<编程珠玑>笔记(三) 四条原则

  第三章作者重在阐述一种编程观念， 即 “data does indeed strcture programs”

  这一章貌似没什么干货，只好把作者的几个例子贴出来，反复看看了。

1  A survey program

	Total	US Citi	Perm Visa	Temp Visa	Male	Female
African American	1289	1239	17	2	684	593
Mexican American	675	577	80	11	448	219
Native American	198	182	5	3	132	64
Spanish American	411	223	152	20	224	179
Asian American	519	312	152	41	247	270
Caucasian	16272	15663	355	33	9367	6836
Other	225	123	78	19	129	92
Totals	19589	18319	839	129	11231	8253

1) the numbers should be stored as an array

ethnicgroup = entry[0]
campus = entry[1]
if entry[2] == refused
    declined[ethnicgroup, 2]++
else
    j = 1 + entry[2]
    count[campus, ethnicgroup, j]++
if entry[3] == refused
    declined[ethnicgroup, 3]++
else
     j = 4 + entry[3]
     count[campus, ethnicgroup, j]++

View Code

   replace above lines with array  offset containing 0, 0, 1, 4, 6, ...

for i = [2, 8]
    if entry[i] == refused
        declined[ethnicgroup, i]++
else
     j = offset[i] + entry[i]
     count[campus, ethnicgroup, j]++

View Code

2) should the array be laid out to its output structure or its input structure?

  

  The left one result in a little more work when the data was read and a little less work when it was written.

 

2  Form-letter programming

Welcome back, Jane!
We hope that you and all the members of the Frank family are constantly
reminding your neighbors there on Maple Street to shop with us.
As usual, we will ship your order to 
    Ms. Jane Q. Frank
    60 Maple Street
    Your Town,  Iowa 12345
   ...  ...

View Code

  behind the scenes a computer looked up your user name in a database record and retrieved fields like

Frank | Jane | Q. | Ms.| 60 | Maple Street | Your Town | Iowa | 12345  

  seperate form letter generator from form letter schema

Welcome back, $1!
We hope that you and all the members of the $0 family are constantly
reminding your neighbors there on $5 to shop with us.
As usual, we will ship your order to 
    $3  $1  $2.  $0
    $4  $5
    $6, $7  $8 
   ...  ... 

View Code

read fields from database
loop from start to end of schema
    c = next character in schema
    if  c != '$'
        printchar  c
    else 
        c = next character in schema
        case c of 
            '$' :         printchar '$'
            '0' - '9' :   printstring field[c]
            default :   error("bad schema")

View Code

 

3  Four principles

1) rework repeated code into arrays

    a long stretch of similar code is often best expressed by the simplest of data structures, the array

2) encapsulate complex structures

    define a sophisticated data structure in abstract terms, and express those operations as a class

3) use advanced tools when possible

    Hypertext, name-value pairs, spreadsheets, databases, languages are powerful tools

4) let the data structure the program

     before writing code, thoroughly understand the input, the output and the intermediate data structures