Summarize Scalars or Matrices by Cross-Classification

summarize is a fast version of summary.formula(formula, method="cross",overall=FALSE) for producing stratified summary statistics and storing them in a data frame for plotting (especially with trellis xyplot and dotplot and Hmisc xYplot). Unlike aggregate, summarize accepts a matrix as its first argument and a multi-valued FUN argument and summarize also labels the variables in the new data frame using their original names. Unlike methods based on tapply, summarize stores the values of the stratification variables using their original types, e.g., a numeric by variable will remain a numeric variable in the collapsed data frame. summarize also retains "label" attributes for variables. summarize works especially well with the Hmisc xYplot function for displaying multiple summaries of a single variable on each panel, such as means and upper and lower confidence limits.

asNumericMatrix converts a data frame into a numeric matrix, saving attributes to reverse the process by matrix2dataframe. It saves attributes that are commonly preserved across row subsetting (i.e., it does not save dim, dimnames, or names attributes).

matrix2dataFrame converts a numeric matrix back into a data frame if it was created by asNumericMatrix.

multivariate, manip, category
summarize(X, by, FUN, …, 
          type=c('variables','matrix'), subset=TRUE,


matrix2dataFrame(x, at=attr(x, 'origAttributes'), restoreAll=TRUE)


a vector or matrix capable of being operated on by the function specified as the FUN argument


one or more stratification variables. If a single variable, by may be a vector, otherwise it should be a list. Using the Hmisc llist function instead of list will result in individual variable names being accessible to summarize. For example, you can specify llist(,sex) or llist(,sex). The latter gives a new temporary name, Age.


a function of a single vector argument, used to create the statistical summaries for summarize. FUN may compute any number of statistics.


extra arguments are passed to FUN

the name to use when creating the main summary variable. By default, the name of the X argument is used. Set to NULL to suppress this name replacement.


Specify type="matrix" to store the summary variables (if there are more than one) in a matrix.


a logical vector or integer vector of subscripts used to specify the subset of data to use in the analysis. The default is to use all observations in the data frame.


by default when type="matrix", the first column of the computed matrix is the name of the first argument to summarize. Set keepcolnames=TRUE to retain the name of the first column created by FUN


a data frame (for asNumericMatrix) or a numeric matrix (for matrix2dataFrame).


List containing attributes of original data frame that survive subsetting. Defaults to attribute "origAttributes" of the object x, created by the call to asNumericMatrix


set to FALSE to only restore attributes label, units, and levels instead of all attributes


For summarize, a data frame containing the by variables and the statistical summaries (the first of which is named the same as the X variable unless is given). If type="matrix", the summaries are stored in a single variable in the data frame, and this variable is a matrix.

asNumericMatrix returns a numeric matrix and stores an object origAttributes as an attribute of the returned object, with original attributes of component variables, the storage.mode.

matrix2dataFrame returns a data frame.

See Also

label, cut2, llist, by

  • summarize
  • asNumericMatrix
  • matrix2dataFrame
s <- summarize(ap>1, llist(size=cut2(sz, g=4), bone), mean,
dotplot(Proportion ~ size | bone, data=s7)
# }
temperature <- rnorm(300, 70, 10)
month <- sample(1:12, 300, TRUE)
year  <- sample(2000:2001, 300, TRUE)
g <- function(x)c(Mean=mean(x,na.rm=TRUE),Median=median(x,na.rm=TRUE))
summarize(temperature, month, g)
mApply(temperature, month, g)

mApply(temperature, month, mean, na.rm=TRUE)
w <- summarize(temperature, month, mean, na.rm=TRUE)
xyplot(temperature ~ month, data=w) # plot mean temperature by month

w <- summarize(temperature, llist(year,month), 
               quantile, probs=c(.5,.25,.75), na.rm=TRUE, type='matrix')
xYplot(Cbind(temperature[,1],temperature[,-1]) ~ month | year, data=w)
mApply(temperature, llist(year,month),
       quantile, probs=c(.5,.25,.75), na.rm=TRUE)

