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GNU Info (gawk.info)Basic High LevelWhat a Program Does =================== At the most basic level, the job of a program is to process some input data and produce results. _______ +------+ / \ +---------+ | Data | -----> < Program > -----> | Results | +------+ \_______/ +---------+ The "program" in the figure can be either a compiled program(1) (such as `ls'), or it may be "interpreted". In the latter case, a machine-executable program such as `awk' reads your program, and then uses the instructions in your program to process the data. When you write a program, it usually consists of the following, very basic set of steps: ______ +----------------+ / More \ No +----------+ | Initialization | -------> < Data > -------> | Clean Up | +----------------+ ^ \ ? / +----------+ | +--+-+ | | Yes | | | V | +---------+ +-----+ Process | +---------+ Initialization These are the things you do before actually starting to process data, such as checking arguments, initializing any data you need to work with, and so on. This step corresponds to `awk''s `BEGIN' rule (Note: The `BEGIN' and `END' Special Patterns.). If you were baking a cake, this might consist of laying out all the mixing bowls and the baking pan, and making sure you have all the ingredients that you need. Processing This is where the actual work is done. Your program reads data, one logical chunk at a time, and processes it as appropriate. In most programming languages, you have to manually manage the reading of data, checking to see if there is more each time you read a chunk. `awk''s pattern-action paradigm (Note: Getting Started with `awk'.) handles the mechanics of this for you. In baking a cake, the processing corresponds to the actual labor: breaking eggs, mixing the flour, water, and other ingredients, and then putting the cake into the oven. Clean Up Once you've processed all the data, you may have things you need to do before exiting. This step corresponds to `awk''s `END' rule (Note: The `BEGIN' and `END' Special Patterns.). After the cake comes out of the oven, you still have to wrap it in plastic wrap to keep anyone from tasting it, as well as wash the mixing bowls and other utensils. An "algorithm" is a detailed set of instructions necessary to accomplish a task, or process data. It is much the same as a recipe for baking a cake. Programs implement algorithms. Often, it is up to you to design the algorithm and implement it, simultaneously. The "logical chunks" we talked about previously are called "records", similar to the records a company keeps on employees, a school keeps for students, or a doctor keeps for patients. Each record has many component parts, such as first and last names, date of birth, address, and so on. The component parts are referred to as the "fields" of the record. The act of reading data is termed "input", and that of generating results, not too surprisingly, is termed "output". They are often referred to together as "Input/Output," and even more often, as "I/O" for short. (You will also see "input" and "output" used as verbs.) `awk' manages the reading of data for you, as well as the breaking it up into records and fields. Your program's job is to tell `awk' what to with the data. You do this by describing "patterns" in the data to look for, and "actions" to execute when those patterns are seen. This "data-driven" nature of `awk' programs usually makes them both easier to write and easier to read. ---------- Footnotes ---------- (1) Compiled programs are typically written in lower-level languages such as C, C++, Fortran, or Ada, and then translated, or "compiled", into a form that the computer can execute directly. automatically generated by info2www version 1.2.2.9 |