如何在 ABAP 里用函数式编程思想打印出非波拉契 Fibonacci（数列）

2023-04-14
四川
本文字数：1236 字
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在 JavaScript 里可以用 ES6 提供的 FunctionGenerator 这种黑科技来打印非波拉契数列，具体细节参考我这篇文章。

在 ABAP 里也有很多种方式实现这个需求。

下面这个 report 分别用递归和 ABAP internal table 的方式实现了非波拉契数列的打印。

REPORT Z_FIBO.
PARAMETERS: N type i,
v1 RADIOBUTTON GROUP v default 'X',
v2 RADIOBUTTON GROUP v.
data: f type i,
t type i.
data: product_guid type comm_product-product_guid.
get run time field t.
case 'X'.
when v1\. perform fibonacci using n changing f.
when v2\. perform fibonacci_2 using n changing f.
endcase.
write: / 'Fibonacci(', n, ') =', f.
get run time field t.
write: / 'Runtime', t, 'microseconds'.
*&---------------------------------------------------------------------*
*& Form fibonacci
*&---------------------------------------------------------------------*
form fibonacci using in type i
changing fib type i.
data: f_1 type i, f_2 type i,
n_1 type i, n_2 type i.
case in.
when 0\. fib = 1.
when 1\. fib = 1.
when others.
n_1 = in - 1.
n_2 = in - 2.
perform fibonacci using n_1 changing f_1.
perform fibonacci using n_2 changing f_2.
fib = f_1 + f_2.
endcase.
endform. "fibonacci
*&---------------------------------------------------------------------*
*& Form fibonacci_2
*&---------------------------------------------------------------------*
form fibonacci_2 using in type i
changing fib type i.
data: f_1 type i, f_2 type i,
n_1 type i, n_2 type i,
l type i.
data: fibo type table of i.
append 1 to fibo. " fibonacci(0)
append 1 to fibo. " fibonacci(1)
n_1 = 1.
n_2 = 2.
l = in - 1.
do l times.
read table fibo index n_1 into f_1.
read table fibo index n_2 into f_2.
fib = f_1 + f_2.
add 1 to n_1\. add 1 to n_2.
append fib to fibo.
enddo.
endform. "fibonacci_2

复制代码

以上两种解决方案相对来说都比较传统，再来看看使用 ABAP 7.40 提供的新关键字 COND 实现的非波拉契数列打印：

REPORT z.
CLASS lcl_fibonacci DEFINITION.
PUBLIC SECTION.
TYPES: zint_tab TYPE TABLE OF int4 WITH EMPTY KEY.
METHODS fibonacci
IMPORTING !n TYPE i RETURNING VALUE(fib_numbers) TYPE zint_tab.
ENDCLASS.
CLASS lcl_fibonacci IMPLEMENTATION.
METHOD fibonacci.
fib_numbers = COND #( WHEN n = 0
THEN VALUE #( ( |0| ) )
WHEN n = 1
THEN VALUE #( ( |0| ) ( |1| ) )
ELSE
VALUE #( LET fn1 = fibonacci( n - 1 )
x = fn1[ lines( fn1 ) ]
y = fn1[ lines( fn1 ) - 1 ]
IN ( LINES OF fn1 ) ( x + y ) ) ).
ENDMETHOD.
ENDCLASS.
START-OF-SELECTION.
cl_demo_output=>display( NEW lcl_fibonacci( )->fibonacci( 10 ) ).