R conditional sum in data frame depending on word in a column

I have a data frame containing words and numeric entries. I want to sum all the entries for which the row entry in the word now is identical.

District name   Population   Child birth rate
A               30,000       .7
A               20,000       .5
B               10,000       .09
B               15,000       .6
C               80,000       .007

I want to sum up the population and child birth rates on the district level. I tried using lapply and sum, but I can't figure it out.

The result to dput(head(mydata) is:

structure(list(District = structure(c(5L, 5L, 5L, 5L, 5L, 5L), .Label =         c("Charlottenburg-Wilmersdorf", 
"Friedrichshain-Kreuzberg", "Lichtenberg", "Marzahn-Hellersdorf", 
"Mitte", "Neukoelln", "Pankow", "Reinickendorf", "Spandau", "Steglitz-Zehlendorf", 
"Tempelhof-Schoeneberg", "Treptow-Koepenick"), class = "factor"), 
Population = c(81205L, 70911L, 5629L, 12328L, 78290L, 84789L
), Overall.crime = c(27864L, 13181L, 943L, 4515L, 15673L, 
16350L), Robbery = c(315L, 195L, 20L, 79L, 232L, 261L), Mugging = c(183L, 
81L, 9L, 54L, 111L, 118L), Assault = c(2016L, 1046L, 51L, 
468L, 1679L, 1718L), Molestation.Stalking = c(480L, 429L, 
16L, 114L, 567L, 601L), Theft = c(13587L, 4961L, 396L, 2019L, 
6725L, 6954L), Car.Theft = c(185L, 149L, 10L, 28L, 159L, 
159L), Bycicle.Theft = c(1444L, 561L, 95L, 123L, 588L, 595L
), Burglary = c(557L, 297L, 37L, 87L, 397L, 528L), Arson = c(36L, 
51L, 7L, 15L, 28L, 56L), Property.Damage = c(2113L, 871L, 
64L, 260L, 1257L, 1172L), Drug.Offenses = c(781L, 538L, 24L, 
87L, 604L, 492L)), .Names = c("District", "Population", "Overall.crime", 
"Robbery", "Mugging", "Assault", "Molestation.Stalking", "Theft", 
"Car.Theft", "Bycicle.Theft", "Burglary", "Arson", "Property.Damage", 
"Drug.Offenses"), row.names = c(NA, 6L), class = "data.frame")

I had spared you all those German names before, but I guess that was stupid since the problem is within the data...

Using ddply gives me following error:

Error in df$Population : object of type 'closure' is not subsettable

Thank you for any help!


Using the data you originally posted did you mean to do this?

df <- read.table( text = "District_name   Population   Child_birth_rate
A               30000       .7
A               20000       .5
B               10000       .09
B               15000       .6
C               80000       .007" , h = TRUE )

aggregate( cbind( Population , Child_birth_rate ) ~ District_name , data = df , sum )
#  District_name Population Child_birth_rate
#1             A      50000            1.200
#2             B      25000            0.690
#3             C      80000            0.007

Is it a good idea to sum the birth rate?

Using your actual data it might be more convenient to use ddply from plyr to aggregate in the a simillar fashion (but you want to use sum and mean on two different columns):

require( plyr )
ddply( mydata , "District" , function(df) c( "Pop" = sum( df$Population), "Robbery" = mean( df$Robbery ) ) )
#  District    Pop    Crime
#1    Mitte 333152 183.6667

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