COBOL 文件訪問(wèn)模式

截至目前，已經(jīng)對(duì)文件組織方案進(jìn)行了討論。對(duì)于每一個(gè)文件組織方案，可以使用不同的訪問(wèn)模式。以下是文件訪問(wèn)模式的類型：

• 順序訪問(wèn)
• 隨機(jī)訪問(wèn)
• 動(dòng)態(tài)訪問(wèn)

本章節(jié)中提到的語(yǔ)法及其各自的術(shù)語(yǔ)，僅涉及它們?cè)诔绦蛑械挠梅āＪ褂眠@些語(yǔ)法的完整程序?qū)⒃谙乱徽掠懻摗?

順序訪問(wèn)

當(dāng)訪問(wèn)模式是按順序的，記錄檢索的方法根據(jù)所選的文件組織而變化。

• 對(duì)于順序文件 ，被插入的記錄仍按相同的順序進(jìn)行訪問(wèn)。

• 對(duì)于索引文件 ，用于獲取記錄的參數(shù)是記錄的鍵值。

• 對(duì)于相對(duì)文件 ，記錄相對(duì)鍵用于檢索記錄。

語(yǔ)法

以下是順序訪問(wèn)模式的語(yǔ)法：

ENVIRONMENT DIVISION.
INPUT-OUTPUT SECTION.
FILE-CONTROL.
   SELECT file-name ASSIGN TO dd-name
   ORGANIZATION IS SEQUENTIAL
   ACCESS MODE IS SEQUENTIAL
	
	
ENVIRONMENT DIVISION.
INPUT-OUTPUT SECTION.
   FILE-CONTROL.
   SELECT file-name ASSIGN TO dd-name
   ORGANIZATION IS INDEXED
   ACCESS MODE IS SEQUENTIAL
   RECORD KEY IS rec-key1
   ALTERNATE RECORD KEY IS rec-key2

		
ENVIRONMENT DIVISION.
INPUT-OUTPUT SECTION.
   FILE-CONTROL.
   SELECT file-name ASSIGN TO dd-name
   ORGANIZATION IS RELATIVE
   ACCESS MODE IS SEQUENTIAL
   RELATIVE KEY IS rec-key1

隨機(jī)訪問(wèn)

當(dāng)訪問(wèn)模式為隨機(jī)（RANMOM）時(shí)，記錄檢索方法根據(jù)所選文件組織而變化。

• 對(duì)于索引文件，記錄可以通過(guò)主鍵或備用鍵的鍵字段中的值來(lái)訪問(wèn)?？梢杂幸粋€(gè)或多個(gè)備用索引。

• 對(duì)于相對(duì)文件，記錄通過(guò)記錄相對(duì)鍵進(jìn)行檢索。

語(yǔ)法

以下是隨機(jī)訪問(wèn)模式的語(yǔ)法：

ENVIRONMENT DIVISION.
INPUT-OUTPUT SECTION.
   FILE-CONTROL.
   SELECT file-name ASSIGN TO dd-name
   ORGANIZATION IS INDEXED
   ACCESS MODE IS RANDOM
   RECORD KEY IS rec-key1
   ALTERNATE RECORD KEY IS rec-key2

		
ENVIRONMENT DIVISION.
INPUT-OUTPUT SECTION.
   FILE-CONTROL.
   SELECT file-name ASSIGN TO dd-name
   ORGANIZATION IS RELATIVE
   ACCESS MODE IS RANDOM
   RELATIVE KEY IS rec-key1

動(dòng)態(tài)訪問(wèn)

動(dòng)態(tài)訪問(wèn)支持在同一程序中的順序和隨機(jī)訪問(wèn)。通過(guò)動(dòng)態(tài)訪問(wèn)，一個(gè)文件定義可用于執(zhí)行順序和隨機(jī)進(jìn)程，例如按順序訪問(wèn)一些記錄和通過(guò)鍵訪問(wèn)其他記錄。

使用相對(duì)和索引文件，動(dòng)態(tài)訪問(wèn)模式允許你在順序訪問(wèn)模式和隨機(jī)訪問(wèn)模式之間來(lái)回切換，同時(shí)使用READ語(yǔ)句上的NEXT短語(yǔ)讀取文件。NEXT和READ的功能將在下一章中進(jìn)行討論。

語(yǔ)法

以下是動(dòng)態(tài)訪問(wèn)模式的語(yǔ)法：

ENVIRONMENT DIVISION.
INPUT-OUTPUT SECTION.
   FILE-CONTROL.
   SELECT file-name ASSIGN TO dd-name
   ORGANIZATION IS SEQUENTIAL
   ACCESS MODE IS DYNAMIC
   RECORD KEY IS rec-key1
   ALTERNATE RECORD KEY IS rec-key2

		
ENVIRONMENT DIVISION.
INPUT-OUTPUT SECTION.
   FILE-CONTROL.
   SELECT file-name ASSIGN TO dd-name
   ORGANIZATION IS RELATIVE
   ACCESS MODE IS DYNAMIC
   RELATIVE KEY IS rec-key1