Why is it a bad habit to parse and evaluate a string code (in R)?

Question

I've been told that it is a bad habit to parse and evaluate a character string

to_run = "for (i in 1:10){print(i);print('Hello World!')}"

eval(parse(text=to_run))

Why is it a bad habit?

It seems to me to be quite a flexible way of programming as we can construct our code in a iterative manner by pasting character strings together. For example it allows to easily deal with objects of various dimensions for example.

if (length(dim(my.array)) == 2){to_run = "A = my.array[1,]"}
if (length(dim(my.array)) == 3){to_run = "A = my.array[1,,]"}
eval(parse(text=to_run))

Among other things, don't you think that this makes your code less flexible and less reusable in the long run? — A5C1D2H2I1M1N2O1R2T1, Dec 19 '13 at 10:23
See, also, [dangers of eval(parse())](http://stackoverflow.com/questions/13649979/what-specifically-are-the-dangers-of-evalparse?lq=1) — alexis_laz, Dec 19 '13 at 10:26

score 2 · Accepted Answer · answered Dec 19 '13 at 10:23

2

Self-modifying code is much more difficult to understand than to write. The example you gave has a perfectly valid R equivalent:

if (length(dim(my.array)) == 2) {
  A = my.array[1,]
} else if (length(dim(my.array)) == 3)
  A = my.array[1,,]

answered Dec 19 '13 at 10:23

krlmlr

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Ok, good point. It makes the code more complex to read and understand. – Remi.b Dec 19 '13 at 10:25

Why is it a bad habit to parse and evaluate a string code (in R)?

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