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Java butt:

Compilrs

Chapter 1

Introduction

1.1Overview

One of the main communication mechanisms between a computer and a person is given by sending and receiving textual messages: the user write an order using the keyboard, and the computer executes it, returning as result an informative message about the actions carried out.

Although the evolution of computers is currently directed

towards the use of innovative and ergonomic user interfaces (such as the mouse, the touchscreens, graphic tablets, etc.), we can say that almost all actions that the user performs on these interfaces are translated sooner or later to sequences of commands that are executed as if they had been entered by keyboard. By

On the other hand, and from the point of view of the IT professional, the work that this performed on the computer is plagued with situations in which there is direct textual communication with the machine: use of an interpreter of commands ( shell ), batch job file construction, programming by various languages, etc. Even word processors like WordPerfect and MS Word stores the documents written by the user using an encoding

structured textual that, every time the document is opened, is recognized, traversed and presented on screen.

For all this, no computer scientist worth his salt can avoid the undoubted

need to know the ins and outs of the tool you use during your work daily and on which the human-machine interaction rests: the translator .

There are a lot of situations where it can be very helpful to know

how the different parts of a compiler work, especially the parts that are in charge of chopping up the source texts and converting them into syntactically valid phrases. For example, a situation of apparent complexity may arise if we have an MS Word document that comes from a merge with a database and is wants, from it, to obtain the original DB. How to solve the problem? Well based on the fact that the document structure is made up of blocks that are repeated; the solution could be:

Convert the document to pure text format.

Process this text with a translator to eliminate superfluous characters

and result in another text in which each field in the database table is in quotes.

The above text is imported with any DBMS.

Other applications of the translator construction can be the creation of

preprocessors for languages that do not have it (for example, to easily work with SQL in C, you can make a preprocessor to introduce embedded SQL), or even converting ASCII 10 (LF) character to HTML "
" to pass text to the web.

In this chapter, we introduce the construction of a compiler and describe

its components, the environment in which they work and some software that facilitate its construction.

1.2Translator concept.

A translator is defined as a program that translates or converts from

a text or program written in a source language to a text or program equivalent written in a target language producing, if possible, error messages . Translators encompass both compilers (in which the target language usually be machine code) as well as interpreters (in which the target language is made up of the atomic actions that the interpreter can execute). Fig . 1 .one shows the basic schema that makes up a compiler / interpreter.

Figure 1.1. Preliminary sketch of a translator

It is important to highlight the speed with which it can be built today

a compiler. In the 1950s, translators were viewed as programs remarkably difficult to write. The first Fortran compiler ( Formula Translator ), for example, required the equivalent of 18 years of individual work (it really didn't take that long since the work was developed in team). Until the theory of automata and formal languages was applied to the

creation of translators, their development has been plagued with problems and errors. Without However, today a basic compiler can be the final project of any university student of Computer Science.

1.2.1 Types of translators

Since the origins of computing, there has been a gulf between the form

how people express their needs and the way a computer is capable to interpret instructions. The translators have tried to bridge this chasm to make work easier for humans, which has led to the application of automata theory to different fields and specific areas of computing, giving rise to the different types of translators that we will see next.

1.2.1.1 Language translators

They translate from one given language to another, such as from English to Spanish.

This type of translator has a multitude of problems, namely:

Need for artificial intelligence and problem with set phrases. He

The problem with artificial intelligence is that it has a lot of artificial and little intelligence, so today it is almost impossible to translate phrases with a deep meaning. As an anecdote, during the cold war, in a attempt to perform automatic translations from Russian to English and vice versa, A prototype was tested by introducing the text in English: The spirit is strong but the flesh is weak whose translation into Russian was switched back to English to see if it matched the original. What was the surprise of the developers when what was obtained was: "The wine is good but the meat It is rotten (in English spirit means both spirit and alcohol). Others examples that are difficult to translate are made up of phrases such as: Piel de chicken "," in case the flies "," molar mallet ", etc.

Difficult to formalize in the specification of the meaning of words.

Change in the meaning of words according to the context. Ex: "to say that, he took a cookie .

In general, the most satisfactory results in language translationnatural have occurred on restricted subsets of language. And even more, on subsets in which there is very little room for ambiguity in the interpretation of texts: legal speeches, technical documentation, etc.

1.2.1.2 Compilers

It is the translator whose input is a sentence in formal language

and as output it has an executable file, that is, it performs a translation of a code high-level machine code (compiler is also understood to be that program which provides an object file instead of the final executable).

1.2.1.3 Interpreters

It's like a compiler, only the output is an execution. The program of

input is recognized and executed at the same time. No physical result (code machine) but logical (one execution). There are languages that can only be interpreted, such as SNOBOL ( StriNg Oriented SimBOlyc Language ), LISP ( LISt Processing ), some versions of BASIC ( Beginner's All-purpose Symbolic Instruction Code ), etc.

Their main advantage is that they allow easy debugging. Between the

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