Loop through a data frame that generate an output based on conditions and values of other columns

Question

A brief version of my df is like the following:

A <- c(10,50,50,10,60,70,50,20,60,10)
B <- c(0,3,2,4,1,3,5,1,1,2)
df<- data.frame(A,B)
df
A   B   
10  0
50  3
50  2
10  4
60  1
70  3
50  5
20  1
60  1
10  2

I want to generate an output C based on some conditions of column A and values in column B.

Conditions:

C=0 when A < 50 or A "just becomes" >= 50
C= abs(B[n+1] - B[n]) when both B[n+1] and B[n] are >= 50

Here is an example of what I want in column C:

A   B   C   Explaination
10  0   0    A <50, so C=0
50  3   0    A just becomes >= 50, so C is still 0
50  2   1    We have two consecutive A >= 50, so C = abs(2-3) = 1
10  4   0    A <50, so C=0
60  1   0    A just becomes >= 50, so C is still 0
70  3   2    We have two consecutive A >= 50, so C = abs(3-1) = 2
50  5   2    consecutive rows with A >= 50, so C = abs(5-3) = 2
20  1   0    A <50, so C=0
60  1   0    A just becomes >= 50, so C is still 0
10  2   0    A <50, so C=0

mathematical.coffee · Accepted Answer · 2015-07-30 00:54:48Z

1

here's one option, though the logic is a little long-winded:

df$C <- ifelse(df$A < 50 | (df$A >= 50 & c(0, head(df$A, -1)) < 50) ,
               0, c(NA, abs(diff(df$B))))

Explanation:

I started with ifelse(df$A < 50, 0, c(NA, abs(diff(df$B)))). This gives a 0 if A < 50 and the differences of the Bs otherwise (diff(B) is one element shorter than B itself so I stick a NA on the front).
However this doesn't account for when A first exceeds 50, where the value should be 0 still. Note c(0, head(df$A, -1)) is just elements 2:end of the A column with a 0 stuck on front - it is the "previous" value of A. So if df$A >= 50 and the previous value is < 50, then this is the first time we have surpassed 50 and we should get the value 0.

I am sure there are other ways to do it too.

answered Jul 30, 2015 at 0:54

mathematical.coffee

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Shenglin Chen · Accepted Answer · 2015-07-30 01:06:50Z

0

library(dplyr)
df%>%mutate(C=ifelse(A<50,0,(ifelse(A>=50 & lag(A)>=50,abs(B-lag(B)),0))))

answered Jul 30, 2015 at 1:06

Shenglin Chen

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