r - function works perfectly fine outside ddply but throws an error inside ddply -

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i trying row-wise operation (by group) on data frame. when run function 1 group, runs fine. however, when put function inside ddply run groups, throws error - argument of length zero.

the function when run separately on data frame 'test':

for (i in 1:(nrow(test) - 5)) {    if (i <= 5) {     test[i, "mppalert"] <- 0       }    firstmpp <- test[i, "tagmppsearchcount"]   lastmpp <- test[i+5, "tagmppsearchcount"]    if ((lastmpp - firstmpp) >= 10) {     test[i+5, "mppalert"] <- 1       } else {     test[i+5, "mppalert"] <- 0       }  }

the above function inside ddply throws error:

error in if (lastmpp - firstmpp >= 10) { : argument of length 0

below ddply code:

mpp_fn <- function(x) {      (i in 1:(nrow(x) - 5)) {      if (i <= 5) {       x[i, "mppalert"] <- 0         }      firstmpp <- x[i, "tagmppsearchcount"]     lastmpp <- x[i+5, "tagmppsearchcount"]      if (lastmpp - firstmpp >= 10) {       x[i+5, "mppalert"] <- 1         } else {       x[i+5, "mppalert"] <- 0         }    }  }  result <- ddply(data, c("shelterid", "inverterid"), mpp_fn(x))

in above code, values of firstmpp , lastmpp resolving null , therefore error, why happening (when runs fine outside ddply)?

update: here output of dput(data):

structure(list(shelterid = c("sh02", "sh02", "sh02", "sh02",  "sh02", "sh02", "sh02", "sh02", "sh02", "sh02", "sh02", "sh02",  "sh02", "sh02", "sh02", "sh02", "sh02", "sh02", "sh02", "sh02",  "sh02", "sh02", "sh02", "sh02", "sh02", "sh02", "sh02", "sh02",  "sh02", "sh02", "sh02", "sh02", "sh02", "sh02", "sh02", "sh02",  "sh03", "sh03", "sh03", "sh03", "sh03", "sh03", "sh03", "sh03",  "sh03", "sh03", "sh03", "sh03", "sh03", "sh03", "sh03", "sh03",  "sh03", "sh03", "sh03", "sh03", "sh03", "sh03", "sh03", "sh03",  "sh03", "sh03", "sh03", "sh03", "sh03", "sh03", "sh03", "sh03",  "sh03", "sh03", "sh03", "sh03"), inverterid = c("i1", "i1", "i1",  "i1", "i1", "i1", "i1", "i1", "i1", "i1", "i1", "i1", "i1", "i1",  "i1", "i1", "i1", "i1", "i2", "i2", "i2", "i2", "i2", "i2", "i2",  "i2", "i2", "i2", "i2", "i2", "i2", "i2", "i2", "i2", "i2", "i2",  "i1", "i1", "i1", "i1", "i1", "i1", "i1", "i1", "i1", "i1", "i1",  "i1", "i1", "i1", "i1", "i1", "i1", "i1", "i2", "i2", "i2", "i2",  "i2", "i2", "i2", "i2", "i2", "i2", "i2", "i2", "i2", "i2", "i2",  "i2", "i2", "i2"), tagmppsearchcount = c(0, 0, 0, 0, 0, 0, 0,  2, 0, 0, 3, 0, 0, 3, 0, 0, 3, 0, 3, 0, 0, 0, 0, 0, 0, 0, 0, 0,  0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0,  0, 0, 0, 0, 0, 3, 0, 3, 0, 0, 4, 0, 0, 4, 0, 4, 0, 0, 5, 0, 0,  5, 0)), .names = c("shelterid", "inverterid", "tagmppsearchcount" ), row.names = c(350l, 351l, 352l, 353l, 354l, 355l, 356l, 357l,  358l, 359l, 360l, 361l, 362l, 363l, 364l, 365l, 366l, 367l, 494l,  495l, 496l, 497l, 498l, 499l, 500l, 501l, 502l, 503l, 504l, 505l,  506l, 507l, 508l, 509l, 510l, 511l, 638l, 639l, 640l, 641l, 642l,  643l, 644l, 645l, 646l, 647l, 648l, 649l, 650l, 651l, 652l, 653l,  654l, 655l, 782l, 783l, 784l, 785l, 786l, 787l, 788l, 789l, 790l,  791l, 792l, 793l, 794l, 795l, 796l, 797l, 798l, 799l), class = "data.frame")

here dplyr solution. doesn't require explicit loops

library(dplyr) data %>%    group_by(shelterid, inverterid) %>%    mutate(     first = lag(tagmppsearchcount, 5),      mppalert = ifelse(       is.na(first),       0,       ifelse(         tagmppsearchcount - first > 10,          1,          0       )     )   )

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