CODEWITHSUNDEEP
rspline
giraffehere
giraffehere

Reputation: 1148

Forcing Spline Through Data Points

I've begun working with spline in R. I would like to create a spline that is forced to go through the data points in my data. Here's some example code:

x <- 1:12
y <- c(2172205,  5332408, 10453117, 16038564, 21109922, 
       27966857, 33563819, 39948003, 42629618, 45226902, 
       46172149, 46805245)
z <- spline(x, y, 36)

> plot(z)
> print(z)
$x
 [1]  1.000000  1.314286  1.628571  1.942857  2.257143  2.571429  2.885714  3.200000  3.514286  3.828571  4.142857
[12]  4.457143  4.771429  5.085714  5.400000  5.714286  6.028571  6.342857  6.657143  6.971429  7.285714  7.600000
[23]  7.914286  8.228571  8.542857  8.857143  9.171429  9.485714  9.800000 10.114286 10.428571 10.742857 11.057143
[34] 11.371429 11.685714 12.000000

$y
 [1]  2172205  2869024  3855824  5086171  6514487  8101175  9809181 11599445 13406684 15147279 16741530 18233441
[13] 19806294 21649308 23806067 26054346 28143697 29930437 31596698 33386543 35470344 37630794 39523543 40845316
[25] 41679456 42321140 43055048 43928388 44777465 45433385 45831051 46053800 46196328 46338922 46528029 46805245

What I would like is to have the values of the individual points in the spline as shown here, but forcing the line through each individual x and y component originally given. So, 5332408 should show up in the y output and 2 should show up in the x.

I realize this could give me a worse, less smooth curve, but that's okay for my purposes. Any help is greatly appreciated and thank you in advance!

Upvotes: 1

Views: 535

Answers (1)

IRTFM
IRTFM

Reputation: 263301

Use splinefun instead of spline to see that you actually do get the points intersecting the curve:

zf <- splinefun(x, y)

print( list(seq(36), y2=zf(seq(36) ) )  )
#----------
[[1]]
 [1]  1  2  3  4  5  6  7  8  9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24
[25] 25 26 27 28 29 30 31 32 33 34 35 36

$y2
 [1]    2172205    5332408   10453117   16038564   21109922   27966857
 [7]   33563819   39948003   42629618   45226902   46172149   46805245
[13]   48751454   53350661   61942754   75867617   96465137  125075200
[19]  163037692  211692499  272379506  346438600  435209667  540032593
[25]  662247264  803193565  964211384 1146640605 1351821115 1581092800
[31] 1835795545 2117269238 2426853763 2765889007 3135714856 3537671196

 y %in% zf(seq(36) )
 [1] TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE TRUE

Upvotes: 2

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