[SPARK-12903] [SparkR] Add covar_samp and covar_pop for SparkR

[yanboliang]

Add covar_samp and covar_pop for SparkR.
Should we also provide cov alias for covar_samp? There is cov implementation at stats.R which masks stats::cov already, but may bring to breaking API change.

cc @sun-rui @felixcheung @shivaram

[yanboliang]

Jenkins, test this please.

[SparkQA]

Test build #49680 has finished for PR 10829 at commit 7c3e718.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[sun-rui]

The number of parameters are fixed to be 2. No need to support varargs.
Something like:
setGeneric("covar_samp", function(col1, col2) {standardGeneric("covar_samp") })

[sun-rui]

@yanboliang, please add a new method for "cov", which is alias to "covar_samp"

[SparkQA]

Test build #49898 has finished for PR 10829 at commit ebde36a.

• This patch fails R style tests.
• This patch merges cleanly.
• This patch adds no public classes.

[yanboliang]

@sun-rui Thanks for your comments. There is a cov implementation at stats.R with different signature:

setMethod("cov",
          signature(x = "DataFrame", col1 = "character", col2 = "character"),
          function(x, col1, col2) {
            statFunctions <- callJMethod(x@sdf, "stat")
            callJMethod(statFunctions, "cov", col1, col2)
          })

And this cov has masked stats::cov, so we can not add cov as alias to covar_samp.

[SparkQA]

Test build #49900 has finished for PR 10829 at commit 16d3fad.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[sun-rui]

@yanboliang, we can have different methods for a generic function. You can try something like follows:

setGeneric("cov", function(x, ...) {standardGeneric("cov") })

setMethod("cov",
          signature(x = "DataFrame", ...),

setMethod("cov",
          signature(x = "characterOrColumn", ...),

[SparkQA]

Test build #49913 has finished for PR 10829 at commit eca64a6.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[SparkQA]

Test build #49954 has finished for PR 10829 at commit a036f95.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[sun-rui]

LGTM

[felixcheung]

looks good

[shivaram]

LGTM. Thanks @yanboliang - Merging this to master