df_compare.R

library(dplyr)

df1 <- mtcars
df2 <- filter(mtcars, mpg >19)

names(df1)
col1 <- 'qsec'
val1 <- 15.5

col2 <- 'qsec'
val2 <- 15.50
row.names(df1[rownum1,])
row.names(df2[rownum2,])
names(df1)[3] <- 'three'
# if only ONE row matched in both data frames, compare
# all columns with the same names
compare_2dfs <- function(x, df1,r1,df2,r2){
  list(df1 =  df1[r1, x],  "df2"=df2[r2, x], "diff"=df1[r1,x]-df2[r2,x])
}    

oneRowOfComparisons <- function(df1, cond1, df2, cond2) {
  r1 <- which(df1[,col1]==val1)
  r2 <- which(df2[, col2] == val2) 
  if(singleRowMatch(df1, cond1) && singleRowMatch(df2, cond2)){
    common_column_names <-  intersect( names(df1), names(df2))
    
    d <- data.frame(
      sapply(common_column_names, compare_2dfs, df1, r1, df2, r2)
    )
    return(unlist(d))
  }
  
}

singleRowMatch <- function(df1, cond1){
  return(TRUE)
}

oneRowOfComparisons(df1,0,df2, 0)

#sapply(row.names(d), function(x){paste(x, names(d), sep="_")} )



filter_(mtcars, "mpg==19.70")