R: finding intersection between two vectors [duplicate]

Question

v1 = c(2, 2.01, 2.02, 2.03, 2.04, 2.05, 2.06, 2.07, 2.08, 2.09, 2.1, 
  2.11, 2.12, 2.13, 2.14, 2.15, 2.16, 2.17, 2.18, 2.19, 2.2, 2.21, 
  2.22, 2.23, 2.24, 2.25, 2.26, 2.27, 2.28, 2.29, 2.3, 2.31, 2.32, 
  2.33, 2.34, 2.35, 2.36, 2.37, 2.38, 2.39, 2.4, 2.41, 2.42, 2.43, 
  2.44, 2.45, 2.46, 2.47, 2.48, 2.49, 2.5, 2.51, 2.52, 2.53, 2.54, 
  2.55, 2.56, 2.57, 2.58, 2.59, 2.6, 2.61, 2.62, 2.63, 2.64, 2.65, 
  2.66, 2.67, 2.68, 2.69, 2.7, 2.71, 2.72, 2.73, 2.74, 2.75, 2.76, 
  2.77, 2.78, 2.79, 2.8, 2.81, 2.82, 2.83, 2.84, 2.85, 2.86, 2.87, 
  2.88, 2.89, 2.9, 2.91, 2.92, 2.93, 2.94, 2.95, 2.96, 2.97, 2.98, 
  2.99)

> intersect(v1, seq(2, 2.99, 0.01))
 [1] 2.00 2.01 2.02 2.04 2.05 2.06 2.08 2.09 2.10 2.12 2.13 2.14 2.16 2.17 2.19 2.20 2.21 2.23 2.24 2.25 2.26 2.27
[23] 2.28 2.29 2.30 2.31 2.33 2.34 2.35 2.37 2.38 2.39 2.41 2.42 2.44 2.45 2.46 2.48 2.49 2.50 2.51 2.52 2.53 2.54
[45] 2.55 2.56 2.57 2.58 2.59 2.60 2.62 2.63 2.64 2.66 2.67 2.69 2.70 2.71 2.72 2.73 2.74 2.75 2.76 2.77 2.78 2.79
[67] 2.80 2.81 2.82 2.83 2.84 2.85 2.87 2.88 2.89 2.91 2.92 2.94 2.95 2.96 2.97 2.98 2.99

I have a vector of length 100 called v1. I want to see the intersection of v1 and a seq(2, 2.99, 0.01) vector (should be just v1 itself). But I get a vector that is only 83 elements long? And clearly 2.03, 2.15 ... are not in the intersection. How is that possible?

Answer 1

This a floating point error in r. See the Floating Point Guide for more information.

This can be seen as the error because this returns what you're looking for:

v1 = c(2, 2.01, 2.02, 2.03, 2.04, 2.05, 2.06, 2.07, 2.08, 2.09, 2.1, 
  2.11, 2.12, 2.13, 2.14, 2.15, 2.16, 2.17, 2.18, 2.19, 2.2, 2.21, 
  2.22, 2.23, 2.24, 2.25, 2.26, 2.27, 2.28, 2.29, 2.3, 2.31, 2.32, 
  2.33, 2.34, 2.35, 2.36, 2.37, 2.38, 2.39, 2.4, 2.41, 2.42, 2.43, 
  2.44, 2.45, 2.46, 2.47, 2.48, 2.49, 2.5, 2.51, 2.52, 2.53, 2.54, 
  2.55, 2.56, 2.57, 2.58, 2.59, 2.6, 2.61, 2.62, 2.63, 2.64, 2.65, 
  2.66, 2.67, 2.68, 2.69, 2.7, 2.71, 2.72, 2.73, 2.74, 2.75, 2.76, 
  2.77, 2.78, 2.79, 2.8, 2.81, 2.82, 2.83, 2.84, 2.85, 2.86, 2.87, 
  2.88, 2.89, 2.9, 2.91, 2.92, 2.93, 2.94, 2.95, 2.96, 2.97, 2.98, 
  2.99)

v2 <- seq(2, 2.99, 0.01)

v1 <- round(v1,2) #rounds to 2 decimal places
v2 <- round(v2,2)

intersect(v1,v2) #returns v1