How do you increment the number returned from llength?

Question

Suppose I capture the number of files in a directory using glob and llength:

# For this example, suppose llength returned 4.
set number_of_images [llength [glob -nocomplain -directory $destination_folder -type f *]]

I would like to use that number and increment it in a foreach loop depending on the number of files in another directory.

foreach file [glob -nocomplain -directory $source_folder -type f *] {
    puts [incr $number_of_images] 

    # I want to start the count at 5, and increment by 1 with each loop. incr fails to 
    # do so, as well as mathfunc::int.
}

My issue is, according to the documentation, llength returns a string:

Treats list as a list and returns a decimal string giving the number of elements in it.

Can I convert it to an int and use it?

Answer 1

In Tcl, everything is a string. Yes you can increment a variable that holds an integer (represented as a decimal string).

The issue is with your use of incr. The incr command is used to increment a variable holding an integer, not an integer value. Simply change your code from:

    puts [incr $number_of_images] 

... to ...

    puts [incr number_of_images]