Comment (computer programming)
In computer programming, a comment is a programmer-readable explanation or annotation in the source code of a computer program. They are added with the purpose of making the source code easier for humans to understand, and are generally ignored by compilers and interpreters. The syntax of comments in various programming languages varies considerably.
Comments are sometimes processed in various ways to generate documentation external to the source code itself by documentation generators, or used for integration with source code management systems and other kinds of external programming tools.
The flexibility provided by comments allows for a wide degree of variability, but formal conventions for their use are commonly part of programming style guides.
OverviewComments are generally formatted as either block comments or line comments.
Block comments delimit a region of source code which may span multiple lines or a part of a single line. This region is specified with a start delimiter and an end delimiter. Some programming languages allow block comments to be recursively nested inside one another, but others do not.
Line comments either start with a comment delimiter and continue until the end of the line, or in some cases, start at a specific column in the source code, and continue until the end of the line.
Some programming languages employ both block and line comments with different comment delimiters. For example, C++ has block comments delimited by
*/that can span multiple lines and line comments delimited by
//. Other languages support only one type of comment. For example, Ada comments are line comments: they start with
--and continue to the end of the line.
UsesHow best to make use of comments is subject to dispute; different commentators have offered varied and sometimes opposing viewpoints.
There are many different ways of writing comments and many commentators offer conflicting advice.
Planning and reviewingComments can be used as a form of pseudocode to outline intention prior to writing the actual code. In this case it should explain the logic behind the code rather than the code itself.
If this type of comment is left in, it simplifies the review process by allowing a direct comparison of the code with the intended results. A common logical fallacy is that code that is easy to understand does what it's supposed to do.
Code descriptionComments can be used to summarize code or to explain the programmer's intent. According to this school of thought, restating the code in plain English is considered superfluous; the need to re-explain code may be a sign that it is too complex and should be rewritten, or that the naming is bad.
Comments may also be used to explain why a block of code does not seem to fit conventions or best practices. This is especially true of projects involving very little development time, or in bug fixing. For example:
' documentation available on server behavior issue, so just coding around it.
vtx = server.mappath
Algorithmic descriptionSometimes source code contains a novel or noteworthy solution to a specific problem. In such cases, comments may contain an explanation of the methodology. Such explanations may include diagrams and formal mathematical proofs. This may constitute explanation of the code, rather than a clarification of its intent; but others tasked with maintaining the code base may find such explanation crucial. This might especially be true in the case of highly specialized problem domains; or rarely used optimizations, constructs or function-calls.
For example, a programmer may add a comment to explain why an insertion sort was chosen instead of a quicksort, as the former is, in theory, slower than the latter. This could be written as follows:
list = ;
// Need a stable sort. Besides, the performance really does not matter.
Resource inclusions, diagrams, and flowcharts consisting of ASCII art constructions can be inserted into source code formatted as a comment. Further, copyright notices can be embedded within source code as comments. Binary data may also be encoded in comments through a process known as binary-to-text encoding, although such practice is uncommon and typically relegated to external resource files.
The following code fragment is a simple ASCII diagram depicting the process flow for a system administration script contained in a Windows Script File running under Windows Script Host. Although a section marking the code appears as a comment, the diagram itself actually appears in an XML CDATA section, which is technically considered distinct from comments, but can serve similar purposes.
script.wsf --> ClientApp
Although this identical diagram could easily have been included as a comment, the example illustrates one instance where a programmer may opt not to use comments as a way of including resources in source code.
MetadataComments in a computer program often store metadata about a program file.
In particular, many software maintainers put submission guidelines in comments to help people who read the source code of that program to send any improvements they make back to the maintainer.
Other metadata includes:
the name of the creator of the original version of the program file and the date when the first version was created,
the name of the current maintainer of the program,
the names of other people who have edited the program file so far,
the URL of documentation about how to use the program,
the name of the software license for this program file,
When an algorithm in some section of the program is based on a description in a book or other reference, comments can be used to give the page number and title of the book or Request for Comments or other reference.
DebuggingA common developer practice is to comment out a code snippet, meaning to add comment syntax causing that block of code to become a comment, so that it will not be executed in the final program. This may be done to exclude certain pieces of code from the final program, or it can be used to find the source of an error. By systematically commenting out and running parts of the program, the source of an error can be determined, allowing it to be corrected.
An example of commenting out code for exclusion purposes is below:
opt_enabled = true;
opt_debug = true;
opt_verbose = true;
The above code fragment suggests that the programmer opted to disable the debugging option for some reason.
Many IDEs allow quick adding or removing such comments with single menu options or key combinations. The programmer has only to mark the part of text they want to comment and choose the appropriate option.
