After learning what the basic data types are, the next step is to learn how to combine them together to start writing a basic program. A basic program consists of conditionals and loops.
In this chapter, you will learn how to use Vimscript data types to write conditionals and loops.
Vimscript relational operators are similar to many programming languages:
a == b equal to
a != b not equal to
a > b greater than
a >= b greater than or equal to
a < b less than
a <= b less than or equal to
For example:
:echo 5 == 5
:echo 5 != 5
:echo 10 > 5
:echo 10 >= 5
:echo 10 < 5
:echo 5 <= 5
Recall that strings are coerced into numbers in an arithmetic expression. Here Vim also coerces strings into numbers in an equality expression. "5foo" is coerced into 5 (truthy):
:echo 5 == "5foo"
" returns true
Also recall that if you start a string with a non-numerical character like "foo5", the string is converted into number 0 (falsy).
echo 5 == "foo5"
" returns false
Vim has more relational operators for comparing strings:
a =~ b
a !~ b
For examples:
let str = "hearty breakfast"
echo str =~ "hearty"
" returns true
echo str =~ "dinner"
" returns false
echo str !~ "dinner"
" returns true
The =~ operator performs a regex match against the given string. In the example above, str =~ "hearty" returns true because str contains the "hearty" pattern. You can always use == and !=, but using them will compare the expression against the entire string. =~ and !~ are more flexible choices.
echo str == "hearty"
" returns false
echo str == "hearty breakfast"
" returns true
Let's try this one. Note the uppercase "H":
echo str =~ "Hearty"
" true
It returns true even though "Hearty" is capitalized. Interesting... It turns out that my Vim setting is set to ignore case (set ignorecase), so when Vim checks for equality, it uses my Vim setting and ignores the case. If I were to turn off ignore case (set noignorecase), the comparison now returns false.
set noignorecase
echo str =~ "Hearty"
" returns false because case matters
set ignorecase
echo str =~ "Hearty"
" returns true because case doesn't matter
If you are writing a plugin for others, this is a tricky situation. Does the user use ignorecase or noignorecase? You definitely do not want to force your users to change their ignore case option. So what do you do?
Luckily, Vim has an operator that can always ignore or match case. To always match case, add a # at the end.
set ignorecase
echo str =~# "hearty"
" returns true
echo str =~# "HearTY"
" returns false
set noignorecase
echo str =~# "hearty"
" true
echo str =~# "HearTY"
" false
echo str !~# "HearTY"
" true
To always ignore case when comparing, append it with ?:
set ignorecase
echo str =~? "hearty"
" true
echo str =~? "HearTY"
" true
set noignorecase
echo str =~? "hearty"
" true
echo str =~? "HearTY"
" true
echo str !~? "HearTY"
" false
I prefer to use # to always match the case and be on the safe side.
Now that you have seen Vim's equality expressions, let's touch a fundamental conditional operator, the if statement.
At minimum, the syntax is:
if {clause}
{some expression}
endif
You can extend the case analysis with elseif and else.
if {predicate1}
{expression1}
elseif {predicate2}
{expression2}
elseif {predicate3}
{expression3}
else
{expression4}
endif
For example, the plugin vim-signify uses a different installation method depending on your Vim settings. Below is the installation instruction from their readme, using the if statement:
if has('nvim') || has('patch-8.0.902')
Plug 'mhinz/vim-signify'
else
Plug 'mhinz/vim-signify', { 'branch': 'legacy' }
endif
Vim has a ternary expression for a one-liner case analysis:
{predicate} ? expressiontrue : expressionfalse
For example:
echo 1 ? "I am true" : "I am false"
Since 1 is truthy, Vim echoes "I am true". Suppose you want to conditionally set the background to dark if you are using Vim past a certain hour. Add this to vimrc:
let &background = strftime("%H") < 18 ? "light" : "dark"
&background is the 'background' option in Vim. strftime("%H") returns the current time in hours. If it is not yet 6 PM, use a light background. Otherwise, use a dark background.
The logical "or" (||) works like many programming languages.
{Falsy expression} || {Falsy expression} false
{Falsy expression} || {Truthy expression} true
{Truthy expression} || {Falsy expression} true
{Truthy expression} || {Truthy expression} true
Vim evaluates the expression and return either 1 (truthy) or 0 (falsy).
echo 5 || 0
" returns 1
echo 5 || 5
" returns 1
echo 0 || 0
" returns 0
echo "foo5" || "foo5"
" returns 0
echo "5foo" || "foo5"
" returns 1
If the current expression evaluates to truthy, the subsequent expression won't be evaluated.
let one_dozen = 12
echo one_dozen || two_dozen
" returns 1
echo two_dozen || one_dozen
" returns error
Note that two_dozen is never defined. The expression one_dozen || two_dozen doesn't throw any error because one_dozen is evaluated first found to be truthy, so Vim doesn't evaluate two_dozen.
The logical "and" (&&) is the complement of the logical or.
