零基础学习COBOL：21天教程

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"《21天学会COBOL（第2版）》是一本适合初学者的COBOL编程入门教程，旨在帮助读者在三周的时间里逐步掌握COBOL语言的基础知识和应用技巧。" COBOL（Common Business Oriented Language，通用商业语言）是一种历史悠久的高级编程语言，尤其在商业系统和数据处理领域广泛应用。这本书分为三个学习周，每个周都有若干个主题日，详细介绍了COBOL的不同方面。第一周主要介绍COBOL的基础知识，包括： 1. 第1天：你的第一个COBOL程序 - 学习如何编写简单的COBOL程序，理解程序结构和基本语法。 2. 第2天：使用变量和常量 - 了解如何声明和使用变量，以及定义常量。 3. 第3天：初步了解结构化COBOL - 探讨如何组织和控制程序流程，引入结构化编程概念。 4. 第4天：决策制作 - 学习如何使用条件语句（如IF-THEN-ELSE）来实现程序中的逻辑判断。 5. 第5天：使用PERFORM，GOTO和IF控制程序 - 进一步探讨流程控制结构，如循环和无条件跳转。 6. 第6天：使用数据和COBOL运算符 - 掌握各种算术、比较和逻辑运算符。 7. 第7天：设计基础 - 学习如何规划和设计COBOL程序，理解良好的编程实践。第二周深入到数据处理和文件操作： 1. 第8天：结构化数据 - 了解数组和复合数据类型的概念。 2. 第9天：文件输入/输出（I/O） - 学习如何读写文本文件和二进制文件。 3. 第10天：打印 - 如何生成报告和输出格式化的数据。 4. 第11天：索引文件I/O - 引入索引文件的概念，提高数据访问效率。 5. 第12天：关于索引文件的更多内容 - 深入研究索引文件的高级特性。 6. 第13天：删除记录和其他索引文件操作 - 学习如何修改和删除索引文件中的记录。 7. 第14天：索引文件复习 - 回顾和巩固所学的索引文件知识。 8. 第15天：数据完整性 - 讨论如何确保数据的一致性和正确性。第三周则关注更复杂的COBOL技术： 1. 第16天：查找和数组 - 学习如何使用查找功能和数组处理大量数据。 2. 第17天：备用键 - 理解备用键的概念及其在数据检索中的作用。 3. 第18天：调用其他程序 - 掌握如何在COBOL程序中嵌套或调用其他子程序。 4. 第19天：复杂的数据录入问题 - 解决实际项目中可能出现的复杂数据处理挑战。 5. 第20天：更复杂的数据录入 - 继续深化对复杂数据处理的理解。 6. 第21天：选择、排序和报告 - 学习如何创建自定义的排序和报告功能。此外，书中还提供了额外的奖励章节，涵盖： 1. 控制断点 - 学习如何使用调试器进行程序调试。 2. COBOL的其他语法 - 探讨COBOL语言的其他特性和语法细节。 3. 全屏I/O - 了解如何处理全屏幕的用户交互。 4. 使用符号调试器 - 实践使用调试工具来诊断和修复程序错误。 5. 内置函数和Y2K问题 - 讨论COBOL中的内置函数，并讨论与千禧年问题相关的兼容性问题。这本书通过系统的学习计划，引导读者逐步建立COBOL编程技能，无论对于希望进入IT行业的新人还是需要更新COBOL知识的专业人士，都是一个非常宝贵的学习资源。

Figure 1.1.

When the user starts a word processing program, the CPU loads the program from a hard disk into memory.

In Figure 1.1, the user has typed a command or clicked a button to start a word processing program. The CPU

locates the program on the disk, loads it into main memory, and then begins executing the instructions in the

word processing program.

In Figure 1.2, the word processing program is running and the user types a command or clicks a button to load a

document for editing. The CPU locates the document on the disk and loads it, ready to be edited, into main

memory.

Figure 1.2.

When the user asks for a document to be edited, the CPU loads the document from a hard disk into memory.

In Figure 1.3, the word processing program is running and the user types a letter "A" to be added to the

document. The CPU collects the letter typed at the keyboard and places it in the document in memory.

Figure 1.3.

When the user types the letter "A," the CPU collects the character from the keyboard and inserts it into main

memory.

In Figure 1.4, the user has requested that the document be saved. The CPU collects all the memory containing

the document and writes it to the hard disk.

Figure 1.4.

When the user asks for the document to be saved, it is pulled from memory and written back to the hard disk.

The CPU, input, output, main memory, and secondary storage all work together to form a computer.

What Is a Program?

A computer is an incredibly stupid device. It doesn't do anything unless and until it is told to do so.

When a computer is first switched on, the CPU starts looking through main memory for an instruction. You can

think of it as being in a perpetual state of readiness.

