如何自动修改代码？

Question

I have a large piece of code consisting of thousands of equations. Example of two connective lines, corresponding to lines 998 and 999 is

sum(x[i]*y[i]for i in 1:n) == 15;
sum(z[i]*x[i]for i in 1:n) == 30;

I would like to replace such lines with the following

sum(x[i]*y[i]for i in 1:n) - 15 >= -s[998];
sum(x[i]*y[i]for i in 1:n) - 15 <=  s[998];

sum(z[i]*x[i]for i in 1:n) - 30 >= -s[999];
sum(z[i]*x[i]for i in 1:n) - 30 <=  s[999];

How to automate this process?

Answer 1

Substitutions

Our first step is to transform each individual line:

sum(x[i]*y[i]for i in 1:n) == 15;

into this:

sum(x[i]*y[i]for i in 1:n) - 15 >= -s[998];

While doing it in one command would be a nice parlor trick, we will do it in several easier to follow steps.

1. == to -

   Here is our first command, it is very simple:

    :%s/==/-<CR>

   • : enters command-line mode.
   • % is the range of lines on which to execute the following command. Here, % is a shortcut for 1,$ (from line 1 to last line), thus "every line".
   • s is the "substitute" command, see :help :s .
   • /== is what you want to substitute.
   • /- is what you want to substitute it with.
   • Press <CR> (ENTER) to execute the command.

   In plain english: "substitute every == with - ".

   We should get something like:

    sum(x[i]*y[i]for i in 1:n) - 15; sum(z[i]*x[i]for i in 1:n) - 30;

2. ; to >= -s[XXX];

   Here is our second command:

    :%s/;/ >= -s[XXX];

   Where we substitute every ; with a generic >= -s[XXX]; . This is also quite simple.

   In plain english: "substitute every ; with >= -s[XXX]; .

   We should get something like:

    sum(x[i]*y[i]for i in 1:n) - 15 >= -s[XXX]; sum(z[i]*x[i]for i in 1:n) - 30 >= -s[XXX];

3. XXX to line number

   Here is our third command:

    :%s/XXX/\=line('.')

   The big change between this command and the other ones is that the replacement part is dynamic. With \= , we use an expression that is evaluated during each execution instead of a fixed string. line('.') is a vimscript function that returns a line number, which is exactly what we want between those brackets.

   In plain english: "substitute every XXX with the current line number".

   We should get something like:

    sum(x[i]*y[i]for i in 1:n) - 15 >= -s[998]; sum(z[i]*x[i]for i in 1:n) - 30 >= -s[999];

Duplication

Here we duplicate each line with a single command:

:g/^/t.|s/-s/ s

• :g is the :help :global command.
• /^/ matches every line so the following command will be executed on every line.
• t is the :help :t command.
• . represents the current line.
• | separates two commands.
• s/-s/ s removes the - before the s on the duplicated line.

In plain english: "mark every line, then copy each marked line below itself, then remove that leading - before the s ".

We should get something like:

sum(x[i]*y[i]for i in 1:n) - 15 >= -s[998];
sum(x[i]*y[i]for i in 1:n) - 15 >=  s[998];
sum(z[i]*x[i]for i in 1:n) - 30 >= -s[999];
sum(z[i]*x[i]for i in 1:n) - 30 >=  s[999];

One last thing

We use one last command to add a line between our blocks:

:g/ s[/put=''

• Every line matching s[ is marked.
• We use :help :put to append an empty line.

In plain english: "put an empty line after each line with s[ ".

We should get something like:

sum(x[i]*y[i]for i in 1:n) - 15 >= -s[998];
sum(x[i]*y[i]for i in 1:n) - 15 >=  s[998];

sum(z[i]*x[i]for i in 1:n) - 30 >= -s[999];
sum(z[i]*x[i]for i in 1:n) - 30 >=  s[999];

What did we learn?

• how to perform simple substitutions with :help :substitute ,
• the notion of :help :range ,
• how to use an expression in the replacement part (the usual gateway drug to vimscript) with :help sub-replace-\= ,
• how to execute arbitrary commands on lines matching a specific pattern with :help :global ,
• how to copy a given line to a given address with :help :t ,
• how to put some text, here an empty string, below the current line with :help :put ,
• how to separate commands with :help :|.