R Linear Interpolation of data into every 1 unit where max(x) is variable and is not a whole number

Question

I have the following two data sets

'''

 Percent <- c(0,30.4,99.6)
 Value1 <- c(100,80.4,70)
 Value2 <- c(0.04,0.06, 0.062)

 DF1 <- data.frame(Percent,Value1,Value2)

 Percent_A <- c(0,10,50.2,70,90.1,130.6,150,180.3)
 Value1_A <- c(100,90,88,70,60,62,62,58)
 Value2_A <- c(0.04,0.042,0.05,0.059,0.06,0.066,0.07,0.074)

 DF2 <- data.frame(Percent_A, Value1_A, Value2_A)

'''

I would like to interpolate these data frames (in reality I have many of these data frames, so it would be ideal if I can use one approach to deal with all data frames). I have some basic understanding of interpolation in R but am running into a couple of problems.

1. I would like these data frames to be interpolated into new data frames with y values given for every single-percentage value. This is causing me issues for two reasons (1: the data frames have two different max(Percent) values; 2: max(Percent) values are frequently not whole numbers). Based on my limited interpolation in R knowledge, the approxfun function seems useful, but I am unsure how to interpolate into every 1 percentage units, where the max known percentage value is not a whole number.

2. It would be great if I could interpolate Value1 and Value2 (in relation to Percent) for a given DF at the same time.

I would love your knowledge and insight on this matter! Thank you for the time and consideration on this topic.

The intended result is of course quite long (please let me know if you'd like more detail):

Intended result:

'''

DF1$Percent <- c(0:100) 
DF1$Value1 <- #interpolated values for percent= 1,2,3....99,100 
DF1$Value2 <- #interpolated values for percent= 1,2,3....99,100

DF2$Percent_A <- c(0:180) 
DF2$Value1_A <- #interpolated values for percent= 1,2,3....99,180 
DF2$Value2_A <- #interpolated values for percent= 1,2,3....99,180

'''

Note: I will then filter DF2 so that only the interpolated values for percent = 1-100 are displayed (as this is all that I am interested in).

Answer 1

You can only interpolate within a range, not beyond it so here is an approach:

Pct1 <- with(DF1, ceiling(min(Percent)):floor(max(Percent)))
DF1.I1 <- with(DF1, approx(Percent, Value1, xout=Pct1))
DF1.I2 <- with(DF1, approx(Percent, Value2, xout=Pct1))
DF1.interp <- data.frame(Percent=Pct1, Value1=DF1.I1$y, Value2=DF1.I2$y)

The maximum DF1$Percent is 99.6 so you cannot interpolate to 100.

Pct2 <- with(DF2, ceiling(min(Percent_A)):floor(max(Percent_A)))
DF2.I1 <- with(DF2, approx(Percent_A, Value1_A, xout=Pct2))
DF2.I2 <- with(DF2, approx(Percent_A, Value2_A, xout=Pct2))
DF2.interp <- data.frame(Percent_A=Pct2, Value1_A=DF2.I1$y, Value2_A=DF2.I2$y)

Now plots of Value 1:

dev.new(width=10, height=6)
par(mfrow=c(1, 2))
plot(Value1~Percent, DF1.interp, type="l")
plot(Value1_A~Percent_A, DF2.interp, type="l")