R For Loop with Certain conditions - r

Question

I have a dataframe (surveillance) with many variables (villages, houses, weeks). I want to eventually do a time-series analysis.
Currently for each village, there are between 1-183 weeks, each of which has several houses associated. I need the following: each village to have a single data point at each week. Thus, I need to sum up a third variable.
Example:
Village Week House Affect
A 3 7 12
B 6 3 0
C 6 2 2
A 3 9 1
A 5 8 0
A 5 2 8
C 7 19 0
C 7 2 1
I tried this and failed. How do I ask R to only sum observations with the same village and week value?
for (i in seq(along=surveillance)) {
if (surveillance$village== surveillance$village& surveillance$week== surveillance$week)
{surveillance$sumaffect <- sum(surveillance$affected)}
}
Thanks

Answer 1

No need for loop. Use ddply or similar
library(plyr)
Village = c("A","B","C","A","A","A","C","C")
Week = c(3,6,6,3,5,5,7,7)
Affect = c(12,0,2,1,0,8,0,1)
df = data.frame(Village,Week,Affect)
View(df)
result = ddply(df,.(Village,Week),summarise, val = sum(Affect))
View(result)
DF:
Village Week Affect
1 A 3 12
2 B 6 0
3 C 6 2
4 A 3 1
5 A 5 0
6 A 5 8
7 C 7 0
8 C 7 1
Result:
Village Week val
1 A 3 13
2 A 5 8
3 B 6 0
4 C 6 2
5 C 7 1

Answer 2

The function aggregate will do what you need.
dfs <- ' Village Week House Affect
1 A 3 7 12
2 B 6 3 0
3 C 6 2 2
4 A 3 9 1
5 A 5 8 0
6 A 5 2 8
7 C 7 19 0
8 C 7 2 1
'
df <- read.table(text=dfs)
Then the aggregation
> aggregate(Affect ~ Village + Week , data=df, sum)
Village Week Affect
1 A 3 13
2 A 5 8
3 B 6 0
4 C 6 2
5 C 7 1
This is an example of a split-apply-combine strategy; if you find yourself doing this often, you should investigate the dplyr (or plyr, its ancestor) or data.table as alternatives to quickly doing this sort of analysis.
EDIT: updated to use sum instead of mean