K&R Exercise 1-20 - Need some clarification

Question

I don't fully understand what the following exercise is asking:

"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. Should n be a variable or a symbolic parameter?"

Could someone clarify the bolded part?

Answer 1

This exercise is asking you to emulate the behavior of tabs by adding the correct amount of spaces, such that the output is still aligned on tab stops.

For example :

"hello\tworld"

should become :

"hello   world"

(the tab has been replaced with three spaces), if tab stops are every 4 columns (ie. n = 4).

Or to clarify by indicating where the tab stops are :

hello   world
^   ^   ^   ^

If tab stops are every 3 columns, then you should get :

hello world
^  ^  ^  ^

(the tab was replaced by only 1 space)

Answer 2

If you take an example of tabstops being set at n=8, for example, the if the input has 1 character, the tab will add 7 spaces (to bring you to column 9). Basically, don't always add n spaces, add the number of spaces that brings you to the appropriate column for your particular value of n.

For example:

         1
1234567890123456789
1------>
123---->
More words----->

"Assume a fixed set of tab stops" is for non-programmers, basically. We're used to a tab always aligning on a multiple of 4,8,etc. But in word processors, the tab stops are configurable... so the first tab would align you on column 6, the second would go to 30 (for example to sloppily center text) and the third would give you column 70 (for page numbers or something). He's just specifying here that we're talking about "programmer" tabstops, not a word processor's tabstops.