# [R] difference

Ashta sewashm at gmail.com
Sat Oct 29 04:28:44 CEST 2016

```Hi all thank you very much for your help. Worked very well for that
data set. I just found out that one of the data sets have another
level and do the same thing, I want to calculate the difference
between successive row values (num)  to the first row value within
city and year.

city, year, num
1, 2001,    25
1, 2001,    75
1, 2001,    150
1, 2002,    35
1, 2002,    65
1, 2002,    120
2, 2001,    25
2, 2001,    95
2, 2001,    150
2, 2002,    35
2, 2002,    110
2, 2002,    120

The result will be

city,year,num,Diff
1, 2001,    25, 0
1, 2001,    75, 50
1, 2001,    150, 125
1, 2002,    35, 0
1, 2002,    65, 30
1, 2002,    120, 85
2, 2001,    25, 0
2, 2001,    95, 70
2, 2001,    150, 125
2, 2002,    35, 0
2, 2002,    110, 75
2, 2002,    120, 85

Thank you again

On Fri, Oct 28, 2016 at 4:08 AM, P Tennant <philipt900 at iinet.net.au> wrote:
> Hi,
>
> You could use an anonymous function to operate on each `year-block' of your
> dataset, then assign the result as a new column:
>
> d <- data.frame(year=c(rep(2001, 3), rep(2002, 3)),
>                 num=c(25,75,150,30,85,95))
>
> d\$diff <- unlist(by(d\$num, d\$year, function(x) x - x[1]))
> d
>
>   year num diff
> 1 2001  25    0
> 2 2001  75   50
> 3 2001 150  125
> 4 2002  30    0
> 5 2002  85   55
> 6 2002  95   65
>
>
> Philip
>
>
> On 28/10/2016 3:20 PM, Ashta wrote:
>>
>> Hi all,
>>
>> I want to calculate the difference  between successive row values to
>> the first row value within year.
>> How do I get that?
>>
>>   Here is    the sample of data
>> Year   Num
>> 2001    25
>> 2001    75
>> 2001   150
>> 2002    30
>> 2002    85
>> 2002    95
>>
>> Desired output
>> Year   Num  diff
>> 2001    25       0
>> 2001    75      50
>> 2001  150    125
>> 2002    30        0
>> 2002    85      55
>> 2002    95      65
>>
>> Thank you.
>>
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