user2716568 - 6 months ago 23

R Question

An example of my dataset is structured as follows:

`dput(head(MovementAnalysis,10))`

structure(list(Name = c("Amber", "Amber", "Amber", "Amber", "Amber",

"Jeff", "Jeff", "Jeff", "Jeff", "Jeff"), Sample = c(1, 2, 3, 4, 5, 1, 2,

3, 4, 5), X = c(26.66, 26.66, 26.65, 26.64, 26.64, 26.47, 26.46, 26.45,

26.43, 26.42), Y = c(-12.38, -12.37, -12.36, -12.36, -12.35, -12.23,

-12.22, -12.22, -12.22, -12.22), .Names = c("Name", "Sample", "X", Y"), row.names = c(NA, 10L), class = "data.frame")

I wish to calculate the Angular Velocity for each

`Name`

`i <- 2`

while(i < length(X) - k){

if (i > k)

{

a <- c(X[i] - X[i-k], Y[i] - Y[i-k])

b <- c(X[i+k] - X[i], Y[i+k] - Y[i])

AngularVelocity <- acos(sum(a * b) / (sqrt(sum(a * a)) * sqrt(sum(b * b)))) * (180 / pi)

}

i <- i+1

}

I attempted to do this in

`dplyr`

`Name`

`Output <- MovementAnalysis %>%`

arrange(Name,Sample) %>%

group_by(Name) %>%

mutate(An = (X - (lag(X)-2) + (Y - (lag(Y)-2))))

Output <- MovementAnalysis %>%

arrange(Name,Sample) %>%

group_by(Name) %>%

mutate(Bn = (X - (lag(X)+2) + (Y - (lag(Y)+2))))

I understand this is a lengthy question, so welcome any feedback on how to improve the question.

I had successfully been using code in the answer below for a while now. However, I am now getting an error when trying the code with a new dataset. An example of this dataset is below:

`# Create list of inviduals, drill number and practical or criterion measure`

ID = c("Gus_D1_Practical", "Gus_D1_Criterion", "Hudson_D1_Practical", "Hudson_D1_Criterion")

# Set the seed

set.seed(300)

# Create a data.frame of dummy peak velocity data from two different tracking systems

ExampleDataset <- data.frame(ID = rep((ID), each = 300),

Sample = rep(1:300, each = 1),

X = runif(300, 4.5, 6.7),

Y = runif(300, 4.1, 8))

# Set the SampleRate

SampleRate <- 100

k <- as.integer(SampleRate)

# Calculate Angular Velocity

library(dplyr)

Output <- ExampleDataset %>%

arrange(ID,Sample) %>%

group_by(ID) %>%

do( { a = diff(cbind(.$X, .$Y),lag=2)

b = tail(a, -k)

a = head(a, -k)

ang_vel = acos(rowSums(a*b)/(sqrt(rowSums(a^2))*sqrt(rowSums(b^2)))) * (180 / pi)

data_frame(Sample=head(tail(.$Sample,-k),-k), ang_vel) }) %>%

right_join(ExampleDataset, by = c("ID","Sample"))

Unfortunately, when I now try to calculate Angular Velocity the following error is returned:

`Error in data_frame_(lazyeval::lazy_dots(...)) :`

arguments imply differing number of rows: 100, 198

Any thoughts on what I may be doing wrong?

Answer

I suspect this is a bit different type of application for `dplyr`

. You might try something like

```
library(dplyr)
Output <- MovementAnalysis %>%
arrange(Name,Sample) %>%
group_by(Name) %>%
do( { a = diff(cbind(.$X, .$Y),lag=2)
b = tail(a, -k)
a = head(a, -k)
ang_vel = acos(rowSums(a*b)/(sqrt(rowSums(a^2))*sqrt(rowSums(b^2)))) * (180 / pi)
data_frame(Sample=head(tail(.$Sample,-k),-k), ang_vel) }) %>%
right_join(MovementAnalysis, by = c("Name","Sample"))
```