A有列a，b，c，d B有列a，b，c，e
merged = merge(A,B,by = c("a","b","c"), all.x = TRUE, sort = FALSE)
A$difference = merged$d - A$d
sum(A$difference) # <- this is unequal 0
这怎么可能？ 所有列都是数值，A中的每个a，b，c值也在B
中这是我的确切代码：
A = d[c("OpTyp","DeltaOffset","Size","Duration")]
B = d.aggregated[c("OpTyp","DeltaOffset","Size","mean_Duration")]
merged = merge(A,B,by = c("OpTyp","DeltaOffset","Size"), all.x = TRUE, sort = FALSE)
merged$difference = A$Duration - merged$Duration
sum(merged$difference)