3. Working with Matrices

Question 1

Naming matrix rows and columns

Answer 1

Assign name attributes to rows of a matrix:

rownames(matrix)

Assign name attributes to columns of a matrix:

colnames(matrix)

Question 2

Matrix operations

Answer 2

Finding Matrix Dimensions

• dim(math_chemistry)

Combining Vectors or Matrices by Row

• rbind(matrix_1, matrix_2)
• rbind(vector_1, vector_2)
• rbind(vector_1, matrix_1)

Combining Vectors or Matrices by Column

• cbind(matrix_1, matrix_2)
• cbind(vector_1, vector_2)
• cbind(vector_1, matrix_1)

Question 3

Indexing matrices by element

Answer 3

Extract a single element:

• matrix[2,5]
• matrix[“Stanford”,”patents”]

Extract multiple elements:

• matrix[c(1,2),c(1,3)]
• matrix[c(“Harvard”,”Stanford”),c(“world_rank”,”influence”)]

Question 4

Indexing matrices by rows and columns

Answer 4

Extract a single row:

• matrix[1,]
• matrix[“Harvard”,]

Extract a single column:

• matrix[,2]
• matrix[,”quality_of_education”]

Extract multiple rows or columns:

• matrix[,c(“quality_of_education”,”influence”,”broad_impact”)]
• matrix[,c(“2,3,4”)]

Question 5

Rank values of a vector

Answer 5

Rank values of a vector:

• rank(vector)

Rank values of a matrix:

• rank(matrix[,”column”])
• rank(matrix[“row”,])

Question 6

Calculate the sum of values in a vector or a matrix

Answer 6

Sum of values in a vector:

• sum(vector)

Sum of values in a matrix:

• sum(matrix[,”column”])
• sum(matrix[“row”,])

Question 7

Concepts

Answer 7

Like vectors, matrices only contain one data type. Unlike vectors, they are two-dimensional.

When adding a vector to a matrix, it’s good practice to make sure the new vector is the same length as the number of rows or columns in the matrix.