Doing Math in R

This is my guide on doing math in R. 

Doing Math in R

Working in R means doing a lot of calculations. This is what R is for and why it is called a statistical programming language. You can do simple math and work with vectors. In fact, there are a few different categories. They are:

• Arithmetic
• Functions
• Vectors
• Matrixes

The arithmetic operators should be familiar to everyone. These are the basic math operators everyone learned when they were kids.

• \[x + y\]               y added to x
• \[x - y\]                y subtracted from x
• \[x * y\]                x multiplied by y
• \[\frac{x}{y}\]        x divided by y
• \[x ^ y\]                x raised to the power of y
• \[x %% y\]           remainder of x divided by y
• \[x% / %y\]          x divided by y but rounded down

Let’s move to the next section, mathematical functions. These are the traditional algebraic functions and they work the same way. 

• abs(x)              takes the absolute value of x
• log x                takes the logarithm of x with base y
• exp(x)              returns the exponential of x
• sqrt(x)              returns the square root of x
• factorial (x)      returns the factorial of x!
• choose(x,y)     returns the number of possible combinations when drawing y elements from x  possibilities

You can take the log of a number like this:

Log 1.5

You can take the log of a series of numbers:

log(2:4)

That function takes the natural log of the numbers 2,3,4

You can specify a base:

log(2:4, base=4)

The other functions work similarly, I will get into them more when we need to.

You can round numbers easily in R. You just use the ‘round()’ function. 

round(454567.2333445, digits=3)

Significant digits can be done just as easily.

signif(333.334455, digits=3)

Trig functions are also available. By default, R gives results in radians. So, if you need a result in degrees, you will have to convert it. I will show you how, though. 

cos(120)

Gives results in radians

cos(95 * pi / 180)

Gives results in degrees

Working With Vectors

A vector is a one-dimensional set of values. It looks like this:

x=c(1,2,3,4,5,6,7,8)

They have to be the same type, such as integers. 

There is a function, ‘str()’, that lets you look at any particular vector and see its properties. Use it like this:

str(x)

To see the length of a vector:

length(x)

Vectors can be several different types:

• Numeric
• Integer
• Logical
• Character
• Datetime
• Factors

You can test a vector to see what kind it is:

is.numeric(x)

is.character(x)

is.logical(x)

Those are all separate tests to determine what kind of vector you have. You will get the output of ‘true’ when you have a match. 

To create vectors you can enter in numbers or use a sequence of numbers. I will show you how to do both. It is common to assign a variable to your data so it is easy to work on it. The variable is ‘x’.

x=c(22,33,44,55,66,77,88,99)

The ‘c’ is a function itself and combines the numbers in the parentheses to make a vector.

You can also use the colon operator to create a sequence of numbers.

x=c(2:7)

This creates a vector with the number 2,3,4,5,6,7

You can include negative numbers too.

x=c(7:-2)

R lets you combine vectors when you need to. If we have:

x=c(1:9)

y=c(11:16)

We can combine them like this:

total=c(x,y)

We can repeat vectors too. We do this with the ‘rep()’ function.

If we want to repeat a vector a set number of times, we do this:

rep(c(1:9), times=4)

When we want to repeat every value:

rep(c(1:9), each=3)

We can also tell R how often to repeat each value:

rep(c(1,9), times=c(3,4)

Looking At Vector Values

Once we have a vector, R lets us look at and work with individual values. The square brackets let us extract a value from the vector. We just indicate the position we want inside of the square brackets. 

X[3]

This gives us the 3rd number from the start of the vector.

We can get more than one position value at once:

x[c(1,2,3)]

This will give us the first 3 positions of the vector. 

You can change the value of a vector.

x=c(1,2,3,4,5)

Let us change the last value from 5 to 3

X[5] = 3

Now, our vector has been changed to what we want to reflect it as.

Making Copies of Vectors

Before working with an important vector set of data, make a copy of it. You do not want to accidentally change it without knowing it. Do it like this:

X.copy = x

Now you can do your work with a little less worry. 

Comparing Values

To compare values in a vector:

X > 5

This gives us logical values. Any time there is a value greater than 5, the output is true.

We can also check positions that are greater than 5.

which(x > 5)

This shows us which positions in the vector are greater than 5.

These are the logical operators in R:

• X == y
• X != y
• X > y
• X >= y
• X < y
• X <= y
•  X & y
• X | y
• !x
• xor(x,y)

More Arithmetic Operations

Once we have a vector set up and kind of know how it works, we can start doing more with it. I recently took a statistics class and I used many of these functions to great effect. It really speeds things up. The idea of a vector is to look at each value in a vector and do something with it. That is what functions do once we have a vector set up. Here are the arithmetic functions that are pretty useful:

• sum(x)               calculates the sum of values in the vector x
• prod(x)               calculates the product of all the values in the vector x
• min(x)                gives the minimum of all values in x
• max(x)               gives the max of all the values in the vector x
• cumsum(x)        gives the cumulative sum of all the values in the vector x
• cumprod(x)        gives the cumulative product of all the values in the vector x
• cumin(x)            gives the minimum for all values in x from the start of the vector to the position indicated
• cummax(x)        gives the maximum for all values in x from the start of the vector until the position indicated
• diff(x)                 gives for every value the difference between that value and the next value in the vector