Automatic documentation generations sometimes store documentation and metadata in comments. These may include insert positions for automatic header file inclusion, commands to set the file's syntax highlighting mode, or the file's revision number. These functional control comments are also commonly referred to as annotations. Keeping documentation within source code comments is considered as one way to simplify the documentation process, as well as increase the chances that the documentation will be kept up to date with changes in the code.
Examples of documentation generators include the programs Javadoc for use with Java, Ddoc for D, Doxygen for C, C++, Java, IDL, Visual Expert for PL/SQL, Transact-SQL, PowerBuilder and PHPDoc for PHP. Forms of docstring are supported by Python, Lisp, Elixir, and Clojure.
C#, F# and Visual Basic.NET implement a similar feature called "XML Comments" which are read by IntelliSense from the compiled.NET assembly.
Syntax extensionOccasionally syntax elements that were originally intended to be comments are re-purposed to convey additional information to a program, such as "conditional comments".
Such "hot comments" may be the only practical solution that maintains backward-compatibility, but are widely regarded as a kludge.
Directive usesThere are cases where the normal comment characters are co-opted to create a special directive for an editor or interpreter.
Two examples of this directing an interpreter are:
- The Unix "shebang" –
#!– used on the first line of a script to point to the interpreter to be used.
- "Magic comments" identifying the encoding a source file is using, e.g. Python's PEP 263.
- !/usr/bin/env python3
- -*- coding: UTF-8 -*-
Somewhat similar is the use of comments in C to communicate to a compiler that a default "fallthough" in a case statement has been done deliberately:
Inserting such a
/* Fall thru */comment for human readers was a already a common convention, but in 2017 the gcc compiler began looking for these, and, if not found, emitting: "warning: this statement may fall through". Compiler-dependent comments which switch off warnings have existed for decades.
Stress reliefSometimes programmers will add comments as a way to relieve stress by commenting about development tools, competitors, employers, working conditions, or the quality of the code itself. The occurrence of this phenomenon can be easily seen from online resources that track profanity in source code.
Normative viewsThere are various normative views and long-standing opinions regarding the proper use of comments in source code. Some of these are informal and based on personal preference, while others are published or promulgated as formal guidelines for a particular community.
Need for commentsExperts have varying viewpoints on whether, and when, comments are appropriate in source code. Some assert that source code should be written with few comments, on the basis that the source code should be self-explanatory or self-documenting. Others suggest code should be extensively commented.
In between these views is the assertion that comments are neither beneficial nor harmful by themselves, and what matters is that they are correct and kept in sync with the source code, and omitted if they are superfluous, excessive, difficult to maintain or otherwise unhelpful.
Comments are sometimes used to document contracts in the design by contract approach to programming.
Level of detailDepending on the intended audience of the code and other considerations, the level of detail and description may vary considerably.
For example, the following Java comment would be suitable in an introductory text designed to teach beginning programming:
String s = "Wikipedia"; /* Assigns the value "Wikipedia" to the variable s. */
This level of detail, however, would not be appropriate in the context of production code, or other situations involving experienced developers. Such rudimentary descriptions are inconsistent with the guideline: "Good comments... clarify intent." Further, for professional coding environments, the level of detail is ordinarily well defined to meet a specific performance requirement defined by business operations.
StylesThere are many stylistic alternatives available when considering how comments should appear in source code. For larger projects involving a team of developers, comment styles are either agreed upon before a project starts, or evolve as a matter of convention or need as a project grows. Usually programmers prefer styles that are consistent, non-obstructive, easy to modify, and difficult to break.
Block commentThe following code fragments in C demonstrate just a tiny example of how comments can vary stylistically, while still conveying the same basic information:
This is the comment body.
- This is the comment body. *
- Variation Two. *
Factors such as personal preference, flexibility of programming tools, and other considerations tend to influence the stylistic variants used in source code. For example, Variation Two might be disfavored among programmers who do not have source code editors that can automate the alignment and visual appearance of text in comments.
Software consultant and technology commentator Allen Holub is one expert who advocates aligning the left edges of comments:
/* This is the style recommended by Holub for C and C++.
* It is demonstrated in Enough Rope, in rule 29.
/* This is another way to do it, also in C.
** It is easier to do in editors that do not automatically indent the second
** through last lines of the comment one space from the first.
** It is also used in Holub's book, in rule 31.
The use of /* and */ as block comment delimiters was inherited from PL/I into the B programming language, the immediate predecessor of the C programming language.
Line commentsLine comments generally use an arbitrary delimiter or sequence of tokens to indicate the beginning of a comment, and a newline character to indicate the end of a comment.
In this example, all the text from the ASCII characters // to the end of the line is ignored.
// This is the comment body.
