plotEqn: Plot Linear Equations

Description

Shows what matrices $A, b$ look like as the system of linear equations, $A x = b$ with two unknowns, x1, x2, by plotting a line for each equation.

Usage

plotEqn(
  A,
  b,
  vars,
  xlim,
  ylim,
  col = 1:nrow(A),
  lwd = 2,
  lty = 1,
  axes = TRUE,
  labels = TRUE,
  solution = TRUE
)

Value

nothing; used for the side effect of making a plot

Arguments

A: either the matrix of coefficients of a system of linear equations, or the matrix cbind(A,b). The A matrix must have two columns.
b: if supplied, the vector of constants on the right hand side of the equations, of length matching the number of rows of A.
vars: a numeric or character vector of names of the variables. If supplied, the length must be equal to the number of unknowns in the equations, i.e., 2. The default is c(expression(x[1]), expression(x[2])).
xlim: horizontal axis limits for the first variable
ylim: vertical axis limits for the second variable; if missing, ylim is calculated from the range of the set of equations over the xlim.
col: scalar or vector of colors for the lines, recycled as necessary
lwd: scalar or vector of line widths for the lines, recycled as necessary
lty: scalar or vector of line types for the lines, recycled as necessary
axes: logical; draw horizontal and vertical axes through (0,0)?
labels: logical, or a vector of character labels for the equations; if TRUE, each equation is labeled using the character string resulting from showEqn, modified so that the xs are properly subscripted.
solution: logical; should the solution points for pairs of equations be marked?

Author

Michael Friendly

References

Fox, J. and Friendly, M. (2016). "Visualizing Simultaneous Linear Equations, Geometric Vectors, and Least-Squares Regression with the matlib Package for R". useR Conference, Stanford, CA, June 27 - June 30, 2016.

Examples

Run this code

# consistent equations
A<- matrix(c(1,2,3, -1, 2, 1),3,2)
b <- c(2,1,3)
showEqn(A, b)
plotEqn(A,b)

# inconsistent equations
b <- c(2,1,6)
showEqn(A, b)
plotEqn(A,b)

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