How to count the number of occurrences of one variable that are shared between levels of another variable

Question

I have data that looks like

id      camera
angelina    cam03
angelina    cam03
angelina    cam03
angelina    cam03
angelina    cam03
angelina    cam03
angeli         


        

Answer 1

crossprod should do it for this:

crossprod(table(mydf))
#       Camera
# Camera 0001 0002 0003
#   0001    4    1    2
#   0002    1    1    0
#   0003    2    0    3

diag can be used to set the diagonal to zero or NA if required. You can do it all in one go with:

`diag<-`(crossprod(table(mydf)), 0)
#       Camera
# Camera 0001 0002 0003
#   0001    0    1    2
#   0002    1    0    0
#   0003    2    0    0

Sample data:

mydf <- data.frame(
    Individual = c("A", "A", "B", "B", "C", "D", "E", "E"),
    Camera = c("0001", "0002", "0001", "0003", "0001", "0003", "0001", "0003"))

EDIT:

In case of duplicated cameras for same individual, you can eliminate the duplicates prior to the crossprod call:

`diag<-`(crossprod(table(unique(mydf2))), 0)
       camera
#camera  cam03 cam15 cam22 cam25
  #cam03     0     1     2     1
  #cam15     1     0     1     1
  #cam22     2     1     0     1
  #cam25     1     1     1     0

Second data:

mydf2 <- structure(list(id = c("angelina", "angelina", "angelina", "angelina", 
"angelina", "angelina", "angelina", "angelina", "angelina", "angelina", 
"angelina", "angelina", "barry", "barry", "barry", "barry", "barry", 
"barry", "barry", "barry", "barry", "barry", "barry", "barry", 
"barry", "barry", "barry", "barry", "barry"), camera = c("cam03", 
"cam03", "cam03", "cam03", "cam03", "cam03", "cam03", "cam03", 
"cam03", "cam03", "cam03", "cam22", "cam03", "cam03", "cam03", 
"cam03", "cam03", "cam03", "cam03", "cam03", "cam03", "cam03", 
"cam22", "cam22", "cam22", "cam22", "cam22", "cam15", "cam25"
)), .Names = c("id", "camera"), class = "data.frame", row.names = c(NA, 
-29L))