icd9GetMajor: Get major (three-digit) part of ICD-9 codes
In icd9: Tools for Working with ICD-9 Codes, and Finding Comorbidities

This is reasonably fast, but calculates all the minors, then throws away the result.

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icd9GetMajor(icd9, isShort)

icd9GetMajorShort(icd9Short)

`icd9`	is a character vector or factor of ICD-9 codes. If fewer than five characters is given in a code, then the digits are greedily assigned to hundreds, then tens, then units, before the decimal parts. E.g. "10" becomes "010", not "0010".
`isShort`	single logical value which determines whether the ICD-9 code provided is in short (TRUE) or decimal (FALSE) form. Where reasonable, this is guessed from the input data.