By Walter Alan Zintz    (articolo originale)     indice

The Vi/Ex Editor, Part 3: Answers to Exercises

Solution to Copy and Decapitalize Problem

One straightforward solution is:

global /^CHAPTER/ mark a | copy $ | 'a substitute /APTER/apter/

which marks the original line, then copies it in its all-caps version to the end of the file, and finally returns to the original line to run the decapitalizing substitution.

Another, not quite so obvious but just as good, is to decapitalize "CHAPTER" first, then copy the decapitalized version to the end of the file, and finally run a substitution command on the current line (which is now the copied line at the end of the file) which changes the capitalization back to "CHAPTER".

Solution to troff Problem

The command line:

1 , 16 global /^/ 217 substitute /n(PDu/\\n(PDu/

does it by running the substitution command 16 times. Each time it inserts a single backslash. (The double backslash in the replacement pattern is necessary because the backslash is a special character even there.)

Hint for Numbering Problem

My solution to this problem has an intermediate stage in which each macro is followed by a string of capital I letters on the same line. The count of the capital I letters on any macro line is equal to the paragraph number. That is, the macro line for the fifth paragraph looks like this in the intermediate stage:

.ppIIIII

Solution to Numbering Problem

The paragraphs can be numbered with just t wo global commands. The first one:

global /^\.pp/ . , $ substitute //.ppI/

goes to each line beginning with a start-of-paragraph macro, then runs a substitute command from that line through the end of the file that puts a capital letter I after each such macro. So the substitute command that runs from the first marked line puts an I after every one of the macros; from the second marked line it puts an I after every such macro except the first; from the third marked line it puts an I after every such macro except the first and the second; and so on. Thus, after this global finishes, you have a string of the letter I after every macro that is equal in number to the paragraph's number. That is, after the macro for the third paragraph you have the string "III"; after paragraph 5 you have the string "IIIII"; etcetera. Already you have Roman numerals (of a very primitive sort) numbering the paragraphs.

A second g lobal command puts those Roman numerals into canonical form:

global /^\.pp/ substitute /IIIII/V/ g \
| substitute /VV/X/ g | substitute /IIII/IV/

(As you'll learn in the next installment of this tutorial, a lower-case letter g at the end of a substitute command tells the editor to perform the substitution as many times as it can on each line, and a backslash at the end of a partial command line means the next line continues the command string.)

To see how our second global command sets things right, consider the case of the 19th paragraph. The next four lines show what the macro line looks like at the start of the command and how it has changed after each of the three substitute commands has done its work:

.ppIIIIIIIIIIIIIIIIIII
.ppVVVIIII
.ppXVIIII
.ppXVIV

(Astute readers will realize that the paragraphs could have been numbered with just one global command. Each macro l ine has all the capital I letters it will get before global leaves it for the next line. So we could have had the command string start by marking the line, next run the substitution that adds a capital I to all remaining macro lines, then return to the line and run the substitutions that produce a true Roman numeral.)