R - apply function across every row

Question

I have the following function to get the week from a given date

getWeek <- function (year, month, day) { date <- as.Date(paste(year, month, day, sep = "-"), "%Y-%b-%d") week <- format(date, "%W") return(week) }

I want to apply the above function to following data.frame x and create a new column in x. I tried using mapply but it gives me NA.

> dput(x) structure(list(year = c(2003L, 2010L, 2012L, 2012L, 2007L), month = structure(c(3L, 10L, 9L, 8L, 6L), .Label = c(" Apr", " Aug", " Dec", " Feb", " Jan", " Jul", " Jun", " Mar", " May", " Nov", " Oct", " Sep" ), class = "factor"), day = c(4L, 3L, 25L, 26L, 18L), Humidity = structure(c(38L, 71L, 73L, 49L, 87L), .Label = c("10", "100", "11", "12", "13", "14", "15", "16", "17", "18", "19", "20", "21", "22", "23", "24", "25", "26", "27", "28", "29", "30", "31", "32", "33", "34", "35", "36", "37", "38", "39", "40", "41", "42", "43", "44", "45", "46", "47", "48", "49", "50", "51", "52", "53", "54", "55", "56", "57", "58", "59", "6", "60", "61", "62", "63", "64", "65", "66", "67", "68", "69", "7", "70", "71", "72", "73", "74", "75", "76", "77", "78", "79", "8", "80", "81", "82", "83", "84", "85", "86", "87", "88", "89", "9", "90", "91", "92", "93", "94", "96", "97", "N/A" ), class = "factor")), .Names = c("year", "month", "day", "Humidity" ), row.names = c(2605L, 80763L, 108420L, 106512L, 54342L), class = "data.frame") > mapply(getWeek, x$year, x$month, x$day) [1] NA NA NA NA NA

I need getWeek to be applied to each row in x? Is mapply the right one to use?

Justin · Accepted Answer · 2013-05-29 15:28:56Z

Since your function is already vectorized, you don't need to use apply at all. Instead just call it with the correct variables (once you fix the error below, your mapply solution will work as well):

with(x, getWeek(year, month, day))

However, you have a leading space before each month. So either you need to use the format string %Y- %b-%d' or remove it:x$month <- gsub('^ ', '', x$month)`

ppham27 · Accepted Answer · 2013-05-29 15:44:27Z

Your getWeek function is broken.

It should look something like this:

getWeek <- function (year, month, day) { date <- as.Date(paste(year, month, day, sep = "-"), "%Y- %b-%d") week <- format(date, "%W") return(week) }

You need to account for the leading space in your months.

Of course,

x$month <- substring(x$month,2)

would fix your issue as well.

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