How can I extract the coefficient of a specific variable in an equation in R?-CodePudding

For example, I have a string like "2 * a 3 * b".

I already have something that checks if a certain variable exists in the expression. So for example I input b, how could I make it return the 3?

CodePudding user response：

Not sure I understand you 100%. But if it is that you have a string and want to see the digit(s) that occur immediately prior to b, then you can use str_extract:

library(stringr)
str_extract(x, "\\d (?=b)")
[1] "3"

This works by the look-ahead (?=b), which assterts that the digit(s) to be extracted must be followed by the character b. If you need the extracted substring in numeric format:

as.numeric(str_extract(x, "\\d (?=b)"))

Data:

x <- "2a 3b"

CodePudding user response：

If we know that the formula is linear, as in the example in the question, then we can take the derivative.

# inputs
s <- "2 * a   3 * b"
var <- "b"

D(parse(text = s), var)
## [1] 3

CodePudding user response：

To extract the coefficient of a specific variable in an equation in R, you can use the lm() function to fit a linear model, and then use the coef() function to extract the coefficients.

For example, if you have an equation y = 2x 3 and you want to extract the coefficient of the x variable, you can use the following code:

# Fit a linear model to the equation
model <- lm(y ~ x)

# Extract the coefficients
coef(model)

This will return the coefficients of the equation, including the coefficient of the x variable, which in this case is 2.