Why is := allowed as an infix operator?

I have come across the popular data.table package and one thing in particular intrigued me. It has an in-place assignment operator

:=

This is not defined in base R. In fact if you didn't load the data.table package, it would have raised an error if you had tried to used it (e.g., a := 2) with the message:

Error: could not find function ":="

Also, why does := work? Why does R let you define := as infix operator while every other infix function has to be surrounded by %%, e.g.

`:=` <- function(a, b) 
 paste(a,b)


"abc" := "def"

Clearly it's not meant to be an alternative syntax to %function.name% for defining infix functions. Is data.table exploiting some parsing quirks of R? Is it a hack? Will it be "patched" in the future?

edited Nov 13 '18 at 12:56

Moody_Mudskipper

22.7k32964

asked Oct 9 '14 at 2:37

xiaodai

4,110114871

5

Please take a look at the data.table documentation, starting maybe with the FAQ.

– Dirk Eddelbuettel
Oct 9 '14 at 2:39

1

@DirkEddelbuettel. I understand how it's used in data.table. But the fact that R allows for such an operator to be defined and not cause a syntax error is what intrigued me. It's a fundamental question about R and maybe how it parses code.

– xiaodai
Oct 9 '14 at 2:44

2

AFAIK it is local to data.table and only works with the [ subsetting. So your question is off-base (not an R quirk) which is why I sent you to the data.table docs which discuss this.

– Dirk Eddelbuettel
Oct 9 '14 at 2:49

1

@DirkEddelbuettel I think you are missing my point. I can define a funciton using := <- function(a,b) paste(a,b); and I can use it by doing "abc" := "def"! But all other infix function are in the form of %in.fn%?. Why?

– xiaodai
Oct 9 '14 at 2:57

2

This Q&A from Matt might be very relevant here as well.

– Arun
Oct 9 '14 at 6:51

add a comment |

I have come across the popular data.table package and one thing in particular intrigued me. It has an in-place assignment operator

:=

This is not defined in base R. In fact if you didn't load the data.table package, it would have raised an error if you had tried to used it (e.g., a := 2) with the message:

Error: could not find function ":="

Also, why does := work? Why does R let you define := as infix operator while every other infix function has to be surrounded by %%, e.g.

`:=` <- function(a, b) 
 paste(a,b)


"abc" := "def"

edited Nov 13 '18 at 12:56

Moody_Mudskipper

22.7k32964

asked Oct 9 '14 at 2:37

xiaodai

4,110114871

5

Please take a look at the data.table documentation, starting maybe with the FAQ.

– Dirk Eddelbuettel
Oct 9 '14 at 2:39

1

@DirkEddelbuettel. I understand how it's used in data.table. But the fact that R allows for such an operator to be defined and not cause a syntax error is what intrigued me. It's a fundamental question about R and maybe how it parses code.

– xiaodai
Oct 9 '14 at 2:44

2

AFAIK it is local to data.table and only works with the [ subsetting. So your question is off-base (not an R quirk) which is why I sent you to the data.table docs which discuss this.

– Dirk Eddelbuettel
Oct 9 '14 at 2:49

1

@DirkEddelbuettel I think you are missing my point. I can define a funciton using := <- function(a,b) paste(a,b); and I can use it by doing "abc" := "def"! But all other infix function are in the form of %in.fn%?. Why?

– xiaodai
Oct 9 '14 at 2:57

2

This Q&A from Matt might be very relevant here as well.

– Arun
Oct 9 '14 at 6:51

add a comment |

I have come across the popular data.table package and one thing in particular intrigued me. It has an in-place assignment operator

:=

This is not defined in base R. In fact if you didn't load the data.table package, it would have raised an error if you had tried to used it (e.g., a := 2) with the message:

Error: could not find function ":="

Also, why does := work? Why does R let you define := as infix operator while every other infix function has to be surrounded by %%, e.g.

