给定特定条件的列元素求和

Question

given a data.frame with 3 columns, I need to sum those elements belonging to the column III, given two conditions: column I and column II must have the same value (by row). For example, if I have this data.frame:

I . II . III

1 2 0.3

1 2 0.4

2 1 0.3

2 3 0.5

3 1 0.5

I want to sum all values in III given I=1 with II=2; I=2 with II=1; I=2 with II=3

Answer 1

If you can convert the matrix to data.frame

Using @Franks' dataset

d1 <- as.data.frame(d)
library(dplyr)
d1%>% 
group_by(`I`, `II`) %>% 
summarize(Sum=sum(`III`))
#     Source: local data frame [4 x 3]
#  Groups: I

#   I II Sum
# 1 1  2 0.7
# 2 2  1 0.3
# 3 2  3 0.5
# 4 3  1 0.5

Answer 2

Turning @beginneR's comment into an answer, if your data frame is df :

aggregate(III ~ I + II, df, sum)
##   I II III
## 1 2  1 0.3
## 2 3  1 0.5
## 3 1  2 0.7
## 4 2  3 0.5

This will calculate the sum of the elements in column III for every combination of values in columns I and II .

Your question is a bit confusing because you refer to a 3 column data frame and then a 5X5 matrix. A matrix is not the same as a data frame.

Answer 3

# Read in sample matrix (3 columns, 5 rows)
d<-as.matrix(read.table(text="
I II III
1 2 0.3
1 2 0.4
2 1 0.3
2 3 0.5
3 1 0.5", header=T))

# get all sums by unique groupings of column I and II
all_group_sums <- tapply(d[,'III'], paste0(d[,'I'], d[,'II']), sum)

# conditions you're interested in
of_interest <- c("12", "21", "23")

# filter tapply result by conditions of interest
all_group_sums[of_interest]

# 12  21  23 
#0.7 0.3 0.5 

Or the same as above, just in one line:

tapply(d[,'III'], paste0(d[,'I'], d[,'II']), sum)[c("12", "21", "23")]
# 12  21  23 
#0.7 0.3 0.5