R help - Mar 2011 - removing data randomly based on previous observations

Hello,

I have a dataset which contains multiple trap pulls over the course of a
fishing season.  Each trap was baited and then returned to the water to be
pulled again.  I am trying to remove that bait from the catch observed on
the next pull (the bait is the same as the catch, lobster).  The traps have
been baited differently throughout the season.

I would like to randomly remove two lobsters from the catch if the bait in
the previous trap was two, or 3, or 4, etc.

My idea was to create a new variable with the bait from the previous
observation.  To do this, I started by creating a column that a text
variable in one column (e.g., "1 short") equals 1 in the new column. 
This
is simple enough with an ifelse function.  However, that is not my only bait
left variable (e.g., I have "2 short," "3 short," etc.).  If
I try to
include any other "yes" results in the ifelse function, it has not
worked
how I have tried it so far.  For example,
>catch$s_bait <- ifelse(bait_left=="1 short"|bait_left=="2
short", c(1,2),0)
this returns repeating 1 and 2s when the bait_left variable=="2
short".
>catch$s_bait <- ifelse(c(bait_left=="4
short"|bait_left=="1 short 1 legal"|
+     bait_left=="5 short"|bait_left=="2
short"|bait_left=="1 short"|
+     bait_left=="3 short"|bait_left=="2 short 1 legal"),
4|1|5|2|1|3|2,0)

this returns a 1 for every variable that matches the bait_left variables
defined in the function.  I have also tried the previous function without
the c() around the test variable and with it around the yes variable, and
neither of those have worked either.

I can do the whole thing separately and then put it all together, however I
am trying to not do that.

Also, another option is the ability to just use ifelse functions
repetitively, however, I am not sure what to use in the "no" argument
so
that everytime I run the function, it does not replace what I have already
attempted to convert.  If I use:

catch$s_bait <- ifelse(bait_left=="1
short",1,)&ifelse(bait_left=="2
short",2,)

it results in the following error: Error in ifelse(bait_left == "1
short",
1, ) :  argument "no" is missing, with no default


Once I figure out how to create the column of bait, I need to apply that to
the next observation (same trap number).  I'm not sure how to do that, or
how to randomly select 1 or more lobsters from the catch of that trap on
that observation to remove it as it may have been bait (I do not know
exactly which were baits from the previous observation).

the first few lines of the database for an example are:
> catch[1:20,c(4:10,25:27)]
         date           bait_left             species
sex       size    N  s_bait l_bait
1    1/4/2011         4 short             Panulirus argus     Male
77       1      0     NA
2  11/18/2010       1 short 1 legal   Panulirus argus     Male     75
1      0     NA
3    9/7/2010         2 legal             Scrawled cowfish
NA      1      0     NA
4    1/4/2011         4 short             White grunt
160     1      0     NA
5    1/4/2011         4 short             Panulirus argus     Female
75      1      0     NA
6    1/4/2011         4 short             Panulirus argus     Male
56      1      0     NA
7    1/4/2011         4 short             Panulirus argus     Male
76      1      0     NA
8  10/18/2010       1 short 1 legal   Panulirus argus     Female  72
1      0     NA
9    1/4/2011         4 short             Panulirus argus     Female
70      1      0     NA
10 11/18/2010      1 short 1 legal   Panulirus argus     Male      82
1       0      NA
11 11/18/2010      1 short 1 legal   Stone crab
NA     1       0     NA
12   1/4/2011         4 short            Stone crab
NA     1       0     NA
13  11/1/2010      1 short 1 legal    Portunus gibbesii
65      1      0     NA
14 11/18/2010     1 short 1 legal    Panulirus argus     Female   72
1      0     NA
15 10/18/2010     1 short 1 legal    Panulirus argus     Male       82
1      0     NA
16  11/1/2010      1 short 1 legal    Panulirus argus     Male      79
1      0     NA
17  11/1/2010      1 short 1 legal    Panulirus argus     Male      89
1      0     NA
18  12/1/2010      1 short 1 legal    Portunus gibbesii
120      1      0     NA
19   1/4/2011         4 short            Stone crab
NA      1      0     NA
20  12/1/2010      1 short 1 legal    White grunt
140     1      0     NA

Number 1 should have a s_bait quantity of 4, and number 2 should have a
s_bait of 1 and a l_bait of 1.

Any answers or ideas or places to look for answers are greatly appreciated.

Thank you

Bryan
-------------------------------------------------
Bryan Danson, M.Sc.
Biological Scientist I
FWC - Marathon, FL

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