How to substract a column by row?

Question

I want to do an easy subtract in R, but I don't know how to solve it. I would like to know if I have to do a loop or if there is a function.

I have a column with numeric variables, and I would like to subtract n by n-1.

Time_Day Diff
10  10
15  5
45  30
60  15

Thus, I would like to find the variable "Diff".

Thank you, I didn't know about this function :)) – Marie Jul 10 '15 at 07:52 — Marie, Jul 10 '15 at 07:52

score 2 · Accepted Answer · answered Jul 10 '15 at 07:51

2

you can also try with package dplyr

library(dplyr)
mutate(df, dif=Time_Day-lag(Time_Day))
#   Time_Day Diff dif
# 1       10   10  NA
# 2       15    5   5
# 3       45   30  30
# 4       60   15  15

answered Jul 10 '15 at 07:51

Mamoun Benghezal

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Thank you for this perfect answer! – Marie Jul 10 '15 at 07:56

score 1 · Answer 2 · answered Jul 10 '15 at 07:52

1

Does this do what you need?

Here we save the column as a variable:

c <- c(10, 15, 45, 60)

Now we add a 0 to the beginning and then cut off the last element:

cm1 <- c(0, c)[1:length(c)]

Now we subtract the two:

dif <- c - cm1

If we print that out, we get what you're looking for:

dif # 10 5 30 15

answered Jul 10 '15 at 07:52

sudo make install

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Thank you, but I prefer the easy way with diff or dplyr :)) – Marie Jul 10 '15 at 07:58
Glad you found your answer! Good luck with your project. :-) – sudo make install Jul 10 '15 at 07:58

Victorp · Answer 3 · 2015-07-10T08:17:30.550

1

With diff :

df <- data.frame(Time_Day = c(10, 15, 45, 60))
df$Diff <- c(df$Time_Day[1], diff(df$Time_Day))
df
##  Time_Day Diff
##1       10   10
##2       15    5
##3       45   30
##4       60   15

It works fine in dplyr too :

library("dplyr")
df <- data.frame(Time_Day = c(10, 15, 45, 60))
df %>% mutate(Diff = c(Time_Day[1], diff(Time_Day)))

edited Jul 10 '15 at 08:17

answered Jul 10 '15 at 07:53

Victorp

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How to substract a column by row?

3 Answers3