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§ 6.1 Smart comments

With the following macro definition in place:

m4_define([m4_ignore],)

you can add comments to your m4 source code that do not appear in the H.T.M.L. output. Conventional H.T.M.L. comments <!-- ... --> can be viewed in a Web browser by selecting the View Source (Internet Explorer) or View Page Source (Mozilla Firefox) menu button. Calls to the m4_ignore macro such as

m4_ignore(I do not show up in the H.T.M.L. code)

do not show up in the H.T.M.L. output and are therefore a safe place to put comments that you don't want the users of your Web pages to read, such as swear words and secret information.

§ 6.2 Number of years since a given date

With the following macro definition in place:

m4_define([m4_years],[m4_eval((((12 * 32 * m4_esyscmd(date +%Y) + 32 * (m4_esyscmd(date +%m) - 1) + m4_esyscmd(date +%d))
- (($1 * 12 * 32) + ($2 * 32) + $3))) / (12 * 32))])

The following macro call: m4_years(1973,06,17) prints out the number of years elapsed since my date of birth, i.e. my age in years.

§ 6.3 Questions and answers in the same source file

When writing Web tutorials (such as this one) it is useful to keep your questions and the answers to those questions in a single source file. In Question 6.15 you will write a macro ANSWER so that answers are not visible in question pages but are visible in the answer pages. For symmetry it is also useful to have a macro called QUESTION so that your single source file can look something like this:

QUESTION(1, What is George Bernard Shaw's middle name?)
ANSWER(1, Bernard)

QUESTION(2, How do you spell H.T.M.L.?)
ANSWER(2, H.T.M.L.)

QUESTION(3, What is the square root of 144?)
ANSWER(3, 12)

QUESTION(4, How many loaves are there in a baker's dozen?)
ANSWER(4, 13)

QUESTION(5, What is the meaning of life?)
ANSWER(5, 42)

§ 6.4 Two versions of your CV

• Your private CV can have information that you don't want to appear on your public CV, such as your home address or other information that is private to you.
• Your private CV will not have the header and footer information generated by m4_begin_html and m4_end_html so that it is more suitable for printing out on a printer.
• In your private CV you can safely define the PATCH_EMAIL macro to do nothing, because your private CV will not a accessible by the Internet. See Question 6.20 for this.

§ 6.5 Automatic numbering of sections

With the following definitions in place:

m4_define([H2_COUNT],0)
m4_define([H2],[m4_define([H2_COUNT],m4_incr([H2_COUNT]) <h2>H2_COUNT. $1 </h2>)])

You can write the following m4 code:

H2(First section)
H2(Second section)
H2(Conclusion)

and it will generate the following H.T.M.L. code:

<h2>1. First section</h2>
<h2>2. Second section</h2>
<h2>3. Conclusion</h2>

§ 6.6 Questions