expression - What is "{" class in R? -


here code:

mf = function(..., expr) {     expr = substitute(expr)     print(class(expr))     print(str(expr))     expr } mf(a = 1, b = 2, expr = {matrix(na, 4, 4)})

output:

[1] "{" length 2 {  matrix(na, 4, 4) }  - attr(*, "srcref")=list of 2   ..$ :class 'srcref'  atomic [1:8] 1 25 1 25 25 25 1 1   .. .. ..- attr(*, "srcfile")=classes 'srcfilecopy', 'srcfile' <environment: 0x7fbcdbce3860>    ..$ :class 'srcref'  atomic [1:8] 1 26 1 41 26 41 1 1   .. .. ..- attr(*, "srcfile")=classes 'srcfilecopy', 'srcfile' <environment: 0x7fbcdbce3860>   - attr(*, "srcfile")=classes 'srcfilecopy', 'srcfile' <environment: 0x7fbcdbce3860>   - attr(*, "wholesrcref")=class 'srcref'  atomic [1:8] 1 0 1 42 0 42 1 1   .. ..- attr(*, "srcfile")=classes 'srcfilecopy', 'srcfile' <environment: 0x7fbcdbce3860>  null { matrix(na, 4, 4) }

apparently result of substitute(expr) produces of class "{". class exactly? why {matrix(na, 4, 4)} of length 2? these strange attrs mean?

the { class block of code. looking @ classes, note difference between these

mf(a = 1, b = 2, expr = {matrix(na, 4, 4)}) # [1] "{" mf(a = 1, b = 2, expr = matrix(na, 4, 4)) # [1] "call"

a class of { can hold multiple statements. length() indicates how many statements in block (including start of block). example

length(quote({matrix(na, 4, 4)})) # [1] 2 length(quote({matrix(na, 4, 4); matrix(na,3,3)})) # [1] 3 length(quote({})) # [1] 1

the attributes "srcref" , "srcfile" how r tracks functions defined trying give informative error messages. can see ?srcfile page more information that.


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