Greek letters in legend in R

Question

I want to have three curves on the same plot with different parameter alpha.

curve(sin(x), from = 1, to = 3, lty = 1, ylim = c(-1, 1))
curve(sin(2 * x), add = TRUE, lty = 2)    
curve(sin(3 * x), add = TRUE, lty = 3)
legend("topright", legend = expression(paste(alpha, " = ", c(1, 2, 3))), lty = 1:3)

In the legend, I want to have three lines with alplha = 1, alpha = 2, alpha = 3. How do I make it correct?

Solution

The better, looped answer comes from here and user20650.

Solution with sapply

The expression function is quite tricky but in conjunction with substitute you can use sapply to loop:

curve(sin(x), from = 1, to = 3, lty = 1, ylim = c(-1, 1))
curve(sin(2 * x), add = TRUE, lty = 2)    
curve(sin(3 * x), add = TRUE, lty = 3)
legend("topright",
       legend = sapply(1:3, function(x) as.expression(substitute(alpha == B,
                                                                 list(B = as.name(x))))),
       lty = 1:3)

Simple Fix

curve(sin(x), from = 1, to = 3, lty = 1, ylim = c(-1, 1))
curve(sin(2 * x), add = TRUE, lty = 2)    
curve(sin(3 * x), add = TRUE, lty = 3)
legend("topright", legend = c(expression(paste(alpha, " = ", 1)),
                              expression(paste(alpha, " = ", 2)),
                              expression(paste(alpha, " = ", 3))), lty = 1:3)

Comments

[JACKY88]

Is it possible to do this in a more automatic way? My real plot has more lines and I have a lot of these plots.

[MrFlick]

I prefer legend("topright", legend = as.expression(lapply(1:3, function(x) bquote(alpha==.(x)))), lty = 1:3)