Write a Program Detab That Replaces Tabs in the Input with the Proper Number of Blanks to Space to the Next Tab Stop

Question

Continuing my K&R journey!

Please see below for my solution to:

Exercise 1-20: Write a program detab that replaces tabs in the input with the proper number of blanks to space to the next tab stop. Assume a fixed set of tab stops, say every n columns.

#include <stdio.h>
#define TAB '\t' // character to look for
#define SPACE '-' // tab replacement char
#define TABSIZE 8 // fixed set of tab stops
#define MAXLINE 8000 // maximum number of chars in a line
static int getLine(char line[]);
static void processLine(char line[], int lineCharCount);
int main() {
 int len; //length of line
 char line[BUFSIZ]; //string constant array for appending ch
 while ((len = getLine(line)) > 0) {
 processLine(line, len);
 }
 return 0;
}
static int getLine(char line[]) {
 int c;
 int i;
 for (i = 0; i < MAXLINE - 1 && (c = getchar()) != EOF && c != '\n'; ++i) {
 line[i] = c;
 }
 if (c == '\n') {
 line[i] = c;
 ++i;
 }
 line[i] = '0円';
 return i;
}
static void processLine(char line[], int lineCharCount) {
 int nonTabCount; // running count of characters up until \t is encountered
 int i; // for loop
 nonTabCount = 0;
 for (i = 0; i < lineCharCount; i++) {
 if (line[i] == TAB) {
 nonTabCount++;
 line[i] = SPACE;
 putchar(line[i]);
 while (TABSIZE - nonTabCount > 0) {
 putchar(SPACE);
 nonTabCount++;
 }
 nonTabCount = 0;
 } else {
 putchar(line[i]);
 nonTabCount++;
 if (nonTabCount >= TABSIZE) {
 nonTabCount = 0;
 }
 }
 }
}

• Reinderien: How is a hyphen the same as a space?
• Eric: Oh dear, I forgot to change back to space..... having hyphens was helpful while building the program. Gosh darnit.

Answer 1

Overall: Aside from bug, a very good post.

---

Nice layout & formatting

Good object names

Bug: Inconsistent size

With char line[BUFSIZ]; and C definition of "The value of the macro BUFSIZ shall be at least 256.", calling getLine(line) is a problem. getLine() assumes the buffer is MAXLINE or 8000.

Better to use the same. Even better, pass the size into getLine().

// static int getLine(char line[]) {
static size_t getLine(char line[], size_t size) {

I like size first to allow static code analysis with:

static size_t getLine(size_t size, char line[size]) {

In this case, best to initialize to handle tiny sizes.

// int c;
int c = 0;

Pedantic: string size

String size can exceed INT_MAX. Better to use size_t to express lineCharCount to handle all strings.

// processLine(char line[], int lineCharCount)
processLine(char line[], size_t lineCharCount)

Then use size_t i; in the function.

Simplify

Simpler and handles unsigned math well - no overflow.

// while (TABSIZE - nonTabCount > 0) {
while (TABSIZE > nonTabCount) {

Printing

I'd print the processed line afterwards and avoid printing in processLine(). Of course that is tricky as the line grows.

Why change line[]?

processLine() has line[i] = SPACE;, but why change only 1 space? If anything the de-tabify process might add multiple spaces and that complicates things. Code instead could putchar(SPACE); and leave line[] unchanged.

If processLine() changed to leave char line[] unchanged, consider making that paramter const char line[].

String?

With processLine(char line[], int lineCharCount), I'd expect the function received a pointier to a string, in which case, the lineCharCount is not needed. Instead iterate until a null character is found.

Yet by passing lineCharCount does allow handling of a line of input with a null character that was read - an advance functionality.

Fixed tab size

"Assume a fixed set of tab stops, say every n columns." --> that sounds like n should be an option at program start to de-tabify at different values. Maybe that is for the next iteration of code.

Documentation

Coding goal deserves to be in the code as a comment.

Advanced: long line

When getLine() fills the buffer, yet the line is still not done, consider how one would re-work the function to convey that incomplete read to the caller and if the remainder of the line should remain unread.

Declare and initialize in one step

// int nonTabCount;
// nonTabCount = 0;
int nonTabCount = 0;

• chux: No review on #define SPACE '-' as OP already id'ed that typo.
• chux: iamericfletcher, Post lacks tests to show that code works. Next time, consider a test harness.
• Eric: I'm confused - in getLine in main(), what would be the size I'm passing into it? Would I declare size_t size; and then use that like so while ((len = getLine(size, line)) > 0)?
• Eric: Also, would I keep char line[BUFSIZ]; the way it is in main()?
• chux: @iamericfletcher More like in main(), char line[BUFSIZ]; ... while ((len = getLine(sizeof line, line)) > 0).