{Falsy Expression} && {Falsy Expression} false
{Falsy expression} && {Truthy expression} false
{Truthy Expression} && {Falsy Expression} false
{Truthy expression} && {Truthy expression} true
For example:
echo 0 && 0
" returns 0
echo 0 && 10
" returns 0
&& evaluates an expression until it sees the first falsy expression. For example, if you have true && true, it will evaluate both and return true. If you have true && false && true, it will evaluate the first true and stop at the first false. It will not evaluate the third true.
let one_dozen = 12
echo one_dozen && 10
" returns 1
echo one_dozen && v:false
" returns 0
echo one_dozen && two_dozen
" returns error
echo exists("one_dozen") && one_dozen == 12
" returns 1
The for loop is commonly used with the list data type.
let breakfasts = ["pancakes", "waffles", "eggs"]
for breakfast in breakfasts
echo breakfast
endfor
It works with nested list:
let meals = [["breakfast", "pancakes"], ["lunch", "fish"], ["dinner", "pasta"]]
for [meal_type, food] in meals
echo "I am having " . food . " for " . meal_type
endfor
You can technically use the for loop with a dictionary using the keys() method.
let beverages = #{breakfast: "milk", lunch: "orange juice", dinner: "water"}
for beverage_type in keys(beverages)
echo "I am drinking " . beverages[beverage_type] . " for " . beverage_type
endfor
Another common loop is the while loop.
let counter = 1
while counter < 5
echo "Counter is: " . counter
let counter += 1
endwhile
To get the content of the current line to the last line:
let current_line = line(".")
let last_line = line("$")
while current_line <= last_line
echo getline(current_line)
let current_line += 1
endwhile
Often your program doesn't run the way you expect it to. As a result, it throws you for a loop (pun intended). What you need is a proper error handling.
When you use break inside a while or for loop, it stops the loop.
To get the texts from the start of the file to the current line, but stop when you see the word "donut":
let line = 0
let last_line = line("$")
let total_word = ""
while line <= last_line
let line += 1
let line_text = getline(line)
if line_text =~# "donut"
break
endif
echo line_text
let total_word .= line_text . " "
endwhile
echo total_word
If you have the text:
one
two
three
donut
four
five
Running the above while loop gives "one two three" and not the rest of the text because the loop breaks once it matches "donut".
The continue method is similar to break, where it is invoked during a loop. The difference is that instead of breaking out of the loop, it just skips that current iteration.
Suppose you have the same text but instead of break, you use continue:
let line = 0
let last_line = line("$")
let total_word = ""
while line <= last_line
let line += 1
let line_text = getline(line)
if line_text =~# "donut"
continue
endif
echo line_text
let total_word .= line_text . " "
endwhile
echo total_word
This time it returns one two three four five. It skips the line with the word "donut", but the loop continues.
Vim has a try, finally, and catch to handle errors. To simulate an error, you can use the throw command.
try
echo "Try"
throw "Nope"
endtry
Run this. Vim will complain with "Exception not caught: Nope error.
Now add a catch block:
try
echo "Try"
throw "Nope"
catch
echo "Caught it"
endtry
Now there is no longer any error. You should see "Try" and "Caught it" displayed.
Let's remove the catch and add a finally:
try
echo "Try"
throw "Nope"
echo "You won't see me"
finally
echo "Finally"
endtry
Run this. Now Vim displays the error and "Finally".
Let's put all of them together:
try
echo "Try"
throw "Nope"
catch
echo "Caught it"
finally
echo "Finally"
endtry
This time Vim displays both "Caught it" and "Finally". No error is displayed because Vim caught it.
Errors come from different places. Another source of error is calling a nonexistent function, like Nope() below:
try
echo "Try"
call Nope()
catch
echo "Caught it"
finally
echo "Finally"
endtry
The difference between catch and finally is that finally is always run, error or not, where a catch is only run when your code gets an error.
You can catch specific error with :catch. According to :h :catch:
catch /^Vim:Interrupt$/. " catch interrupts (CTRL-C)
catch /^Vim\\%((\\a\\+)\\)\\=:E/. " catch all Vim errors
catch /^Vim\\%((\\a\\+)\\)\\=:/. " catch errors and interrupts
catch /^Vim(write):/. " catch all errors in :write
catch /^Vim\\%((\\a\\+)\\)\\=:E123:/ " catch error E123
catch /my-exception/. " catch user exception
catch /.*/ " catch everything
catch. " same as /.*/
Inside a try block, an interrupt is considered a catchable error.
try
catch /^Vim:Interrupt$/
sleep 100
endtry
In your vimrc, if you use a custom colorscheme, like gruvbox, and you accidentally delete the colorscheme directory but still have the line colorscheme gruvbox in your vimrc, Vim will throw an error when you source it. To fix this, I added this in my vimrc:
try
colorscheme gruvbox
catch
colorscheme default
endtry
Now if you source vimrc without gruvbox directory, Vim will use the colorscheme default.
In the previous chapter, you learned about Vim basic data types. In this chapter, you learned how to combine them to write basic programs using conditionals and loops. These are the building blocks of programming.
Next, let's learn about variable scopes.