如何在R中的二进制文件中替换值？

Question

我必须使用（自定义）二进制文件格式。数据对于我的RAM来说太大了，我只需要导入其中的一小部分，进行一些计算并用新值覆盖/替换该部分（因此，我不想导入所有内容，更改特定部分并编写所有内容返回）。

我尝试了seek和writeBin的组合，但这会生成一个小文件，其新值以零开头：

fn <- tempfile()

writeBin(1L:3L, fn, size = 1L)
readBin(fn, what = "integer", size = 1L, n = 3L)
#> [1] 1 2 3

fh <- file(fn, "wb")
isSeekable(fh)
#> [1] TRUE
seek(fh, 1L, origin = "start", rw = "write")
#> [1] 0
# swap the sign of the second value
writeBin(-2L, fh, size = 1L)
close(fh)

readBin(fn, what = "integer", size = 1L, n = 3L)
#> [1]  0 -2

unlink(fn)

---

使用ab模式附加到文件也无济于事：

fn <- tempfile()

writeBin(1L:3L, fn, size = 1L)
readBin(fn, what = "integer", size = 1L, n = 3L)
#> [1] 1 2 3

fh <- file(fn, "ab")
isSeekable(fh)
#> [1] TRUE
seek(fh, 1L, origin = "start", rw = "write")
#> [1] 3
# swap the sign of the second value
writeBin(-2L, fh, size = 1L)
close(fh)

readBin(fn, what = "integer", size = 1L, n = 3L)
#> [1] 1 2 3

unlink(fn)

我的预期输出为1 -2 3。

R中是否有此方法，还是我必须使用C？

Answer 1

经过一番反复尝试后，我自己找到了解决方案：使用模式"r+b"支持数据替换。

fn <- tempfile()

writeBin(1L:3L, fn, size = 1L)
readBin(fn, what = "integer", size = 1L, n = 3L)
#> [1] 1 2 3

fh <- file(fn, "r+b")
isSeekable(fh)
#> [1] TRUE
seek(fh, 1L, origin = "start", rw = "write")
#> [1] 0
# swap the sign of the second value
writeBin(-2L, fh, size = 1L)
close(fh)

readBin(fn, what = "integer", size = 1L, n = 3L)
#> [1]  1 -2  3

unlink(fn)