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 'prcomp' returns a list with class '"prcomp"' containing the
     following components:

    sdev: the standard deviations of the principal components (i.e.,
          the square roots of the eigenvalues of the
          covariance/correlation matrix, though the calculation is
          actually done with the singular values of the data matrix).
…………

>pca <- prcomp(~ Murder + Assault + Rape, data = USArrests, scale = TRUE)
>summary(pca)
Importance of components:
                         PC1   PC2    PC3
Standard deviation     1.536 0.677 0.4282
Proportion of Variance 0.786 0.153 0.0611
Cumulative Proportion  0.786 0.939 1.0000
>names(pca)
[1] "sdev"     "rotation" "center"   "scale"    "x"        "call" 

>pca$sdev
[1] 1.5357670 0.6767949 0.4282154
> (x <-  pca$sdev)
[1] 1.5357670 0.6767949 0.4282154

我是初学，可能是sdev()，就向loading()一样吧

不太明白你的意思。
> getS3method("prcomp", "default")
function (x, retx = TRUE, center = TRUE, scale. = FALSE, tol = NULL,
    ...)
{
    x <- as.matrix(x)
    x <- scale(x, center = center, scale = scale.)
    cen <- attr(x, "scaled:center")
    sc <- attr(x, "scaled:scale")
    if (any(sc == 0))
        stop("cannot rescale a constant/zero column to unit variance")
    s <- svd(x, nu = 0)
    s$d <- s$d/sqrt(max(1, nrow(x) - 1))
    if (!is.null(tol)) {
        rank <- sum(s$d > (s$d[1L] * tol))
        if (rank < ncol(x)) {
            s$v <- s$v[, 1L:rank, drop = FALSE]
            s$d <- s$d[1L:rank]
        }
    }
    dimnames(s$v) <- list(colnames(x), paste("PC", seq_len(ncol(s$v)),
        sep = ""))
    r <- list(sdev = s$d, rotation = s$v, center = if (is.null(cen)) FALSE else cen,
        scale = if (is.null(sc)) FALSE else sc)
    if (retx)
        r$x <- x %*% s$v
    class(r) <- "prcomp"
    r
}

list(sdev = s$d, rotation = s$v, center = if (is.null(cen)) FALSE else cen,
        scale = if (is.null(sc)) FALSE else sc)
函数prcomp返回的结果其实是一个列表，既然如此，当然可以利用names函数察看列表中每一项的名称，进而可以用‘$’操作符提取你需要的信息了。

  谢谢Janus!!