T const& x) {
while (f != l && *f != x) ++f;
return f;
}

25. template <typename I, typename T>
I find(I f, I l, T const& x) {
while (f != l && *f != x) ++f;
return f;
}

27. string s = “Hello, World!”;
auto c = find(begin(s), end(s), '?'));
print(c, end(s));

28. template <typename I, typename T>
I find(I f, I l, T const& x) {
while (f != l && *f != x) ++f;
return f;
}

29. template <typename I, typename T>
I find(I f, I l, T const& x) {
while (f != l && *f != x) ++f;
return f;
}

30. template <typename I, typename T>
I find(I f, I l, T const& x) {
while (f != l && *f != x) ++f;
return f;
}

31. template <typename I, typename T>
I find(I f, I l, T const& x) {
while (f != l && *f != x) ++f;
return f;
}

32. template <typename I, typename T>
I find(I f, I l, T const& x) {
while (f != l && *f != x) ++f;
return f;
}

33. template <typename I, typename T>
I find(I f, I l, T const& x) {
while (f != l && *f != x) ++f;
return f;
}

34. template <typename I, typename T>
I find(I f, I l, T const& x) {
while (f != l && *f != x) ++f;
return f;
}

35. template <typename I, typename T>
I find(I f, I l, T const& x) {
while (f != l && *f != x) ++f;
return f;
}

36. template <typename I, typename T>
I find(I f, I l, T const& x) {
while (f != l && *f != x) ++f;
return f;
}

37. template <typename I, typename T>
I find(I f, I l, T const& x) {
while (f != l && *f != x) ++f;
return f;
}

38. template <typename I, typename T>
I find(I f, I l, T const& x) {
while (f != l && *f != x) ++f;
return f;
}

39. template <typename I, typename T>
I find(I f, I l, T const& x) {
while (f != l && *f != x) ++f;
return f;
}

40. template <typename I, typename T>
I find(I f, I l, T const& x) {
while (f != l && *f != x) ++f;
return f;
}

41. template <typename I, typename T>
I find(I f, I l, T const& x) {
while (f != l && *f != x) ++f;
return f;
}

42. ¿Cómo podemos
mejorar el algoritmo?

43. Búsqueda Lineal
con Centinela

44. Range find(Range, E)(Range r, E e) {
auto c = r[$ - 1];
r[$ - 1] = e;
size_t i = 0;
{
scope(exit) r[$ - 1] = c;
for (;; ++i)
if (r[i] == e) break;
}
if (i + 1 == r.length && c != e)
++i;
return r[i .. $];
}

45. Range find(Range, E)(Range r, E e) {
auto c = r[$ - 1];
r[$ - 1] = e;
size_t i = 0;
for (;; ++i)
if (r[i] == e) break;
r[$ - 1] = c;
if (i + 1 == r.length && c != e)
++i;
return r[i .. $];
}

46. H e l l o , _ W o r l d !
$
,element:
range
string s = “Hello, World!”;
auto r = find(s, ',');
print(r);

47. Range find(Range, E)(Range r, E e) {
auto c = r[$ - 1];
r[$ - 1] = e;
H e l l o , _ W o r l d !
$
,element:
range
$ - 1

48. Range find(Range, E)(Range r, E e) {
auto c = r[$ - 1];
r[$ - 1] = e;
H e l l o , _ W o r l d !
$
,element:
range
$ - 1
!c:

49. Range find(Range, E)(Range r, E e) {
auto c = r[$ - 1];
r[$ - 1] = e;
H e l l o , _ W o r l d ,
$
,element:
range
!c:

50. Range find(Range, E)(Range r, E e) {
...
size_t i = 0;
for (;; ++i)
if (r[i] == e) break;
H e l l o , _ W o r l d ,
$
,
=
range

51. Range find(Range, E)(Range r, E e) {
...
size_t i = 0;
for (;; ++i)
if (r[i] == e) break;
H e l l o , _ W o r l d ,
$
,
=
range

52. Range find(Range, E)(Range r, E e) {
...
size_t i = 0;
for (;; ++i)
if (r[i] == e) break;
H e l l o , _ W o r l d ,
$
,
=
range

53. Range find(Range, E)(Range r, E e) {
...
size_t i = 0;
for (;; ++i)
if (r[i] == e) break;
H e l l o , _ W o r l d ,
$
,
=
range

54. Range find(Range, E)(Range r, E e) {
...
size_t i = 0;
for (;; ++i)
if (r[i] == e) break;
H e l l o , _ W o r l d ,
$
,
=
range

55. Range find(Range, E)(Range r, E e) {
...
size_t i = 0;
for (;; ++i)
if (r[i] == e) break;
H e l l o , _ W o r l d ,
$
,
=
range

56. Range find(Range, E)(Range r, E e) {
...
r[$ - 1] = c;
if (i + 1 == r.length && c != e)
++i;
return r[i .. $];
}
H e l l o , _ W o r l d !
$
$ - 1
!c:
range

58. string s = “Hello, World!”;
auto r = find(s, '?');
print(r);

59. Range find(Range, E)(Range r, E e) {
auto c = r[$ - 1];
r[$ - 1] = e;
H e l l o , _ W o r l d !
$
?element:
range
$ - 1

60. Range find(Range, E)(Range r, E e) {
auto c = r[$ - 1];
r[$ - 1] = e;
H e l l o , _ W o r l d !
$
?element:
range
$ - 1
!c:

61. Range find(Range, E)(Range r, E e) {
auto c = r[$ - 1];
r[$ - 1] = e;
H e l l o , _ W o r l d ?
$
?element:
range
!c:

