Select {VGAM} | R Documentation

## Select Variables for a Formula Response or the RHS of a Formula

### Description

Select variables from a data frame whose names begin with a certain character string.

### Usage

    Select(data = list(), prefix = "y",
           lhs = NULL, rhs = NULL, rhs2 = NULL, rhs3 = NULL,
           as.character = FALSE, as.formula.arg = FALSE, tilde = TRUE,
           exclude = NULL, sort.arg = TRUE)

### Arguments

`data` | A data frame or a matrix.
`prefix` | A vector of character strings, or a logical. If a character then the variables chosen from `data` begin with the value of `prefix`. If a logical then only `TRUE` is accepted and all the variables in `data` are chosen.
`lhs` | A character string. The response of a formula.
`rhs` | A character string. Included as part of the RHS a formula. Set `rhs = "0"` to suppress the intercept.
`rhs2, rhs3` | Same as `rhs` but appended to its RHS, i.e., `paste0(rhs, " + ", rhs2, " + ", rhs3)`. If used, `rhs` should be used first, and then possibly `rhs2` and then possibly `rhs3`.
`as.character` | Logical. Return the answer as a character string?
`as.formula.arg` | Logical. Is the answer a formula?
`tilde` | Logical. If `as.character` and `as.formula.arg` are both `TRUE` then include the tilde in the formula?
`exclude` | Vector of character strings. Exclude these variables explicitly.
`sort.arg` | Logical. Sort the variables?

### Details

This is meant as a utility function to avoid manually: (i) making a `cbind` call to construct a big matrix response, and (ii) constructing a formula involving a lot of terms. The savings can be made because the variables of interest begin with some prefix, e.g., with the character `"y"`.

### Value

If `as.character = FALSE` and `as.formula.arg = FALSE` then a matrix such as `cbind(y1, y2, y3)`. If `as.character = TRUE` and `as.formula.arg = FALSE` then a character string such as `"cbind(y1, y2, y3)"`.

If `as.character = FALSE` and `as.formula.arg = TRUE` then a `formula` such as `lhs ~ y1 + y2 + y3`. If `as.character = TRUE` and `as.formula.arg = TRUE` then a character string such as `"lhs ~ y1 + y2 + y3"`. See the examples below. By default, if no variables beginning the the value of `prefix` is found then a `NULL` is returned. Setting `prefix = " "` is a way of selecting no variables.

### Note

This function is a bit experimental at this stage and may change in the short future. Some of its utility may be better achieved using `subset` and its `select` argument, e.g., `subset(pdata, TRUE, select = y01:y10)`.

For some models such as `posbernoulli.t` the order of the variables in the `xij` argument is crucial, therefore care must be taken with the argument `sort.arg`. In some instances, it may be good to rename variables `y1` to `y01`, `y2` to `y02`, etc. when there are variables such as `y14`.

Currently `subsetcol()` and `Select()` are identical. One of these functions might be withdrawn in the future.

### Author(s)

T. W. Yee.

### See Also

`vglm`, `cbind`, `subset`, `formula`, `fill`.

### Examples

    Pneumo <- pneumo
    colnames(Pneumo) <- c("y1", "y2", "y3", "x2")  # The "y" variables are response
    Pneumo$x1 <- 1; Pneumo$x3 <- 3; Pneumo$x <- 0; Pneumo$x4 <- 4  # Add these

    Select(data = Pneumo)  # Same as with(Pneumo, cbind(y1, y2, y3))
    Select(Pneumo, "x")
    Select(Pneumo, "x", sort = FALSE, as.char = TRUE)
    Select(Pneumo, "x", exclude = "x1")
    Select(Pneumo, "x", exclude = "x1", as.char = TRUE)
    Select(Pneumo, c("x", "y"))
    Select(Pneumo, "z")  # Now returns a NULL
    Select(Pneumo, " ")  # Now returns a NULL
    Select(Pneumo, prefix = TRUE, as.formula = TRUE)
    Select(Pneumo, "x", exclude = c("x3", "x1"), as.formula = TRUE,
           lhs = "cbind(y1, y2, y3)", rhs = "0")
    Select(Pneumo, "x", exclude = "x1", as.formula = TRUE, as.char = TRUE,
           lhs = "cbind(y1, y2, y3)", rhs = "0")

    # Now a 'real' example:
    Huggins89table1 <- transform(Huggins89table1, x3.tij = t01)
    tab1 <- subset(Huggins89table1,
                   rowSums(Select(Huggins89table1, "y")) > 0)
    # Same as
    # subset(Huggins89table1, y1 + y2 + y3 + y4 + y5 + y6 + y7 + y8 + y9 + y10 > 0)

    # Long way to do it:
    fit.th <-
       vglm(cbind(y01, y02, y03, y04, y05, y06, y07, y08, y09, y10) ~ x2 + x3.tij,
            xij = list(x3.tij ~ t01 + t02 + t03 + t04 + t05 + t06 + t07 + t08 +
                                t09 + t10 - 1),
            posbernoulli.t(parallel.t = TRUE ~ x2 + x3.tij),
            data = tab1, trace = TRUE,
            form2 = ~ x2 + x3.tij + t01 + t02 + t03 + t04 + t05 + t06 + t07 + t08 +
                                    t09 + t10)
    # Short way to do it:
    Fit.th <- vglm(Select(tab1, "y") ~ x2 + x3.tij,
                   xij = list(Select(tab1, "t", as.formula = TRUE,
                                     sort = FALSE, lhs = "x3.tij", rhs = "0")),
                   posbernoulli.t(parallel.t = TRUE ~ x2 + x3.tij),
                   data = tab1, trace = TRUE,
                   form2 = Select(tab1, prefix = TRUE, as.formula = TRUE))

[Package VGAM version 1.1-1 Index]