Often such a comment has to begin at far left and extend to the whole line. However in many languages, it is also possible to put a comment inline with a command line, to add a comment to it – as in this Perl example:
print $s. "\n"; # Add a newline character after printing
If a language allows both line comments and block comments, programming teams may decide upon a convention of using them differently: e.g. line comments only for minor comments, and block comments to describe higher-level abstractions.
TagsProgrammers may use informal tags in comments to assist in indexing common issues. They may then be able to be searched for with common programming tools, such as the Unix grep utility or even syntax-highlighted within text editors. These are sometimes referred to as "codetags" or "tokens".
Such tags differ widely, but might include:
- BUG – a known bug that should be corrected.
- FIXME – should be corrected.
- HACK – a workaround.
- TODO – something to be done.
- UNDONE – a reversal or "roll back" of previous code.
- XXX – warn other programmers of problematic or misguiding code
ComparisonTypographic conventions to specify comments vary widely. Further, individual programming languages sometimes provide unique variants. For a detailed review, please consult the programming language comparison article.
AdaThe Ada programming language uses '--' to indicate a comment up to the end of the line.
-- the air traffic controller task takes requests for takeoff and landing
task type Controller is
-- task entries for synchronous message passing
entry Request_Takeoff ;
⍝to indicate a comment up to the end of the line.
⍝ Now add the numbers:
c←a+b ⍝ addition
In dialects that have the
⊢primitives, comments can often be inside or separate statements, in the form of ignored strings:
d←2×c ⊣'where'⊢ c←a+ 'bound'⊢ b
display dialog myGreeting & " world!"
-- Show the greeting
BASICIn this classic early BASIC code fragment the REM keyword is used to add comments.
10 REM This BASIC program shows the use of the PRINT and GOTO Statements.
15 REM It fills the screen with the phrase "HELLO"
20 PRINT "HELLO"
30 GOTO 20
In later Microsoft BASICs, including Quick Basic, Q Basic, Visual Basic, Visual Basic.NET, and VB Script; and in descendants such as Free BASIC and Gam bas any text on a line after an ' character is also treated as a comment.
An example in Visual Basic.NET:
Public Class Form1
Private Sub Button1_Click Handles Button1.Click
' The following code is executed when the user
' clicks the button in the program's window.
rem comments still exist.
MessageBox.Show 'Show a pop-up window with a greeting
CThis C code fragment demonstrates the use of a prologue comment or "block comment" to describe the purpose of a conditional statement. The comment explains key terms and concepts, and includes a short signature by the programmer who authored the code.
* Check if we are over our maximum process limit, but be sure to
* exclude root. This is needed to make it possible for login and
* friends to set the per-user process limit to something lower
* than the amount of processes root is running. -- Rik
if >= p->rlim.rlim_cur
&& !capable && !capable)
Since C99, it has also been possible to use the // syntax from C++, indicating a single-line comment.
Cisco IOS and IOS-XE configurationThe exclamation point may be used to mark comments in a Cisco router's configuration mode, however such comments are not saved to non-volatile memory, nor are they displayed by the "show run" command.
It is possible to insert human-readable content that is actually part of the configuration, and may be saved to the NVRAM startup-config via:
- The "description" command, used to add a description to the configuration of an interface or of a BGP neighbor
- The "name" parameter, to add a remark to a static route
- The "remark" command in access lists
! Paste the text below to reroute traffic manually
ip route 0.0.0.0 0.0.0.0 gi0/2 name ISP2
no ip route 0.0.0.0 0.0.0.0 gi0/1 name ISP1
ColdFusionuses comments similar to HTML comments, but instead of two dashes, it uses three. These comments are caught by the ColdFusion engine and not printed to the browser.
C Lines that begin with 'C' are comments
Note that the columns of a line are otherwise treated as four fields: 1 to 5 is the label field, 6 causes the line to be taken as a continuation of the previous statement; and declarations and statements go in 7 to 72.
Fortran 90This Fortran code fragment demonstrates how comments are used in that language, with the comments themselves describing the basic formatting rules.
!* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
!* All characters after an exclamation mark are considered as comments *
!* * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * * *
print '', 'Hello world' ! Fortran 90 introduced the option for inline comments.
HaskellLine comments in Haskell start with '--' until the end of line, and multiple line comments start with ''.
-- and this is a comment on one line
putStrLn "Wikipedia" -- this is another comment
Haskell also provides a literate programming method of commenting known as "Bird Style". In this all lines starting with > are interpreted as code, everything else is considered a comment. One additional requirement is that you always leave a blank line before and after the code block:
In Bird-style you have to leave a blank before the code.
> fact :: Integer -> Integer
> fact 0 = 1
> fact = * fact n
And you have to leave a blank line after the code as well.