`:=` <- function(a, b) 
 paste(a,b)


"abc" := "def"

edited Nov 13 '18 at 12:56

Moody_Mudskipper

22.7k32964

asked Oct 9 '14 at 2:37

xiaodai

4,110114871

I have come across the popular data.table package and one thing in particular intrigued me. It has an in-place assignment operator

:=

This is not defined in base R. In fact if you didn't load the data.table package, it would have raised an error if you had tried to used it (e.g., a := 2) with the message:

Error: could not find function ":="

Also, why does := work? Why does R let you define := as infix operator while every other infix function has to be surrounded by %%, e.g.

`:=` <- function(a, b) 
 paste(a,b)


"abc" := "def"

r data.table colon-equals

edited Nov 13 '18 at 12:56

Moody_Mudskipper

22.7k32964

asked Oct 9 '14 at 2:37

xiaodai

4,110114871

edited Nov 13 '18 at 12:56

Moody_Mudskipper

22.7k32964

asked Oct 9 '14 at 2:37

xiaodai

4,110114871

edited Nov 13 '18 at 12:56

Moody_Mudskipper

22.7k32964

edited Nov 13 '18 at 12:56

Moody_Mudskipper

22.7k32964

edited Nov 13 '18 at 12:56

Moody_Mudskipper

22.7k32964

asked Oct 9 '14 at 2:37

xiaodai

4,110114871

asked Oct 9 '14 at 2:37

xiaodai

4,110114871

asked Oct 9 '14 at 2:37

xiaodai

4,110114871

5

Please take a look at the data.table documentation, starting maybe with the FAQ.

– Dirk Eddelbuettel
Oct 9 '14 at 2:39

1

@DirkEddelbuettel. I understand how it's used in data.table. But the fact that R allows for such an operator to be defined and not cause a syntax error is what intrigued me. It's a fundamental question about R and maybe how it parses code.

– xiaodai
Oct 9 '14 at 2:44

2

AFAIK it is local to data.table and only works with the [ subsetting. So your question is off-base (not an R quirk) which is why I sent you to the data.table docs which discuss this.

– Dirk Eddelbuettel
Oct 9 '14 at 2:49

1

@DirkEddelbuettel I think you are missing my point. I can define a funciton using := <- function(a,b) paste(a,b); and I can use it by doing "abc" := "def"! But all other infix function are in the form of %in.fn%?. Why?

– xiaodai
Oct 9 '14 at 2:57

2

This Q&A from Matt might be very relevant here as well.

– Arun
Oct 9 '14 at 6:51

add a comment |

5

Please take a look at the data.table documentation, starting maybe with the FAQ.

– Dirk Eddelbuettel
Oct 9 '14 at 2:39

1

@DirkEddelbuettel. I understand how it's used in data.table. But the fact that R allows for such an operator to be defined and not cause a syntax error is what intrigued me. It's a fundamental question about R and maybe how it parses code.

– xiaodai
Oct 9 '14 at 2:44

2

AFAIK it is local to data.table and only works with the [ subsetting. So your question is off-base (not an R quirk) which is why I sent you to the data.table docs which discuss this.

– Dirk Eddelbuettel
Oct 9 '14 at 2:49

1

@DirkEddelbuettel I think you are missing my point. I can define a funciton using := <- function(a,b) paste(a,b); and I can use it by doing "abc" := "def"! But all other infix function are in the form of %in.fn%?. Why?

– xiaodai
Oct 9 '14 at 2:57

2

This Q&A from Matt might be very relevant here as well.

– Arun
Oct 9 '14 at 6:51

Please take a look at the data.table documentation, starting maybe with the FAQ.

– Dirk Eddelbuettel
Oct 9 '14 at 2:39

@DirkEddelbuettel. I understand how it's used in data.table. But the fact that R allows for such an operator to be defined and not cause a syntax error is what intrigued me. It's a fundamental question about R and maybe how it parses code.

– xiaodai
Oct 9 '14 at 2:44

AFAIK it is local to data.table and only works with the [ subsetting. So your question is off-base (not an R quirk) which is why I sent you to the data.table docs which discuss this.

