207 lines
6.7 KiB
Markdown
207 lines
6.7 KiB
Markdown
---
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title: File Handling
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localeTitle: Manejo de archivos
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---
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## Manejo de archivos
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### Introducción
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Si ha escrito el programa C `helloworld` anteriormente, ¡ya ha hecho el archivo IO en C! ¡Felicidades! tada
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```c
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/* A simple hello world in C. */
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#include <stdlib.h>
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// Import IO functions.
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#include <stdio.h>
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int main() {
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// This printf is where all the file IO magic happens!
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// How exciting!
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printf("Hello, world!\n");
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return EXIT_SUCCESS;
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}
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```
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El manejo de archivos es la parte más importante de un programador. En lenguaje C usamos un puntero de estructura de un tipo de archivo para declarar un archivo
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```c
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FILE *fp;
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```
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C proporciona una serie de funciones incorporadas para realizar operaciones básicas de archivos
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**fopen ()** **\-** **crea un nuevo archivo o abre un archivo existente**
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**fclose ()** **\-** **cierra un archivo**
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**getc ()** **\-** **lee un carácter de un archivo**
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**putc ()** **\-** **escribe un carácter en un archivo**
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**fscanf ()** **\-** **lee un conjunto de datos de un archivo**
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**fprintf ()** **\-** **escribe un conjunto de datos en un archivo**
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**getw ()** **\-** **lee un entero desde un archivo**
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**putw ()** **\-** **escribe un entero en un archivo**
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**fseek ()** **\-** **establece la posición para desear el punto**
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**ftell ()** **\-** **da la posición actual en el archivo**
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**rebobinar ()** **:** **establece la posición al punto de inicio**
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### Abriendo un archivo
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La función **fopen ()** se utiliza para crear un archivo o abrir un archivo existente
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`c fp = fopen(const char filename,const char mode);`
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En C hay muchos modos para abrir un archivo. **r** **\-** **abrir un archivo en modo de lectura**
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**w** **\-** **abre o crea un archivo de texto en modo de escritura**
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**a** **\-** **abre un archivo en modo agregado**
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**r +** **\-** **abre un archivo en modo de lectura y escritura**
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**a +** **\-** **abre un archivo en modo de lectura y escritura**
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**w +** **\-** **abre un archivo en modo de lectura y escritura**
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Aquí hay un ejemplo de leer y escribir datos en un archivo
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\`\` \`c #incluir
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# incluir
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principal() { ARCHIVO \* fp; char ch; fp = fopen ("hello.txt", "w"); printf ("Introducir datos"); while ((ch = getchar ())! = EOF) { putc (ch, fp); } fclose (fp); fp = fopen ("hello.txt", "r");
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while ((ch = getc (fp)! = EOF) printf ("% c", ch);
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fclose (fp); }
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```
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Now you might be thinking, "this justs prints text to my screen. How is this file IO?"
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The answer isn't obvious at first, and needs some understanding about the UNIX system.
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Under a UNIX system, everything is treated as a file, meaning you can read and write from it.
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This means that your printer can be abstracted as a file since all you do with a printer is write with it.
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It is also useful to think of these files as streams, since as you'll see later, you can redirect them with the shell.
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So how does this relate to `helloworld` and file IO?
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When you call `printf`, you are really just writing to a special file called `stdout`, short for __standard output__.
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`stdout` represents, well, the standard output as decided by your shell, which is usually the terminal.
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This explains why it printed to your screen.
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There are two other streams (ie files) that are available to you with effort, `stdin` and `stderr`.
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`stdin` represents the __standard input__, which your shell usually attaches to the keyboard.
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`stderr` represents the __standard error__ output, which your shell usually attaches to the terminal.
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### Rudimentary File IO, or How I Learnt to Lay Pipes
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Enough theory, let's get down to business by writing some code!
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The easist way to write to a file is to redirect the output stream using the output redirect tool, `>`.
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If you want to append, you can use `>>`. _N.b. these redirection operators are in_ `bash` _and similar shells._
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```
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intento
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# Esto saldrá a la pantalla ...
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./Hola Mundo
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# ... pero esto va a escribir en un archivo!
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./helloworld> hello.txt
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```
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The contents of `hello.txt` will, not surprisingly, be
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```
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¡Hola Mundo!
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```
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Say we have another program called `greet`, similar to `helloworld`, that greets you given your name.
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```
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do
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# incluir
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# incluir
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int main () { // Inicializar una matriz para mantener el nombre. nombre de personaje \[20\]; // Lee una cadena y guárdala en un nombre. scanf ("% s", nombre); // Imprimir el saludo. printf ("Hola,% s!", nombre); devuelve EXIT\_SUCCESS; }
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```
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Instead of reading from the keyboard, we can redirect `stdin` to read from a file using the `<` tool.
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```
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intento
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# Escribe un archivo que contenga un nombre.
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echo kamala> nombre.txt
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# Esto leerá el nombre del archivo e imprimirá el saludo en la pantalla.
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./greet <nombre.txt
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# \==> Hola, Kamala!
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# Si también desea escribir el saludo en un archivo, puede hacerlo usando ">".
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```
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### The Real Deal
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The above methods only worked for the most basic of cases. If you wanted to do bigger and better things, you will probably want to work with files from within C instead of through the shell.
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To accomplish this, you will use a function called `fopen`. This function takes two string parameters, the first being the file name and the second being the mode.
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Mode is basically permissions, so `r` for read, `w` for write, `a` for append. You can also combine them, so `rw` would mean you could read and write to the file. There are more modes, but these are the most used.
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After you have a `FILE` pointer, you can use basically the same IO commands you would've used, except that you have to prefix them with `f` and the first argument will be the file pointer.
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For example, `printf`'s file version is `fprintf`.
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Here's a program called `greetings` that reads a from a file containing a list of names and writes to another file the greetings.
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```
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do
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# incluir
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# incluir
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int main () { // Crear punteros de archivo. ARCHIVO \* nombres = fopen ("names.txt", "r"); ARCHIVO \* greet = fopen ("greet.txt", "w");
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```
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// Check that everything is OK.
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if (!names || !greet) {
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fprintf(stderr, "File opening failed!\n");
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return EXIT_FAILURE;
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}
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// Greetings time!
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char name[20];
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// Basically keep on reading untill there's nothing left.
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while (fscanf(names, "%s\n", name) > 0) {
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fprintf(greet, "Hello, %s!\n", name);
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}
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// When reached the end, print a message to the terminal to inform the user.
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if (feof(names)) {
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printf("Greetings are done!\n");
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}
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return EXIT_SUCCESS;
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```
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}
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```
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Suppose `names.txt` contains the following:
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```
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Kamala Logan Villancico
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```
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Then after running `greetings` the file `greet.txt` will contain:
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```
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Hola, Kamala! Hola logan Hola carol \`\` \`
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Super impresionante, ¿verdad? :sonreír:
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### Más información:
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* [Página de Wikilibros en archivo IO](https://en.wikibooks.org/wiki/C_Programming/File_IO) |