Literate programming can also be done in Haskell, using LaTeX. The code environment can be used instead of the Richard Bird's style:
In LaTeX style this is equivalent to the above example, the code environment could be defined in the LaTeX preamble. Here is a simple definition:
% the LaTeX source file
The \verb|fact n| function call computes $n!$ if $n\ge 0$, here is a definition:\\
fact :: Integer -> Integer
fact 0 = 1
fact = * fact n
Here more explanation using \LaTeX markup
JavaThis Java code fragment shows a block comment used to describe the
setToolTipTextmethod. The formatting is consistent with Sun Microsystems Javadoc standards. The comment is designed to be read by the Javadoc processor.
* This is a block comment in Java.
* The setToolTipText method registers the text to display in a tool tip.
* The text is displayed when the cursor lingers over the component.
* @param text The string to be displayed. If 'text' is null,
* the tool tip is turned off for this component.
public void setToolTipText
var iNum = 100;
var iTwo = 2; // A comment at the end of line
LuaThe Lua programming language uses double-hyphens,
--, for single line comments in a similar way to Ada, Eiffel, Haskell, SQL and VHDL languages. Lua also has block comments, which start with
--and run until a closing
print -- print the result
A common technique to comment out a piece of code, is to enclose the code between
--, as below:
-- no action
In this case, it's possible to reactivate the code by adding a single hyphen to the first line:
In the first example, the
--in the first line starts a long comment, and the two hyphens in the last line
are still inside that comment. In the second example, the sequence
---starts an ordinary, single-line
comment, so that the first and the last lines become independent comments. In this case, the
outside comments. In this case, the last line becomes an independent comment, as it starts with
Long comments in Lua can be more complex than these, as you can read in the section called "Long strings" c.f. Programming in Lua.
MATLABIn MATLAB's programming language, the '%' character indicates a single-line comment. Multi line comments are also available via % brackets and can be nested, e.g.
% These are the derivatives for each term
d = ;
seq = d.*.^n./
% We add-up to get the Taylor approximation
approx = sum
OCamluses nestable comments, which is useful when commenting a code block.
PascalIn Niklaus Wirth's pascal family of languages, comments are opened with .
columnDifference := testColumn - column;
PerlLine comments in Perl, and many other scripting languages, begin with a hash symbol.
- A simple example
print $s. "\n"; # Add a newline character after printing
Instead of a regular block commenting construct, Perl uses Plain Old Documentation, a markup language for literate programming, for instance:
Create a new list object. Properties may be specified through a hash
reference like this:
my $list = Pod::List->new;
See the individual methods/properties for details.
Rakuuses the same line comments and POD Documentation comments as regular Perl, but adds a configurable block comment type: "multi-line / embedded comments".
These start with a hash character, followed by a backtick, and then some opening bracketing character, and end with the matching closing bracketing character. The content can not only span multiple lines, but can also be embedded inline.
PHPComments in PHP can be either in C++ style, or use hashes. PHPDoc is a style adapted from Javadoc and is a common standard for documenting PHP code.
PowerShellComments in Windows PowerShell
- Single line comment
Write-Host "Goodbye, world!"
PythonInline comments in Python use the hash character, as in the two examples in this code:
- This program prints "Hello World" to the screen
Block comments, as defined in this article, don't technically exist in Python. A bare string literal represented by a triple-quoted string can be used but is not ignored by the interpreter in the same way that "#" comment is. In the examples below, the triple double-quoted strings act in this way as comments, but are also treated as docstrings:
Assuming this is file mymodule.py, then this string, being the
first statement in the file, will become the "mymodule" module's
docstring when the file is imported.
"""The class's docstring"""
"""The method's docstring"""
"""The function's docstring"""
RubyComments in Ruby.
Single line commenting:
puts "This is not a comment"
- this is a comment
puts "This is not a comment"
whatever goes in these lines
is just for the human reader
puts "This is not a comment"
SQLStandard comments in SQL are in single-line-only form, using two dashes:
-- This is a single line comment
-- followed by a second line
WHERE Authors.name = 'Smith'; -- Note: we only want 'smith'
-- this comment appears after SQL code
Alternatively, a comment format syntax identical to the "block comment" style used in the syntax for C and Java is supported by Transact-SQL, MySQL, SQLite, PostgreSQL, and Oracle.
MySQL also supports comments from the hash character to the end of the line.
SwiftSingle-line comments begin with two forward-slashes :
// This is a comment.
/* This is also a comment
but is written over multiple lines. */
/* This is the start of the first multiline comment.
/* This is the second, nested multiline comment. */
This is the end of the first multiline comment. */
XMLComments in XML are introduced with