R: Define a Functional Data Object

fd {fda} | R Documentation

## Define a Functional Data Object

### Description

This is the constructor function for objects of the `fd` class. Each function that sets up an object of this class must call this function. This includes functions `Data2fd`, `smooth.basis`, `density.fd`, and so forth that estimate functional data objects that smooth or otherwise represent data. Ordinarily, users of the functional data analysis software will not need to call this function directly, but these notes are valuable to understanding the components of a `list` of class `fd`.

### Usage

    fd(coef=NULL, basisobj=NULL, fdnames=NULL)

### Arguments

`coef` | a vector, matrix, or three-dimensional array of coefficients. The first dimension (or elements of a vector) corresponds to basis functions. A second dimension corresponds to the number of functional observations, curves or replicates. If `coef` is a vector, it represents only a single functional observation. If `coef` is an array, the third dimension corresponds to variables for multivariate functional data objects. A functional data object is "univariate" if `coef` is a vector or matrix and "multivariate" if it is a three-dimensional array. if(is.null(coef)) coef <- rep(0, basisobj[['nbasis']])
`basisobj` | a functional basis object defining the basis `if(is.null(basisobj)){ if(is.null(coef)) basisobj <- basisfd() else { rc <- range(coef) if(diff(rc)==0) rc <- rc+0:1 nb <- max(4, nrow(coef)) basisobj <- create.bspline.basis(rc, nbasis = nb) } }`
`fdnames` | A list of length 3, each member being a string vector containing labels for the levels of the corresponding dimension of the discrete data. The first dimension is for argument values, and is given the default name "time", the second is for replications, and is given the default name "reps", and the third is for functions, and is given the default name "values".

### Details

To check that an object is of this class, use function `is.fd`.

Normally only developers of new functional data analysis functions will actually need to use this function.

### Value

A functional data object (i.e., having class `fd`), which is a list with components named `coefs`, `basis`, and `fdnames`.

### Source

Ramsay, James O., and Silverman, Bernard W. (2006), Functional Data Analysis, 2nd ed., Springer, New York.

Ramsay, James O., and Silverman, Bernard W. (2002), Applied Functional Data Analysis, Springer, New York

### See Also

`smooth.basis` `smooth.basisPar` `Data2fd` `density.fd` `create.bspline.basis` `arithmetic.fd`

### Examples

    ##
    ## default
    ##
    fd()

    ##
    ## The simplest b-spline basis:  order 1, degree 0, zero interior knots:
    ##       a single step function
    ##
    bspl1.1 <- create.bspline.basis(norder=1, breaks=0:1)
    fd.bspl1.1 <- fd(0, basisobj=bspl1.1)

    fd.bspl1.1a <- fd(basisobj=bspl1.1)

    all.equal(fd.bspl1.1, fd.bspl1.1a)

    # TRUE

    ## Not run:
    fd.bspl1.1b <- fd(0)
    Error in fd(0) :
      Number of coefficients does not match number of basis functions.

    ... because fd by default wants to create a cubic spline

    ## End(Not run)
    ##
    ## Cubic spline:  4  basis functions
    ##
    bspl4 <- create.bspline.basis(nbasis=4)
    plot(bspl4)
    parab4.5 <- fd(c(3, -1, -1, 3)/3, bspl4)
    # = 4*(x-.5)^2
    plot(parab4.5)

    ##
    ## Fourier basis
    ##
    f3 <- fd(c(0,0,1), create.fourier.basis())
    plot(f3)
    # range over +/-sqrt(2), because
    # integral from 0 to 1 of cos^2 = 1/2
    # so multiply by sqrt(2) to get
    # its square to integrate to 1.

    ##
    ## subset of an fd object
    ##
    gaitbasis3 <- create.fourier.basis(nbasis=5)
    gaitfd3    <- Data2fd(gait, basisobj=gaitbasis3)
    gaitfd3[1]

[Package fda version 2.4.8 Index]