# Compute the median and outer quartiles.  The outer quartiles are
# displayed using "error bars"
dfr <- expand.grid(month=1:12, year=c(1997,1998), reps=1:100)
y <- abs(month-6.5) + 2*runif(length(month)) + year-1997
s <- summarize(y, llist(month,year), smedian.hilow,
mApply(y, llist(month,year), smedian.hilow,

xYplot(Cbind(y,Lower,Upper) ~ month, groups=year, data=s, 
       keys='lines', method='alt')
# Can also do:
s <- summarize(y, llist(month,year), quantile, probs=c(.5,.25,.75),
xYplot(Cbind(y, Q1, Q3) ~ month, groups=year, data=s, keys='lines')
# To display means and bootstrapped nonparametric confidence intervals
# use for example:
s <- summarize(y, llist(month,year),
xYplot(Cbind(y, Lower, Upper) ~ month | year, data=s)

# For each subject use the trapezoidal rule to compute the area under
# the (time,response) curve using the Hmisc trap.rule function
x <- cbind(time=c(1,2,4,7, 1,3,5,10),response=c(1,3,2,4, 1,3,2,4))
subject <- c(rep(1,4),rep(2,4))
summarize(x, subject, function(y) trap.rule(y[,1],y[,2]))

# }
# Another approach would be to properly re-shape the mm array below
# This assumes no missing cells.  There are many other approaches.
# mApply will do this well while allowing for missing cells.
m <- tapply(y, list(year,month), quantile, probs=c(.25,.5,.75))
mm <- array(unlist(m), dim=c(3,2,12), 
# aggregate will help but it only allows you to compute one quantile
# at a time; see also the Hmisc mApply function
dframe <- aggregate(y, list(Year=year,Month=month), quantile, probs=.5)

# Compute expected life length by race assuming an exponential
# distribution - can also use summarize
g <- function(y) { # computations for one race group
  futime <- y[,1]; event <- y[,2]
  sum(futime)/sum(event)  # assume event=1 for death, 0=alive
mApply(cbind(followup.time, death), race, g)

# To run mApply on a data frame:
xn <- asNumericMatrix(x)
m <- mApply(xn, race, h)
# Here assume h is a function that returns a matrix similar to x

# Get stratified weighted means
g <- function(y) wtd.mean(y[,1],y[,2])
summarize(cbind(y, wts), llist(sex,race), g,'y')
mApply(cbind(y,wts), llist(sex,race), g)

# Compare speed of mApply vs. by for computing 
d <- data.frame(sex=sample(c('female','male'),100000,TRUE),
                y1=runif(100000), y2=runif(100000))
g <- function(x) {
  y <- c(median(x[,'y1']-x[,'y2']),
         med.sum =median(x[,'y1']+x[,'y2']))
  names(y) <- c('med.diff','med.sum')

system.time(by(d, llist(sex=d$sex,country=d$country), g))
             x <- asNumericMatrix(d)
             a <- subsAttr(d)
             m <- mApply(x, llist(sex=d$sex,country=d$country), g)
             x <- asNumericMatrix(d)
             summarize(x, llist(sex=d$sex, country=d$country), g)

# An example where each subject has one record per diagnosis but sex of
# subject is duplicated for all the rows a subject has.  Get the cross-
# classified frequencies of diagnosis (dx) by sex and plot the results
# with a dot plot

count <- rep(1,length(dx))
d <- summarize(count, llist(dx,sex), sum)
Dotplot(dx ~ count | sex, data=d)
# }
d <- list(x=1:10, a=factor(rep(c('a','b'), 5)),
          b=structure(letters[1:10], label='label for b'),
          d=c(rep(TRUE,9), FALSE), f=pi*(1 : 10))
x <- asNumericMatrix(d)
attr(x, 'origAttributes')


# Run summarize on a matrix to get column means
x <- c(1:19,NA)
y <- 101:120
z <- cbind(x, y)
g <- c(rep(1, 10), rep(2, 10))
summarize(z, g, colMeans, na.rm=TRUE,'x')
# Also works on an all numeric data frame
summarize(, g, colMeans, na.rm=TRUE,'x')
# }
Documentation reproduced from package Hmisc, version 4.1-1, License: GPL (>= 2)

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