62. Range find(Range, E)(Range r, E e) {
...
size_t i = 0;
for (;; ++i)
if (r[i] == e) break;
H e l l o , _ W o r l d ?
$
?
=
range

63. Range find(Range, E)(Range r, E e) {
...
size_t i = 0;
for (;; ++i)
if (r[i] == e) break;
H e l l o , _ W o r l d
$
?
=
range

64. Range find(Range, E)(Range r, E e) {
...
size_t i = 0;
for (;; ++i)
if (r[i] == e) break;
H e l l o , _ W o r l d ?
$
?
=
range

65. Range find(Range, E)(Range r, E e) {
...
size_t i = 0;
for (;; ++i)
if (r[i] == e) break;
H e l l o , _ W o r l d ?
$
?
=
range

66. Range find(Range, E)(Range r, E e) {
...
size_t i = 0;
for (;; ++i)
if (r[i] == e) break;
H e l l o , _ W o r l d ?
$
?
=
range

67. Range find(Range, E)(Range r, E e) {
...
size_t i = 0;
for (;; ++i)
if (r[i] == e) break;
H e l l o , _ W o r l d ?
$
?
=
range

68. Range find(Range, E)(Range r, E e) {
...
size_t i = 0;
for (;; ++i)
if (r[i] == e) break;
H e l l o , _ W o r l d ?
$
?
=
range

69. Range find(Range, E)(Range r, E e) {
...
size_t i = 0;
for (;; ++i)
if (r[i] == e) break;
H e l l o , _ W o r l d ?
$
?
=
range

70. Range find(Range, E)(Range r, E e) {
...
size_t i = 0;
for (;; ++i)
if (r[i] == e) break;
H e l l o , _ W o r l d ?
$
?
=
range

71. Range find(Range, E)(Range r, E e) {
...
size_t i = 0;
for (;; ++i)
if (r[i] == e) break;
H e l l o , _ W o r l d ?
$
?
=
range

72. Range find(Range, E)(Range r, E e) {
...
size_t i = 0;
for (;; ++i)
if (r[i] == e) break;
H e l l o , _ W o r l d ?
$
?
=
range

73. Range find(Range, E)(Range r, E e) {
...
size_t i = 0;
for (;; ++i)
if (r[i] == e) break;
H e l l o , _ W o r l d ?
$
?
=
range

74. Range find(Range, E)(Range r, E e) {
...
size_t i = 0;
for (;; ++i)
if (r[i] == e) break;
H e l l o , _ W o r l d ?
$
?
=
range

75. Range find(Range, E)(Range r, E e) {
...
r[$ - 1] = c;
if (i + 1 == r.length && c != e)
++i;
return r[i .. $];
}
H e l l o , _ W o r l d !
$
$ - 1
!c:
range
?element:

76. Range find(Range, E)(Range r, E e) {
...
r[$ - 1] = c;
if (i + 1 == r.length && c != e)
++i;
return r[i .. $];
}
H e l l o , _ W o r l d !
$
?
=
range

77. Range find(Range, E)(Range r, E e) {
...
r[$ - 1] = c;
if (i + 1 == r.length && c != e)
++i;
return r[i .. $];
}
H e l l o , _ W o r l d !
$
range

79. ¿Qué imprime?
int[] a = [1, 2, 3, 4];
auto r = find(a, 3);
print(r);

80. int[] a = [1, 2, 3, 4];
auto r = find(a, 3);
print(r);
> [3, 4]
>

81. Todo muy lindo, ¿no?

82. ¿Qué imprime?
int[] a = [];
auto r = find(a, 3);
print(r);

83. int[] a = [];
auto r = find(a, 3);
print(r);
> core.exception.RangeError@alexandrescu.d(27):
Range violation
>

84. Range find(Range, E)(Range r, E e) {
auto c = r[$ - 1];
r[$ - 1] = e;
...
}

85. Range find(Range, E)(Range r, E e) {
auto c = r[$ - 1]; <<<<<<<<<
r[$ - 1] = e;
...
}

87. ¿Qué imprime?
Usando find() sin Centinela
string s = “Hello, World!”;
auto r = find(s, 'W');
print(r);

88. > World!
>
string s = “Hello, World!”;
auto r = find(s, 'W');
print(r);

89. ¿Qué imprime?
Usando find() con Centinela
string s = “Hello, World!”;
auto r = find(s, 'W');
print(r);

90. > World!
>
string s = “Hello, World!”;
auto r = find(s, 'W');
print(r);
¿?

91. > World!
> o, World!
> W, World!
> Xorld!
> X?%$j!
>

94. Para poder usar
el find() con
Centinela de forma
segura, tengo que
usar mecanismos de
sincronización de Threads
(Mutexes)

95. Entonces, ¿Es últil
el algoritmo de
búsqueda lineal
con Centinela?

96. Sí,es útil.
El problema con la
implementación de Andrei
es que está cambiando
la semántica
de un algoritmo conocido,
donde uno no espera que se
modifiquen datos.

97. Conclusión: Especificar y usar
nombres correctos.
En la próxima charla veremos
como implementarla correctamente
en C++ usando Concepts.

98. ¡Gracias!
Fernando Pelliccioni
componentsprogramming.com/
@ferpelliccioni
fpelliccioni@gmail.com

99. - Video:
https://www.youtube.com/watch?v=AxnotgLql0k
- Filminas:
http://www.slideshare.net/AndreiAlexandrescu2/accu-keynote-by-andrei-alexandrescu
- Donald Knuth llama a los centinelas "dummy records" y los usa en su
Algorithm Q (Quick sequential search)
The Art of Computer Programming, Vol. 3 "Sorting and Searching",
2nd Edition, Chapter 6.1 "Sequential Searching".
Referencias: