For-structure, Loop-until-structure

In this tutorial the goal is to remove items from a sequence that are divisible by two.

for-structure

To start we try using a for-structure:

sequence set = {1,2,3,4,5,6,7,8}  
for i = 1 to length(set) do  
  if remainder(set[i],2)=0 then  
     set = remove(set, i)  
  end if  
end for  
 
-- error 
-- subscript value 6 is out of bounds, reading from a sequence of length 4  

It is not unreasonable to wish that this example works. The problem is that the for-structure evaluates the loop limits just once and locks them into place. The the sequence keeps changing length; it's like the end of a wagging dog's tail that the for-structure can't keep up with.

You must remember that a for-structure is intended to loop a pre-determined number of times and the limits are fixed before looping commences.

exit keyword

However, you can use the exit keyword to jump out of a for-structure before an error is encountered. Since you know the error is due a "subscript value out of bounds" you can write:

sequence set = {1,2,3,4,5,6,7,8}  
for i = 1 to length(set) do  
  if remainder(set[i],2)=0 then  
     set = remove(set, i)  
  end if  
  if i>length(set) then exit end if 
end for  
? set 
--> {1,3,5,7} 

backwards

A better solution is to traverse the sequence backwards:

sequence set = {1,2,3,4,5,6,7,8} 
for i=length(set) to 1 by -1 do 
    if remainder(set[i],2)=0 then 
        set = remove(set, i) 
    end if 
end for 
? set 
--> {1,3,5,7} 

loop-until-structure

Another solution is to use the loop-until-structure were the test condition is recalculated for each iteration:

sequence set = {1,2,3,4,5,6,7,8} 
integer i=1 
loop do 
    if remainder(set[i],2)=0 then 
        set = remove(set, i) 
    end if 
    i += 1 
    until i > length(set)  
end loop 
? set 
--> {1,3,5,7}