– Dirk Eddelbuettel
Oct 9 '14 at 2:49

@DirkEddelbuettel I think you are missing my point. I can define a funciton using := <- function(a,b) paste(a,b); and I can use it by doing "abc" := "def"! But all other infix function are in the form of %in.fn%?. Why?

– xiaodai
Oct 9 '14 at 2:57

This Q&A from Matt might be very relevant here as well.

– Arun
Oct 9 '14 at 6:51

add a comment |

2 Answers
2

active

oldest

votes

It is something that the base R parser recognizes and seems to parse as a left assign (at least in terms or order of operations and such). See the C source code for more details.

as.list(parse(text="a:=3")[[1]])
# [[1]]
# `:=`
# 
# [[2]]
# a
# 
# [[3]]
# [1] 3

As far as I can tell it's undocumented (as far as base R is concerned). But it is a function/operator you can change the behavior of

`:=`<-function(a,b) a+b
3 := 7
# [1] 10

As you can see there really isn't anything special about the ":" part itself. It just happens to be the start of a compound token.

answered Oct 9 '14 at 2:54

MrFlick

122k11140168

12

@BondedDust But data.table doesn't own that function. They really are relying on something lying around in the parser over which they have no control. Another package could redefine c if they like. That's essentially what they are doing (there just happens to be no default implementation for :=)

– MrFlick
Oct 9 '14 at 3:00

1

The OP did mention the data.table package so I would argue that in that setting := is "owned" by data.table. I suppose I would agree with you if you argued that the period-function ( .() ) has separate (localalized) "ownership" by bquote and plyr functions. And to my point, redefining c() is also a really bad idea.

– 42-
Oct 9 '14 at 3:02

15

@BondedDust But let's say that data.table wanted to define ~= as new infix operator. They cannot do that because the parser would not recognize it (ie parse(text="a~=3") would generate an error). The "specialness" of := is completely independent of the the data.table package. It is a big exception to the rule that custom infix operators require % which seems to be the spirit of the OP's question as I read it.

– MrFlick
Oct 9 '14 at 3:07

3

+1 Nice job digging into the source!

– Joshua Ulrich
Oct 9 '14 at 15:19

1

I'm guessing at some point R Core was thinking of implementing := as an alternative to <- since some languages use that as the assignment operator, but then dropped the idea after the parser was written.

– BrodieG
Apr 19 '17 at 22:03

|
show 4 more comments

It's not just a colon operator but rather := is a single operator formed by the colon and equal sign (just as the combination of "<" and "-" forms the assignment operator in base R). The := operator is an infix function that is defined to be part of the evaluation of the "j" argument inside the [.data.table function. It creates or assigns a value to a column designated by its LHS argument using the result of evaluating its RHS.

answered Oct 9 '14 at 2:56

42-

213k15260401

4

To the downvoters who are not explaining their concerns. This answer was written to respond to the question as originally written. You can see its original title and content by clicking on the "edited ...." link.

– 42-
Oct 9 '14 at 17:39

add a comment |

Your Answer

StackExchange.ifUsing("editor", function ()
StackExchange.using("externalEditor", function ()
StackExchange.using("snippets", function ()
StackExchange.snippets.init();
);
);
, "code-snippets");

StackExchange.ready(function()
var channelOptions =
tags: "".split(" "),
id: "1"
;
initTagRenderer("".split(" "), "".split(" "), channelOptions);

StackExchange.using("externalEditor", function()
// Have to fire editor after snippets, if snippets enabled
if (StackExchange.settings.snippets.snippetsEnabled)
StackExchange.using("snippets", function()
createEditor();
);

else
createEditor();

