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# Installing Bison

## Installation of Bison

Install Bison by running the following commands:

> `./configure --prefix=/usr \`
> `--datadir=/usr/share/bison &&`
> `make &&`
> `make install`

Some programs don't know about bison and try to find the yacc program (bison is a (better) alternative for yacc). So to please those few programs out there we'll create a yacc script that calls bison and have it emulate yacc's output file name conventions).

Create a new file `/usr/bin/yacc` by running the following:

`cat > /usr/bin/yacc << "EOF"`
#!/bin/sh
# Begin /usr/bin/yacc
/usr/bin/bison -y "$@"
# End /usr/bin/yacc
`EOF`
`chmod 755 /usr/bin/yacc`

## Command explanations

`--datadir=/usr/share/bison:` This install the bison grammar files in /usr/share/bison rather than /usr/share.

## Contents

The Bison package contains the bison program.

## Description

Bison is a parser generator, a replacement for YACC. YACC stands for Yet Another Compiler Compiler. What is Bison then? It is a program that generates a program that analyses the structure of a textfile. Instead of writing the actual program you specify how things should be connected and with those rules a program is constructed that analyses the textfile.

There are alot of examples where structure is needed and one of them is the calculator.

Given the string :

> 1 + 2 * 3

You can easily come to the result 7. Why ? Because of the structure. You know how to interpretet the string. The computer doesn't know that and Bison is a tool to help it understand by presenting the string in the following way to the compiler:

> +
> / \
> * 1
> / \
> 2 3

You start at the bottom of a tree and you come across the numbers 2 and 3 which are joined by the multiplication symbol, so the computers multiplies 2 and 3. The result of that multiplication is remembered and the next thing that the computer sees is the result of 2*3 and the number 1 which are joined by the add symbol. Adding 1 to the previous result makes 7. In calculating the most complex calculations can be broken down in this tree format and the computer just starts at the bottom and works it's way up to the top and comes with the correct answer. Of course, Bison isn't only used for calculators alone.

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