| head | R Documentation |
Returns the first or last parts of a vector, matrix, table, data frame or function. Since head() and tail() are generic functions, they may also have been extended to other classes.
head(x, ...) ## Default S3 method: head(x, n = 6L, ...) ## S3 method for class 'matrix' head(x, n = 6L, ...) # is exported as head.matrix() ## NB: The methods for 'data.frame' and 'array' are identical to the 'matrix' one ## S3 method for class 'ftable' head(x, n = 6L, ...) ## S3 method for class 'function' head(x, n = 6L, ...) tail(x, ...) ## Default S3 method: tail(x, n = 6L, keepnums = FALSE, addrownums, ...) ## S3 method for class 'matrix' tail(x, n = 6L, keepnums = TRUE, addrownums, ...) # exported as tail.matrix() ## NB: The methods for 'data.frame', 'array', and 'table' ## are identical to the 'matrix' one ## S3 method for class 'ftable' tail(x, n = 6L, keepnums = FALSE, addrownums, ...) ## S3 method for class 'function' tail(x, n = 6L, ...)
x | an object |
n | an integer vector of length up to |
keepnums | in each dimension, if no names in that dimension are present, create them using the indices included in that dimension. Ignored if |
addrownums | deprecated - |
... | arguments to be passed to or from other methods. |
For vector/array based objects, head() (tail()) returns a subset of the same dimensionality as x, usually of the same class. For historical reasons, by default they select the first (last) 6 indices in the first dimension ("rows") or along the length of a non-dimensioned vector, and the full extent (all indices) in any remaining dimensions. head.matrix() and tail.matrix() are exported.
The default and array(/matrix) methods for head() and tail() are quite general. They will work as is for any class which has a dim() method, a length() method (only required if dim() returns NULL), and a [ method (that accepts the drop argument and can subset in all dimensions in the dimensioned case).
For functions, the lines of the deparsed function are returned as character strings.
When x is an array(/matrix) of dimensionality two and more, tail() will add dimnames similar to how they would appear in a full printing of x for all dimensions k where n[k] is specified and non-missing and dimnames(x)[[k]] (or dimnames(x) itself) is NULL. Specifically, the form of the added dimnames will vary for different dimensions as follows:
k=1 (rows): "[n,]" (right justified with whitespace padding)
k=2 (columns): "[,n]" (with no whitespace padding)
k>2 (higher dims): "n", i.e., the indices as character values
Setting keepnums = FALSE suppresses this behaviour.
As data.frame subsetting (‘indexing’) keeps attributes, so do the head() and tail() methods for data frames.
An object (usually) like x but generally smaller. Hence, for arrays, the result corresponds to x[.., drop=FALSE]. For ftable objects x, a transformed format(x).
For array inputs the output of tail when keepnums is TRUE, any dimnames vectors added for dimensions >2 are the original numeric indices in that dimension as character vectors. This means that, e.g., for 3-dimensional array arr, tail(arr, c(2,2,-1))[ , , 2] and tail(arr, c(2,2,-1))[ , , "2"] may both be valid but have completely different meanings.
Patrick Burns, improved and corrected by R-Core. Negative argument added by Vincent Goulet. Multi-dimension support added by Gabriel Becker.
head(letters) head(letters, n = -6L) head(freeny.x, n = 10L) head(freeny.y) head(iris3) head(iris3, c(6L, 2L)) head(iris3, c(6L, -1L, 2L)) tail(letters) tail(letters, n = -6L) tail(freeny.x) ## the bottom-right "corner" : tail(freeny.x, n = c(4, 2)) tail(freeny.y) tail(iris3) tail(iris3, c(6L, 2L)) tail(iris3, c(6L, -1L, 2L)) ## iris with dimnames stripped a3d <- iris3 ; dimnames(a3d) <- NULL tail(a3d, c(6, -1, 2)) # keepnums = TRUE is default here! tail(a3d, c(6, -1, 2), keepnums = FALSE) ## data frame w/ a (non-standard) attribute: treeS <- structure(trees, foo = "bar") (n <- nrow(treeS)) stopifnot(exprs = { # attribute is kept identical(htS <- head(treeS), treeS[1:6, ]) identical(attr(htS, "foo") , "bar") identical(tlS <- tail(treeS), treeS[(n-5):n, ]) ## BUT if I use "useAttrib(.)", this is *not* ok, when n is of length 2: ## --- because [i,j]-indexing of data frames *also* drops "other" attributes .. identical(tail(treeS, 3:2), treeS[(n-2):n, 2:3] ) }) tail(library) # last lines of function head(stats::ftable(Titanic)) ## 1d-array (with named dim) : a1 <- array(1:7, 7); names(dim(a1)) <- "O2" stopifnot(exprs = { identical( tail(a1, 10), a1) identical( head(a1, 10), a1) identical( head(a1, 1), a1 [1 , drop=FALSE] ) # was a1[1] in R <= 3.6.x identical( tail(a1, 2), a1[6:7]) identical( tail(a1, 1), a1 [7 , drop=FALSE] ) # was a1[7] in R <= 3.6.x }) Add the following code to your website.
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