[R] How to visualise what code is processed within a for loop

[MacQueen, Don]

Your code doesn't make sense to me in a couple of ways.

Inside the loop, the first line assigns a value to an object named "t".
Then, the second line does the same thing, assigns a value to an object named "t".

The value of the object named "t" after the second line will be the output of the ifelse() expression, whatever that is. This has the effect of making the first line irrelevant. Whatever value t has after the first line is replaced by whatever it gets from the second line.

It looks like the first line inside the loop is constructing the name of a data frame column, and storing that name as a character string. However, the second line doesn't use that name at all. If your goal is to update the contents of a column, you need to assign something to that column in the next line. Instead you assign it to the object named "t".

What you're looking for will be more along the lines of this:

    for (i in 1:10){
      nm <- paste0("V", i)
      d0[[nm]] <- ifelse( regexpr(d1[i,1], d0$X0) > 0, 1, 0)
    }

This may not a complete solution, since I have no idea what the contents or structure of d1 are, or what the regexpr() is expected to return.

And notice the use of double brackets, [[ and ]]. This is one way to reference a column of a  data frame when you have the column's name stored in a variable. Another way is d0[ , nm]


A couple of additional comments:

 "t" is a poor choice of object name, because it is one of R's built-in functions (immediately after starting a fresh session of R, with nothing left over from any previous session, type help("r") and see what you get).

 ifelse() is intended for use on vectors, not scalars, and it looks like maybe you're using it on a scalar (can't be sure about this, though)

For example, ifelse() is designed for this kind of usage:
> ifelse( c(TRUE, FALSE, TRUE) , 1:3, 11:13)
[1]  1 12  3

Although it works ok for these
> ifelse(TRUE, 3, 4)
[1] 3
> ifelse(FALSE, 3, 4)
[1] 4
They are not really what it is intended for.

--
Don MacQueen
Lawrence Livermore National Laboratory
7000 East Ave., L-627
Livermore, CA 94550
925-423-1062
Lab cell 925-724-7509
 
 
On 4/24/18, 12:30 AM, "R-help on behalf of Luca Meyer" <r-help-bounces using r-project.org on behalf of lucam1968 using gmail.com> wrote:

    Hi,
    
    I am trying to debug the following code:
    
    for (i in 1:10){
      t <- paste("d0$V",i,sep="")
      t <- ifelse(regexpr(d1[i,1],d0$X0)>0,1,0)
    }
    
    and I would like to see what code is actually processing R, how can I do
    that?
    
    More to the point, I am trying to update my variables d0$V1 to d0$V10
    according to the presence or absence of some text (contained in the file
    d1) within the d0$X0 variable.
    
    The code seem to run ok, if I add print(table(t)) within the loop I can see
    that the ifelse procedure is working and to some cases within the d0$V1 to
    d0$V10 variable range a 1 is assigned. But when checking my d0$V1 to d0$V10
    after the for loop they are all still equal to zero...
    
    Thanks,
    
    Luca
    
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