);

function createEditor()
StackExchange.prepareEditor(
heartbeatType: 'answer',
autoActivateHeartbeat: false,
convertImagesToLinks: true,
noModals: true,
showLowRepImageUploadWarning: true,
reputationToPostImages: 10,
bindNavPrevention: true,
postfix: "",
imageUploader:
brandingHtml: "Powered by u003ca class="icon-imgur-white" href="https://imgur.com/"u003eu003c/au003e",
contentPolicyHtml: "User contributions licensed under u003ca href="https://creativecommons.org/licenses/by-sa/3.0/"u003ecc by-sa 3.0 with attribution requiredu003c/au003e u003ca href="https://stackoverflow.com/legal/content-policy"u003e(content policy)u003c/au003e",
allowUrls: true
,
onDemand: true,
discardSelector: ".discard-answer"
,immediatelyShowMarkdownHelp:true
);

);

draft saved

draft discarded

StackExchange.ready(
function ()
StackExchange.openid.initPostLogin('.new-post-login', 'https%3a%2f%2fstackoverflow.com%2fquestions%2f26269423%2fwhy-is-allowed-as-an-infix-operator%23new-answer', 'question_page');

);

Post as a guest

Name

Required, but never shown

2 Answers
2

active

oldest

votes

2 Answers
2

active

oldest

votes

It is something that the base R parser recognizes and seems to parse as a left assign (at least in terms or order of operations and such). See the C source code for more details.

as.list(parse(text="a:=3")[[1]])
# [[1]]
# `:=`
# 
# [[2]]
# a
# 
# [[3]]
# [1] 3

As far as I can tell it's undocumented (as far as base R is concerned). But it is a function/operator you can change the behavior of

`:=`<-function(a,b) a+b
3 := 7
# [1] 10

As you can see there really isn't anything special about the ":" part itself. It just happens to be the start of a compound token.

answered Oct 9 '14 at 2:54

MrFlick

122k11140168

12

@BondedDust But data.table doesn't own that function. They really are relying on something lying around in the parser over which they have no control. Another package could redefine c if they like. That's essentially what they are doing (there just happens to be no default implementation for :=)

– MrFlick
Oct 9 '14 at 3:00

1

The OP did mention the data.table package so I would argue that in that setting := is "owned" by data.table. I suppose I would agree with you if you argued that the period-function ( .() ) has separate (localalized) "ownership" by bquote and plyr functions. And to my point, redefining c() is also a really bad idea.

– 42-
Oct 9 '14 at 3:02

15

@BondedDust But let's say that data.table wanted to define ~= as new infix operator. They cannot do that because the parser would not recognize it (ie parse(text="a~=3") would generate an error). The "specialness" of := is completely independent of the the data.table package. It is a big exception to the rule that custom infix operators require % which seems to be the spirit of the OP's question as I read it.

– MrFlick
Oct 9 '14 at 3:07

3

+1 Nice job digging into the source!

– Joshua Ulrich
Oct 9 '14 at 15:19

1

I'm guessing at some point R Core was thinking of implementing := as an alternative to <- since some languages use that as the assignment operator, but then dropped the idea after the parser was written.

– BrodieG
Apr 19 '17 at 22:03

|
show 4 more comments

It is something that the base R parser recognizes and seems to parse as a left assign (at least in terms or order of operations and such). See the C source code for more details.

as.list(parse(text="a:=3")[[1]])
# [[1]]
# `:=`
# 
# [[2]]
# a
# 
# [[3]]
# [1] 3

As far as I can tell it's undocumented (as far as base R is concerned). But it is a function/operator you can change the behavior of

`:=`<-function(a,b) a+b
3 := 7
# [1] 10

As you can see there really isn't anything special about the ":" part itself. It just happens to be the start of a compound token.

answered Oct 9 '14 at 2:54

MrFlick

122k11140168

12

@BondedDust But data.table doesn't own that function. They really are relying on something lying around in the parser over which they have no control. Another package could redefine c if they like. That's essentially what they are doing (there just happens to be no default implementation for :=)

– MrFlick
Oct 9 '14 at 3:00

1

The OP did mention the data.table package so I would argue that in that setting := is "owned" by data.table. I suppose I would agree with you if you argued that the period-function ( .() ) has separate (localalized) "ownership" by bquote and plyr functions. And to my point, redefining c() is also a really bad idea.

