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Hi, I have a matrix a = matrix (1:16, 4, 4) b = c (2,3) I want to find out which rows of a, where a[,1] equals any values of b? I know that if b is only one value, e.g, b=2, then what I want is a[a[,1] == 2,] But what about if it is not one value but a vector of values? Thanks much in advance. -- Waverley @ Palo Alto

...vector, and reorder it to match with an erratic sequence of x,y coordinates. Here's an example that shows what I want and how close I've got (but for some reason my mind just can't get the final step at the moment!): #Define a grid: xy100 <- expand.grid(1:10, 1:10) # Define a matrix ofvalues mat <- matrix(log(xy100[,1]) * log(xy100[,2])+1, ncol = 10)/2 #Plot some the matrix with image and the same data as xy plot: par(mfrow = c(2,2)) image(mat) plot(xy100, cex = c(mat), pch = 20) #define the subset of coordinates I'm interested in: shortXY <- expand.grid(1:5, 1:5) #and sh...