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I have the following lists:

set w 1 2 3 4 5 6 7
set x 1 2 3 4 5 8 9
set y 1 2 3 4 0 9 1
set z 1 2 3 4 5 6 7

I want to compare all the lists - w.r.t corresponding indices - and find out the common elements and append those common elements to a new list. If I compare the above lists, I see that 1 2 3 4 are common across all the lists and have the same index, so my output should be:

1 2 3 4

If there are no common elements(even at 0'th index) my new list will be empty.

I first start off by creating a list of lists:

set l1 [list $w $x $y $z]

Then I create a nested loop to compare the lists and extract my common elements, I will be using list 'x' as my reference list:

for set j 0 $j < [llength $x] incr j 
 for set i 1 $i < [llength $l1] incr i 

 set a [lindex [lindex $l1 $i] $j]

 if $a == [lindex $x $j] && [lindex $l2 $j] == 
 lappend l2 $a
 else 
 break
 
 

What I'm getting is:

1 2 3 4 5

You are effectively only comparing list x against list x, and the actual output from your code above (assuming list l2 is initially empty) is actually:

1 2 3 4 5 8 9

You might ask:

Why is it comparing list x against list x?

Your inner loop starts at index 1 (set i 1), which is list x in l1.

You might further ask:

Why are the other lists not being compared?

Once you have appended something to l2, lindex $l2 $j for the next list will never be empty, so the inner loop will break.

So, how to do it?

I would probably use something like this:

set w 1 2 3 4 5 6 7
set x 1 2 3 4 5 8 9
set y 1 2 3 4 0 9 1
set z 1 2 3 4 5 6 7

set l1 [list $w $x $y $z]
set l2 [list]

set num [llength $x]

for set i 0 $i < $num incr i 
 # This variable will tell us how many matched. 0 indicating none.
 set same 0

 for set j 0 $j < [llength $l1] incr j 
 # lindex accepts multiple indices, see the manual
 # using x as reference, if the current list's ith element is the same as x's ith element...
 if [lindex $l1 $j $i] == [lindex $x $i] 
 incr same
 
 

 # if same reached 4, means 4 matched
 if $same == 4 
 lappend l2 [lindex $x $i]
 

Result:

1 2 3 4

You could make the inner loop break if the elements do not match, so it won't loop unnecessarily.

Or instead of counting the number of matches, you could check if the inner loop broke with something like:

for set i 0 $i < $num incr i 
 set broke 0

 for set j 0 $j < [llength $l1] incr j 
 if [lindex $l1 $j $i] != [lindex $x $i] 
 set broke 1
 break
 
 

 # if it did not break, then we know all matched
 if $broke == 0 
 lappend l2 [lindex $x $i]
 

Similar implementation, but using an array to store the unique elements of the slice

set lists [list $w $x $y $z]
set common [list]

for set i 0 $i < [llength $w] incr i 
 array unset elements
 foreach list $lists 
 set elements([lindex $list $i]) dummyvalue
 
 set unique [array names elements]
 if [llength $unique] == 1 
 lappend common $unique
 


puts $common ;# => 1 2 3 4