Converting a data frame to matrix

Hello:

I have a tab delim file with 100 rows and 100 columns.

I have numerical values in this table. What I want is to create an image
color map with color gradation.

my values range from -5 to 0. max value is 0.

to acheive this, I need to convert my data.frame into matrix.

I tried following :

mydf <- read.table('mytable',sep='\t',header=T)
mydf1 <- mydf[,2:100]
colnames(mydf1) <- as.character(mydf[,1])

here comes the trouble:

mymat <- as.matrix(mydf1)

-- numerical values become character values.  -1 becomes "-1"

mymat <- data.matrix(mydf1)

-- whole matrix is filled with 1,2 and 3.

Is there any other way I can do this.

say row names are city schools
colnames  are student names