Extracting the value of an element from Dataframe in R

Question

I am very new to R and trying to extract a specific element from a data frame and compare it with an integer.

I had a table saved in text file.

data-file image

I used the following to read the table.

mydata = read.table("file.txt");

In my case I want to compare an element say, say the first element of USERPOR (which is 1.0) with an integer 1.0 (so the comparison should return true).

The code I wrote was

mydata[[2,7]]

[1] 1.000
Levels: 1.000 10.0000 2.000 3.000 4.00 5.00 6.000 7.000 8.000 9.000 USERPROR

However, when I compared them, I got 'FALSE'. Can anybody tell why is that so?

> mydata[[2,7]]==1.0

[1] FALSE

Russ Lenth · Accepted Answer · 2014-08-20 17:24:06Z

2

Hmmmm. First, elements of a data.frame are ordinarily accessed using single brackets -- like mydata[2,7]. Double brackets will access a column, e.g. mydata[[2]] will return the second column. Thus, mydata[[7]][2] is the same as mydata[2,7].

Second, since your output includes a Levels: list, it appears that that variable is stored as a factor having levels of "1.000", "10.0000", ... "USERPROR" (odd enough that I'm guessing the data are entered incorrectly). Accordingly, I believe that in your example, mydata[2,7] == "1.000" would return TRUE.

In general, if you want to compare a numeric value with an integer, don't use a comparison value like 1.0, because thje .0 part forces it to be stored as floating-point, not integer. If the data are stored as floating-point, there may be enough roundoff that a number computed as 1.0 is not exactly equal to an integer 1. The reliable way to test it is to use round(mydata[2,7]) == 1.

answered Aug 20, 2014 at 17:24

Russ Lenth

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5 Comments

Batool Over a year ago

I tried round(mydata[2,7]) == 1. It is giving me the following error. Error in Math.factor(1L) : round not meaningful for factors. Please see my edited question.

Russ Lenth Over a year ago

Right. That's because what you have there is a factor, like I stated earlier - not a floating-point value. I repeat, I think your first issue is that the data are not stored correctly. Get the data in there right first -- otherwise, it's garbage in, garbage out.

Russ Lenth Over a year ago

Here's the problem. Using read.table to read the data, but include the argument header=TRUE. It appears the variable names are being included as part of the data.

Batool Over a year ago

Yes "header=true" fixed the problem. So the actual issue was the way the data was read. Thank you so much! I literally spent hours on this.

Russ Lenth Over a year ago

Good. I don't know why they defaulted to header=FALSE in read.table, but they did.

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