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A general description is here.
‘nchar(x, *)’'s argument ‘keepNA’ governing how the result for ‘NA’s in ‘x’ is determined, gets a new default ‘keepNA = NA’ which returns ‘NA’ where ‘x’ is ‘NA’, except for ‘type = "width"’ which still returns ‘2’, the formatting / printing width of ‘NA’.
‘nchar(x, *)’ and ‘nzchar(x)’ gain a new argument ‘keepNA’ which governs how the result for ‘NA’s in ‘x’ is determined. For ‘nzchar()’ in general and ‘nchar()’ in the R 3.2.x series, the default remains ‘FALSE’ which is fully back compatible. From R 3.3.0, ‘nchar()’'s default will change to ‘keepNA = NA’ and you are advised to consider this for code portability.