'data.frame':   958000 obs. of  5 variables:
 $ date : Factor w/ 1916 levels "2013-01-01","2013-01-02",..: 1 2 3 4 5 6 7 8 9 10 ...
 $ store: int  1 1 1 1 1 1 1 1 1 1 ...
 $ item : int  1 1 1 1 1 1 1 1 1 1 ...
 $ sales: num  13 11 14 13 10 12 10 9 12 9 ...
 $ id   : Factor w/ 45001 levels "0","1","10","100",..: 45001 45001 45001 45001 45001 45001 45001 45001 45001 45001 ...`

set.seed(1234)
n = nrow(df_all)
index = sample(1:n, size = round(0.7*n), replace=T)
train = df_all[index, ]
test = df_all[-index, ]

trainm <- sparse.model.matrix(sales ~ ., data= train)[,-1]

6 x 46917 sparse Matrix of class "dgCMatrix"
   [[ suppressing 20 column names ‘date2013-01-02’, ‘date2013-01-03’, ‘date2013-01-04’ ... ]]

108928 . . . . . . . . . . . . . . . . . . . .
596163 . . . . . . . . . . . . . . . . . . . .
583686 . . . . . . . . . . . . . . . . . . . .
597198 . . . . . . . . . . . . . . . . . . . .
824757 . . . . . . . . . . . . . . . . . . . .
613418 . . . . . . . . . . . . . . . . . . . .

108928 ......
596163 ......
583686 ......
597198 ......
824757 ......
613418 ......

 .....suppressing columns in show(); maybe adjust 'options(max.print= *, width = *)'
 ..............................