The computer is designed so that a small portion of main memory is permanent; it retains its contents even

when the power is switched off. This permanent memory is placed at the location where the CPU begins

searching for its first instruction after powering up. Consequently, it finds this permanent instruction

immediately. This permanent area of memory contains a sequence of instructions that the computer executes on

power-up. The instructions look something like this:

Test the monitor.●

Test the keyboard.●

Test all of the main memory and display the results on the monitor.●

Test any other devices that need to be tested, including disk drives.●

Load the operating system program from secondary storage (the disk) into main memory. The operating●

system is a master program that controls a computer's basic functions and allows other programs to

access the computer's resources, such as the disk drives, printer, keyboard, and screen. (In practice, this

step is a little more complicated, but the principle is correct.) For an MS-DOS-compatible computer, the

operating system is MS-DOS (Microsoft Disk Operating System).

Jump to the first, previously loaded instruction at the beginning of the MS-DOS operating system.●

From this point on, the CPU is executing instructions within the MS-DOS operating system.

If you could read the first few instructions of the MS-DOS operating system in English, they might look

something like what you see in Table 1.1.

Table 1.1. The first few instructions of the MS-DOS operating system as they might appear in

English.

Instruction Number Instruction Comment

001

Display the prompt Put the > prompt on the screen.

002

Wait for a keypress

003

Was a key pressed?

004

If not, GO TO 002

If no key was pressed, go back and try again.

005

Get the key value

006

Save the key value

Store the value in main memory.

007

Display the key value

008

Was the value (ENTER)?

Determine whether the Enter key was pressed,

signaling the end of command input.

009

If not, GO TO 002

The user is still typing, so keep getting keypresses.

010

GO TO do the command

The user pressed Enter, so jump to the instruction that

will try to execute the command. This part of the

program is not shown.

The set of instructions is written in English to represent the steps of a program. The program is executed so

quickly that you see no visible delay between typing the key and seeing it appear on-screen, even though the

action of saving the key value occurs between the keystroke and the display.

What Is a Programming Language?

The CPU expects instructions to arrive as numeric codes. These numeric codes are not easily read by human

beings. A programming language is a set of English-like instructions that includes a set of rules (syntax) for

putting the instructions together to create commands.

A translator changes the English-like commands into numeric codes that the computer can understand. The

most common type of translator is a compiler. The compiler is a program that reads the English-like commands

in a file and then creates another file containing computer-readable numeric codes or commands.

In the previous example, the CPU cannot understand the English-like instruction WAS A KEY PRESSED?,

but a programming language might accept this as a valid command and translate it into codes that the CPU can

recognize.

The term program is used loosely to refer to the actual application that is executed by the CPU, as well as the

file of English-like commands originally written by the programmer before it was translated into the program

that the CPU executes.

In strict terms, the English-like commands in a file are called source code, and the translated numeric codes

placed in the output file are a runable program called executable code. The computer cannot directly execute

source code as if it were a program. However, even experienced programmers will say, "I wrote a program to

calculate the month-end balance." What they really mean is, "I wrote a source code file containing English-like

commands that, when compiled, will produce an executable program file that, when run, will calculate the

month-end balance." It is definitely easier to say, "I wrote a program."

What Is COBOL?

COBOL is a programming language especially aimed at solving business problems. You will see as you work

through this book that COBOL solves a lot more than just business problems and can be used as a solution to

many data processing problems.

New Term: COBOL is an acronym for Common Business Oriented Language.

NOTE: COBOL was developed by the Conference on Data Systems Languages (CODASYL),

convened in 1959 by the Department of Defense. COBOL compilers became available in 1960, but

they were not standardized. The American National Standards Institute (ANSI) standardized a

version of COBOL in 1968. The language was revised and updated by ANSI in 1974 and again in

1985. These standards sometimes are called COBOL or COBOL-68, COBOL-74, or COBOL-85.

Most compilers are now COBOL-85 standard, but there still are a few COBOL-74 versions out

there. This book is written against the COBOL-85 standard, but you will have no trouble using a

COBOL-74 compiler for any of the examples.

Because the year 2000 problem might be one of the reasons that you are studying this book, it is

worth noting that legacy code could be written in any of the earlier COBOL standards. If you are

taking this course to bring yourself up to speed for an update effort, you will need to learn the

quirks and differences of the particular COBOL version that you will be working on. This book

covers the core of COBOL-85 and will give you about 95% of any version of COBOL that you

might work with. Throughout the book there are tips on differences that you might find in other

versions of COBOL.

The future of COBOL is fairly bright. A new COBOL standard is being drafted even as I write this.

This standard is intended to take COBOL into the future and certainly well beyond the year 2000.

The amount of money that is being invested in correcting date problems in existing COBOL code

is a sure indicator that no one is planning to dispose of COBOL in the near future.

Approximately 90% of all COBOL code runs in a character-based environment, which means that

most COBOL screens are 80 columns wide by 24 or 25 characters high and do not contain

graphics. Although there are versions of COBOL on the market that act in a Windows-like

environment, this book is not intended to be a course in COBOL for Windows. This means that

throughout the book, you will be running or executing your programs on an MS-DOS computer or

in an MS-DOS window that has been opened on a Windows computer. The MS-DOS window is an

80 x 24 character window and represents the kind of display that you might see when coding

COBOL on a mainframe or minicomputer.

