File Read and Write in tcl

I saw some previous posts related to opening a file to perform read and write operations but I din't get an answer for my task. I want to append some results to a file (should create a new file if it does not exist).
However, if the file already has result then append should be skipped and move on to next search for next result. I wrote a script for this and I have problem in reading the file. The script is something like this:

proc example {} {
    set a [result1 result2  ... result n]
    set op [open "sample_file" "a+"]
    set file_content ""
    while { ![eof $op] } {
        gets $op line
        lappend file_content $line
    }
    foreach result $a {
        if {[lsearch $file_content $result] == -1} {
            puts $op $result
        }
    }
    close $op
}

Note: In this script I find variable "line" to be empty {""}. I guess I have trouble in reading the file. Please help me with this

---

What you forgot, is to seek to the beginning of the file before reading:

proc example {} {
    set a {result1 result2  ... result n}; # <== curly braces, not square
    set op [open "sample_file" "a+"]
    set file_content ""
    seek $op 0; # <== need to do this because of a+ mode
    while { ![eof $op] } {
        gets $op line
        lappend file_content $line
    }
    foreach result $a {
        if {[lsearch $file_content $result] == -1} {
            puts $op $result
        }
    }
    close $op
}

Update

You can simplify the reading (while loop and all), with one single read statement:

proc example {} {
    set a {result1 result2  result3}
    set op [open "sample_file" "a+"]
    seek $op 0
    set file_content [read $op]
    foreach result $a {
        if {[lsearch $file_content $result] == -1} {
            puts $op $result
        }
    }
    close $op
}

example

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