– 42-
Oct 9 '14 at 3:02

15

@BondedDust But let's say that data.table wanted to define ~= as new infix operator. They cannot do that because the parser would not recognize it (ie parse(text="a~=3") would generate an error). The "specialness" of := is completely independent of the the data.table package. It is a big exception to the rule that custom infix operators require % which seems to be the spirit of the OP's question as I read it.

– MrFlick
Oct 9 '14 at 3:07

3

+1 Nice job digging into the source!

– Joshua Ulrich
Oct 9 '14 at 15:19

1

I'm guessing at some point R Core was thinking of implementing := as an alternative to <- since some languages use that as the assignment operator, but then dropped the idea after the parser was written.

– BrodieG
Apr 19 '17 at 22:03

|
show 4 more comments

It is something that the base R parser recognizes and seems to parse as a left assign (at least in terms or order of operations and such). See the C source code for more details.

as.list(parse(text="a:=3")[[1]])
# [[1]]
# `:=`
# 
# [[2]]
# a
# 
# [[3]]
# [1] 3

As far as I can tell it's undocumented (as far as base R is concerned). But it is a function/operator you can change the behavior of

`:=`<-function(a,b) a+b
3 := 7
# [1] 10

As you can see there really isn't anything special about the ":" part itself. It just happens to be the start of a compound token.

answered Oct 9 '14 at 2:54

MrFlick

122k11140168

It is something that the base R parser recognizes and seems to parse as a left assign (at least in terms or order of operations and such). See the C source code for more details.

as.list(parse(text="a:=3")[[1]])
# [[1]]
# `:=`
# 
# [[2]]
# a
# 
# [[3]]
# [1] 3

As far as I can tell it's undocumented (as far as base R is concerned). But it is a function/operator you can change the behavior of

`:=`<-function(a,b) a+b
3 := 7
# [1] 10

As you can see there really isn't anything special about the ":" part itself. It just happens to be the start of a compound token.

answered Oct 9 '14 at 2:54

MrFlick

122k11140168

answered Oct 9 '14 at 2:54

MrFlick

122k11140168

answered Oct 9 '14 at 2:54

MrFlick

122k11140168

answered Oct 9 '14 at 2:54

MrFlick

122k11140168

12

@BondedDust But data.table doesn't own that function. They really are relying on something lying around in the parser over which they have no control. Another package could redefine c if they like. That's essentially what they are doing (there just happens to be no default implementation for :=)

– MrFlick
Oct 9 '14 at 3:00

1

The OP did mention the data.table package so I would argue that in that setting := is "owned" by data.table. I suppose I would agree with you if you argued that the period-function ( .() ) has separate (localalized) "ownership" by bquote and plyr functions. And to my point, redefining c() is also a really bad idea.

– 42-
Oct 9 '14 at 3:02

15

@BondedDust But let's say that data.table wanted to define ~= as new infix operator. They cannot do that because the parser would not recognize it (ie parse(text="a~=3") would generate an error). The "specialness" of := is completely independent of the the data.table package. It is a big exception to the rule that custom infix operators require % which seems to be the spirit of the OP's question as I read it.

– MrFlick
Oct 9 '14 at 3:07

3

+1 Nice job digging into the source!

– Joshua Ulrich
Oct 9 '14 at 15:19

1

I'm guessing at some point R Core was thinking of implementing := as an alternative to <- since some languages use that as the assignment operator, but then dropped the idea after the parser was written.

– BrodieG
Apr 19 '17 at 22:03

|
show 4 more comments

12

@BondedDust But data.table doesn't own that function. They really are relying on something lying around in the parser over which they have no control. Another package could redefine c if they like. That's essentially what they are doing (there just happens to be no default implementation for :=)

– MrFlick
Oct 9 '14 at 3:00

1

The OP did mention the data.table package so I would argue that in that setting := is "owned" by data.table. I suppose I would agree with you if you argued that the period-function ( .() ) has separate (localalized) "ownership" by bquote and plyr functions. And to my point, redefining c() is also a really bad idea.