The "Hello World" Program

The "Hello world" program has become almost trite. Writing a program that prints "Hello world"

on-screen usually is the first program you learn in any language. Listing 1.1 is a basic COBOL program that

will display "Hello world". The format of a COBOL program is covered in the following sections.

TYPE: Listing 1.1. "Hello world" in COBOL.

000100 IDENTIFICATION DIVISION.

000200 PROGRAM-ID. HELLO.

000300 ENVIRONMENT DIVISION.

000400 DATA DIVISION.

000500 PROCEDURE DIVISION.

000600

000700 PROGRAM-BEGIN.

000800 DISPLAY "Hello world".

000900

001000 PROGRAM-DONE.

001100 STOP RUN.

ANALYSIS: A COBOL program always contains four divisions. These four divisions always have the same

names:

IDENTIFICATION DIVISION●

ENVIRONMENT DIVISION●

DATA DIVISION●

PROCEDURE DIVISION●

In Listing 1.1, lines 000100 and 000200 are the IDENTIFICATION DIVISION. This division is used to

identify basic information about the program. In this example, the IDENTIFICATION DIVISION contains

only the PROGRAM-ID, HELLO.

Line 000300 is the ENVIRONMENT DIVISION, which is used to identify the environment in which the

program is running. Remember that COBOL is intended to run on many different types of machines, and this

section is used to handle the differences between various computers. In this case, the program has no

specialized machine requirements, so the ENVIRONMENT DIVISION is empty.

Line 000400 is the DATA DIVISION, which will contain any data that the program operates on. This

program has no data, so the DATA DIVISION is empty.

Lines 000500 through 001100 are the PROCEDURE DIVISION. This is the meat of the program--the part

that does the work intended by the programmer. The PROCEDURE DIVISION contains two paragraphs at line

000700 (PROGRAM-BEGIN) and line 001000 (PROGRAM-DONE). The term paragraph has a special

definition in COBOL that will be covered a bit later in today's lesson. All the actual work in this program is

done by line 000800.

If you have not yet installed your software, review Appendix C, "Editing, Compiling, and Linking," and the

installation instructions in your software documentation; then complete the installation procedure. Make sure

that you end up with a C> prompt under MS-DOS, and create your working directories as described in

Appendix C. Change to your working directory before you begin editing hello.cbl.

It is extremely important that you type (edit), compile, and, if necessary, link this program to produce a running

program. If you can't get this program to run, the remainder of this book will be an exercise in theory with no

practical application. If you are using some other editor and COBOL combination, consult your local

documentation or someone who is experienced with the system. Don't pass over this exercise without getting it

to run. It is the simplest program to edit, compile, link, and run.

Before you start typing, you should note a couple of things. The first six character positions of each line are

used for line numbering. The seventh character position is always blank. The commands at lines 000800 and

001100 begin at position 12. Everything else starts at position 8.

The editor that comes with Micro Focus Personal COBOL actually skips positions 1 through 7 and leaves the

cursor positioned at column 8. If you are using this editor, use the left arrow key to move to column 1 to type

the line numbers and the lines. Columns 1 through 7 are traditionally used for line numbering in COBOL. Most

larger systems will use line numbers. The practice of skipping or not requiring line numbers is common with PC

COBOL compilers and very modern compilers, but at the moment it is the exception rather than the rule. This

book uses line numbers in keeping with the practice of most larger systems, and to provide a convenient way to

refer to lines of code when a program is being analyzed in the text.

It's preferable to use spaces instead of the Tab key while editing, and you should break the habit of using the

Tab key. Many personal computer-based COBOL compilers can handle tab characters, but the language was not

designed originally to allow tabs in the source code file, and their presence can cause trouble on larger

machines.

If you are using Micro Focus Personal COBOL, start your editor by typing the following line and pressing

Enter:

pcobol hello.cbl

The extension .cbl is the default extension for many MS-DOS based COBOL compilers. On the VAX

minicomputer manufactured by Digital Equipment Corporation, the default extension is .COB, and you would

use a command such as:

EDIT HELLO.COB

DO/DON'T:

DO type each line exactly as it appears in Listing 1.1. Review your work carefully.

DON'T make typing errors. Some of the typing errors that can cause serious problems when you

are compiling include misspelling the name of a DIVISION (for example, INDENTIFICATION

DIVISION), adding an unnecessary hyphen (for example, DATA-DIVISION), or omitting any of

the periods. Note that everything ends with a period (.); in fact, line 000200 has two periods in it.

New Term: Checking source code created with an editor for typographical and other errors before compiling it

is referred to as desk-checking.

New Term: COBOL compilers are prone to produce cascading errors. A cascading error is one or more errors

(sometimes hundreds) generated by the compiler when the problem really is one simple error earlier in the

program. Code containing a missing period can produce a stream of apparent errors on some compilers that can

be traced back in the program to that single error. The missing period itself might not even be mentioned as an

error by the compiler, but it causes later problems that do show up.

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