# --Managing Matrices

2-dimensional rectangular layout in R. Each element of matrix must be same data type such as numeric, character, etc.

## #Creating Matrices

```r
# numeric matrix
m1 <- matrix(1:6, nrow = 2, ncol = 3)
m1
##      [,1] [,2] [,3]
## [1,]    1    3    5
## [2,]    2    4    6
str(m1)
##  int [1:2, 1:3] 1 2 3 4 5 6

attributes(m1)
## $dim
## [1] 2 3

# a character matrix
m2 <- matrix(letters[1:6], nrow = 2, ncol = 3)

m2
##      [,1] [,2] [,3]
## [1,] "a"  "c"  "e" 
## [2,] "b"  "d"  "f"

# structure of m2 is simply character vector with 2x3 dimension
str(m2)
##  chr [1:2, 1:3] "a" "b" "c" "d" "e" "f"

attributes(m2)
## $dim
## [1] 2 3

v1 <- 1:4
v2 <- 5:8

cbind(v1, v2)
##      v1 v2
## [1,]  1  5
## [2,]  2  6
## [3,]  3  7
## [4,]  4  8

rbind(v1, v2)
##    [,1] [,2] [,3] [,4]
## v1    1    2    3    4
## v2    5    6    7    8

# bind several vectors together
v3 <- 9:12

cbind(v1, v2, v3)
##      v1 v2 v3
## [1,]  1  5  9
## [2,]  2  6 10
## [3,]  3  7 11
## [4,]  4  8 12
```

## #Adding on to Matrices

&#x20;Use `cbind()` and `rbind()`functions for adding onto matrices as well.&#x20;

```r
m1 <- cbind(v1, v2)
m1
##      v1 v2
## [1,]  1  5
## [2,]  2  6
## [3,]  3  7
## [4,]  4  8

# add a new column
cbind(m1, v3)
##      v1 v2 v3
## [1,]  1  5  9
## [2,]  2  6 10
## [3,]  3  7 11
## [4,]  4  8 12

# or add a new row
rbind(m1, c(4.1, 8.1))
##       v1  v2
## [1,] 1.0 5.0
## [2,] 2.0 6.0
## [3,] 3.0 7.0
## [4,] 4.0 8.0
## [5,] 4.1 8.1
```

## #Adding Attributes to Matrices

```r
# basic matrix
m2 <- matrix(1:12, nrow = 4, ncol = 3)

m2
##      [,1] [,2] [,3]
## [1,]    1    5    9
## [2,]    2    6   10
## [3,]    3    7   11
## [4,]    4    8   12

# the dimension attribute shows this matrix has 4 rows and 3 columns
attributes(m2)
## $dim
## [1] 4 3
```

```r
# add row names as an attribute
rownames(m2) <- c("row1", "row2", "row3", "row4")

m2
##      [,1] [,2] [,3]
## row1    1    5    9
## row2    2    6   10
## row3    3    7   11
## row4    4    8   12

# attributes displayed will now show the dimension, list the row names
# and will show the column names as NULL
attributes(m2)
## $dim
## [1] 4 3
## 
## $dimnames
## $dimnames[[1]]
## [1] "row1" "row2" "row3" "row4"
## 
## $dimnames[[2]]
## NULL

# add column names
colnames(m2) <- c("col1", "col2", "col3")
m2
##      col1 col2 col3
## row1    1    5    9
## row2    2    6   10
## row3    3    7   11
## row4    4    8   12

attributes(m2)
## $dim
## [1] 4 3
## 
## $dimnames
## $dimnames[[1]]
## [1] "row1" "row2" "row3" "row4"
## 
## $dimnames[[2]]
## [1] "col1" "col2" "col3"
```

```r
dimnames(m2)[[1]] <- c("row_1", "row_2", "row_3", "row_4")
m2
##       col1 col2 col3
## row_1    1    5    9
## row_2    2    6   10
## row_3    3    7   11
## row_4    4    8   12

# column names are contained in the second list item
dimnames(m2)[[2]] <- c("col_1", "col_2", "col_3")
m2
##       col_1 col_2 col_3
## row_1     1     5     9
## row_2     2     6    10
## row_3     3     7    11
## row_4     4     8    12
```

```r
comment(m2) <- "adding a comment to a matrix"

attributes(m2)
## $dim
## [1] 4 3
## 
## $dimnames
## $dimnames[[1]]
## [1] "row_1" "row_2" "row_3" "row_4"
## 
## $dimnames[[2]]
## [1] "col_1" "col_2" "col_3"
## 
## 
## $comment
## [1] "adding a comment to a matrix"
```

## #Subsetting Matrices

```r
m2
##       col_1 col_2 col_3
## row_1     1     5     9
## row_2     2     6    10
## row_3     3     7    11
## row_4     4     8    12
```

```r
# subset for rows 1 and 2 but keep all columns
m2[1:2, ]
##       col_1 col_2 col_3
## row_1     1     5     9
## row_2     2     6    10

# subset for columns 1 and 3 but keep all rows
m2[ , c(1, 3)]
##       col_1 col_3
## row_1     1     9
## row_2     2    10
## row_3     3    11
## row_4     4    12

# subset for both rows and columns
m2[1:2, c(1, 3)]
##       col_1 col_3
## row_1     1     9
## row_2     2    10

# use a vector to subset
v <- c(1, 2, 4)
m2[v, c(1, 3)]
##       col_1 col_3
## row_1     1     9
## row_2     2    10
## row_4     4    12

# use names to subset
m2[c("row_1", "row_3"), ]
##       col_1 col_2 col_3
## row_1     1     5     9
## row_3     3     7    11
```

```r
# simplifying results in a named vector
m2[, 2]
## row_1 row_2 row_3 row_4 
##     5     6     7     8

# preserving results in a 4x1 matrix
m2[, 2, drop = FALSE]
##       col_2
## row_1     5
## row_2     6
## row_3     7
## row_4     8
```

## #Exercises

1. What are the attributes of the built-in `VADeaths` data matrix?
2. Subset this matrix for only male death rates.
3. Subset for males death rates over the age of 60.
4. See if you can calculate averages for each column and row.
5. Can you figure out how to add these averages to your table so the output looks like:


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