– 42-
Oct 9 '14 at 3:02

15

@BondedDust But let's say that data.table wanted to define ~= as new infix operator. They cannot do that because the parser would not recognize it (ie parse(text="a~=3") would generate an error). The "specialness" of := is completely independent of the the data.table package. It is a big exception to the rule that custom infix operators require % which seems to be the spirit of the OP's question as I read it.

– MrFlick
Oct 9 '14 at 3:07

3

+1 Nice job digging into the source!

– Joshua Ulrich
Oct 9 '14 at 15:19

1

I'm guessing at some point R Core was thinking of implementing := as an alternative to <- since some languages use that as the assignment operator, but then dropped the idea after the parser was written.

– BrodieG
Apr 19 '17 at 22:03

@BondedDust But data.table doesn't own that function. They really are relying on something lying around in the parser over which they have no control. Another package could redefine c if they like. That's essentially what they are doing (there just happens to be no default implementation for :=)

– MrFlick
Oct 9 '14 at 3:00

The OP did mention the data.table package so I would argue that in that setting := is "owned" by data.table. I suppose I would agree with you if you argued that the period-function ( .() ) has separate (localalized) "ownership" by bquote and plyr functions. And to my point, redefining c() is also a really bad idea.

– 42-
Oct 9 '14 at 3:02

@BondedDust But let's say that data.table wanted to define ~= as new infix operator. They cannot do that because the parser would not recognize it (ie parse(text="a~=3") would generate an error). The "specialness" of := is completely independent of the the data.table package. It is a big exception to the rule that custom infix operators require % which seems to be the spirit of the OP's question as I read it.

– MrFlick
Oct 9 '14 at 3:07

+1 Nice job digging into the source!

– Joshua Ulrich
Oct 9 '14 at 15:19

I'm guessing at some point R Core was thinking of implementing := as an alternative to <- since some languages use that as the assignment operator, but then dropped the idea after the parser was written.

– BrodieG
Apr 19 '17 at 22:03

|
show 4 more comments

answered Oct 9 '14 at 2:56

42-

213k15260401

4

To the downvoters who are not explaining their concerns. This answer was written to respond to the question as originally written. You can see its original title and content by clicking on the "edited ...." link.

– 42-
Oct 9 '14 at 17:39

add a comment |

answered Oct 9 '14 at 2:56

42-

213k15260401

4

To the downvoters who are not explaining their concerns. This answer was written to respond to the question as originally written. You can see its original title and content by clicking on the "edited ...." link.

– 42-
Oct 9 '14 at 17:39

add a comment |

answered Oct 9 '14 at 2:56

42-

213k15260401

answered Oct 9 '14 at 2:56

42-

213k15260401

answered Oct 9 '14 at 2:56

42-

213k15260401

answered Oct 9 '14 at 2:56

42-

213k15260401

answered Oct 9 '14 at 2:56

42-

213k15260401

4

To the downvoters who are not explaining their concerns. This answer was written to respond to the question as originally written. You can see its original title and content by clicking on the "edited ...." link.

– 42-
Oct 9 '14 at 17:39

add a comment |

4

To the downvoters who are not explaining their concerns. This answer was written to respond to the question as originally written. You can see its original title and content by clicking on the "edited ...." link.

– 42-
Oct 9 '14 at 17:39

To the downvoters who are not explaining their concerns. This answer was written to respond to the question as originally written. You can see its original title and content by clicking on the "edited ...." link.

– 42-
Oct 9 '14 at 17:39

add a comment |

draft saved

draft discarded

Thanks for contributing an answer to Stack Overflow!

Please be sure to answer the question. Provide details and share your research!

But avoid …

Asking for help, clarification, or responding to other answers.

Making statements based on opinion; back them up with references or personal experience.

To learn more, see our tips on writing great answers.

draft saved

draft discarded

Post as a guest

Name

Required, but never shown

Name

Required, but never shown

Name

Required, but never shown

This page is only for reference, If you need detailed information, please check here

搜尋此網誌

Pfthb