pushBack {base} | R Documentation

## Push Text Back on to a Connection

### Description

Functions to push back text lines onto a connection, and to enquire how many lines are currently pushed back.

### Usage

    pushBack(data, connection, newLine = TRUE,
             encoding = c("", "bytes", "UTF-8"))
    pushBackLength(connection)
    clearPushBack(connection)

### Arguments

`data` | a character vector.
`connection` | A connection.
`newLine` | logical. If true, a newline is appended to each string pushed back.
`encoding` | character string, partially matched. See details.

### Details

Several character strings can be pushed back on one or more occasions. The occasions form a stack, so the first line to be retrieved will be the first string from the last call to `pushBack`. Lines which are pushed back are read prior to the normal input from the connection, by the normal text-reading functions such as `readLines` and `scan`.

Pushback is only allowed for readable connections in text mode.

Not all uses of connections respect pushbacks, in particular the input connection is still wired directly, so for example parsing commands from the console and `scan("")` ignore pushbacks on `stdin`.

When character strings with a marked encoding (see `Encoding`) are pushed back they are converted to the current encoding if `encoding = ""`. This may involve representing characters as <U+xxxx> if they cannot be converted. They will be converted to UTF-8 if `encoding = "UTF-8"` or left as-is if `encoding = "bytes"`.

### Value

`pushBack` and `clearPushBack()` return nothing, invisibly.

`pushBackLength` returns the number of lines currently pushed back.

### See Also

`connections`, `readLines`.

### Examples

    zz <- textConnection(LETTERS)
    readLines(zz, 2)
    pushBack(c("aa", "bb"), zz)
    pushBackLength(zz)
    readLines(zz, 1)
    pushBackLength(zz)
    readLines(zz, 1)
    readLines(zz, 1)
    close(zz)

[Package base version 3.3.2 Index]