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myFunction <- function(x){ for(i in 0:23) { if(i<10){ timer <- paste("T0",i, sep ="") tempr <- grepl(timer,x) tempr <- table(tempr)["TRUE"] timeCount <- c(timer,tempr) } else{ timer <- paste("T",i,sep="") tempr <- grepl(timer,x) tempr <- table(tempr)["TRUE"] timeCount <- c(timer,tempr) } } return(timeCount) } tmp <- myFunction(test$timestamp)

What I am trying to do is in the function I am looping for 23 times and generating two values timer (contains values like T00, T01, T02...T23) and tempr (contains values like 23, 24, 25...). Now I want to store all the 23 respective values for the two variables in a dataframe so that my final output is 

TimeZ Freq T00 33 T01 12 T02 22 T04 34 . . . . . . T23 23
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There are several lessons to be learned here. You can use seq(0,23) to construct the initial sequence, make a 1-row dataframe for each value, and use do.call with rbind to join them. You don't need paste or the if because you can use sprintf to add the 0's as needed. Finally, you don't need to use table on your grepl because which does the same thing and is more succinct.

myFunction <- function(x) do.call(rbind, lapply(seq(0, 23), function(i) data.frame(TimeZ = timer <- sprintf("T%02d", i), Freq = which(grepl(timer, x))[1]) )) # example: myFunction(sprintf("T%02d", sample(seq(0,23)))) # TimeZ Freq # 1 T00 14 # 2 T01 24 # 3 T02 2 # 4 T03 7 # 5 T04 19 # ---- # TimeZ Freq # 20 T19 9 # 21 T20 21 # 22 T21 22 # 23 T22 15 # 24 T23 13

If we are allowed to assume that x contains T00, T01, ..., T23 (every single one of them), then it becomes even shorter:

 myFunction <- function(x) data.frame(TimeZ = sprintf("T%02d", seq(0, 23)), Freq = order(x))
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X will be a value like "2008-08-13T23:04:10Z".... So I was using grepl to see if T00, T01...is present or not...
Your code is efficient and working but I have to make some minor changes before getting the desired result.. I have to write "Freq = table((grepl(timer, x)))["TRUE"]" in place of "Freq = which(grepl(timer, x))[1]"

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