Recipe 3.8 Expanding and Compressing Tabs
Recipe 3.8 Expanding and Compressing TabsProblem
You need to convert space characters to tab
SolutionUse my Tabs class or its subclass EnTab . Discussion
Example 3-6 is a listing of
EnTab
, complete with a sample main program. The program works a line at a time. For each character on the line, if the character is a space, we see if we can coalesce it with previous spaces to output a single tab character. This program depends on the
Tabs
class, which we'll come to shortly. The
Tabs
class is used to decide which column
Example 3-6. Entab.java
/**
* EnTab: replace blanks by tabs and blanks. Transmuted from K&R Software Tools
* book into C. Transmuted again, years later, into Java. Totally rewritten to
* be line-at-a-time instead of char-at-a-time.
*
* @author Ian F. Darwin, http://www.darwinsys.com/
* @version $Id: ch03.xml,v 1.3 2004/05/04 18:03:14 ian Exp $
*/
public class EnTab {
/** The Tabs (tab logic handler) */
protected Tabs tabs;
/**
* Delegate tab spacing information to tabs.
*
* @return
*/
public int getTabSpacing() {
return tabs.getTabSpacing();
}
/**
* Main program: just create an EnTab object, and pass the standard input
* or the named file(s) through it.
*/
public static void main(String[] argv) throws IOException {
EnTab et = new EnTab(8);
if (argv.length == 0) // do standard input
et.entab(new BufferedReader(new InputStreamReader(System.in)),
System.out);
else
for (int i = 0; i < argv.length; i++) { // do each file
et.entab(new BufferedReader(new FileReader(argv[i])),
System.out);
}
}
/**
* Constructor: just save the tab values.
*
* @param n
* The number of spaces each tab is to replace.
*/
public EnTab(int n) {
tabs = new Tabs(n);
}
public EnTab() {
tabs = new Tabs();
}
/**
* entab: process one file, replacing blanks with tabs.
*
* @param is A BufferedReader opened to the file to be read.
* @param out a PrintWriter to send the output to.
*/
public void entab(BufferedReader is, PrintWriter out) throws IOException {
String line;
int c, col = 0, newcol;
// main loop: process entire file one line at a time.
while ((line = is.readLine()) != null) {
out.println(entabLine(line));
}
}
/**
* entab: process one file, replacing blanks with tabs.
*
* @param is A BufferedReader opened to the file to be read.
* @param out A PrintStream to write the output to.
*/
public void entab(BufferedReader is, PrintStream out) throws IOException {
entab(is, new PrintWriter(out));
}
/**
* entabLine: process one line, replacing blanks with tabs.
*
* @param line -
* the string to be processed
*/
public String entabLine(String line) {
int N = line.length(), outCol = 0;
StringBuffer sb = new StringBuffer();
char ch;
int consumedSpaces = 0;
for (int inCol = 0; inCol < N; inCol++) {
ch = line.charAt(inCol);
// If we get a space, consume it, don't output it.
// If this takes us to a tab stop, output a tab character.
if (ch == ' ') {
Debug.println("space", "Got space at " + inCol);
if (!tabs.isTabStop(inCol)) {
consumedSpaces++;
} else {
Debug.println("tab", "Got a Tab Stop "+ inCol);
sb.append('\t');
outCol += consumedSpaces;
consumedSpaces = 0;
}
continue;
}
// We're at a non-space; if we're just past a tab stop, we need
// to put the "leftover" spaces back out, since we consumed
// them above.
while (inCol-1 > outCol) {
Debug.println("pad", "Padding space at "+ inCol);
sb.append(' ');
outCol++;
}
// Now we have a plain character to output.
sb.append(ch);
outCol++;
}
// If line ended with trailing (or only!) spaces, preserve them.
for (int i = 0; i < consumedSpaces; i++) {
Debug.println("trail", "Padding space at end # " + i);
sb.append(' ');
}
return sb.toString();
}
}
As the comments state, this code was
The program that goes in the
Example 3-7. DeTab.java
public class DeTab {
Tabs ts; // iniitialized in Constructor
public static void main(String[] argv) throws IOException {
DeTab dt = new DeTab(8);
dt.detab(new BufferedReader(new InputStreamReader(System.in)),
new PrintWriter(System.out));
}
/** detab one file (replace tabs with spaces)
* @param is - the file to be processed
* @param out - the updated file
*/
public void detab(BufferedReader is, PrintWriter out) throws IOException {
String line;
char c;
int col;
while ((line = is.readLine()) != null) {
out.println(detabLine(line));
}
}
/** detab one line (replace tabs with spaces)
* @param line - the line to be processed
* @return the updated line
*/
public String detabLine(String line) {
char c;
int col;
StringBuffer sb = new StringBuffer();
col = 0;
for (int i = 0; i < line.length(); i++) {
// Either ordinary character or tab.
if ((c = line.charAt(i)) != '\t') {
sb.append(c); // Ordinary
++col;
continue;
}
do { // Tab, expand it, must put >=1 space
sb.append(' ');
} while (!ts.isTabStop(++col));
}
return sb.toString();
}
}
The Tabs class provides two methods, settabpos() and istabstop() . Example 3-8 lists the source for the Tabs class. Example 3-8. Tabs.java
public class Tabs {
/** tabs every so often */
public final static int DEFTABSPACE = 8;
/** the current tab stop setting. */
protected int tabSpace = DEFTABSPACE;
/** The longest line that we worry about tabs for. */
public final static int MAXLINE = 250;
/** the current tab stops */
protected boolean[] tabstops;
/** Construct a Tabs object with a given tab stop settings */
public Tabs(int n) {
if (n <= 0)
n = 1;
tabstops = new boolean[MAXLINE];
tabSpace = n;
settabs();
}
/** Construct a Tabs object with a default tab stop settings */
public Tabs() {
this(DEFTABSPACE);
}
/** settabs - set initial tab stops */
private void settabs() {
for (int i = 0; i < tabstops.length; i++) {
tabstops[i] = ((i+1) % tabSpace) == 0;
Debug.println("tabs", "Tabs[" + i + "]=" + tabstops[i]);
}
}
/**
* @return Returns the tabSpace.
*/
public int getTabSpacing() {
return tabSpace;
}
/** isTabStop - returns true if given column is a tab stop.
* If current input line is too long, we just put tabs wherever,
* no exception is thrown.
* @param col - the current column number
*/
public boolean isTabStop(int col) {
if (col > tabstops.length-1) {
tabstops = new boolean[tabstops.length * 2;
settabs();
}
return tabstops[col];
}
}
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