[R] Logical vectors
Joshua Wiley
jwiley.psych at gmail.com
Thu Nov 4 07:46:15 CET 2010
On Wed, Nov 3, 2010 at 10:50 PM, Stephen Liu <satimis at yahoo.com> wrote:
> Hi folks,
>
> Pls help me to understand follow;
>
> An Introduction to R
>
> 2.4 Logical vectors
> http://cran.r-project.org/doc/manuals/R-intro.html#R-and-statistics
>
> 1)
>> x
> [1] 1 2 3 4 5
a vector, x, is defined with 5 elements, {1, 2, 3, 4, 5}
>> temp <- x != 1
perform the logical test that x does not equal 1 returning either TRUE or FALSE.
1 = 1 so TRUE, 2 != 1 so FALSE, etc. next we assign *the results* of
the logical test to the vector 'temp'
>> temp
> [1] FALSE TRUE TRUE TRUE TRUE
print the vector to screen
>>
>
>
> 2)
>> x
> [1] 1 2 3 4 5
note that x has not changed here, we assigned to temp, not to x.
>> temp <- x > 1
now we assign the results of the logical test, x > 1
{1 = 1 so FALSE, 2 > 1 so TRUE, 3 > 1 so TRUE, 4 > 1 so TRUE, 5 > 1 so TRUE}
we assign these results to a vector, 'temp'. This *new* assignment
overwrites the old vector 'temp'
>> temp
> [1] FALSE TRUE TRUE TRUE TRUE
print temp to screen, this is the results of our second logical test (x > 1).
>
>
> Why NOT
>> temp
> [1] TRUE FALSE FALSE FALSE FALSE
My best guess of where you got confused is that we assigned the
results to 'temp', so 'x' remained unchanged {1, 2, 3, 4, 5}, or that
you confused '<-' which is the assignment operator in R, to "less than
negative..." *OR* "less than or equal". We could write this
equivalently:
> 1:5 > 1
[1] FALSE TRUE TRUE TRUE TRUE
this was the logical test, whose results were assigned to the vector, "temp".
> assign(x = "temp", value = 1:5 > 1)
using the assign function (not often recommended) to avoid any
confusion with the assignment operator, "<-".
> temp
[1] FALSE TRUE TRUE TRUE TRUE
print to screen
HTH,
Josh
>
> ?
>
>
> TIA
>
> B.R.
> Stephen L
>
>
>
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--
Joshua Wiley
Ph.D. Student, Health Psychology
University of California, Los Angeles
http://www.joshuawiley.com/
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