Help with linked lists

[paulm]

What is it you are trying to achieve?
insert a single character into my list "A,B,C,D,E" alphabetically (all pointers-> data are strings length of 1)


What is the problem you are having?
the proc just ends up deleting the test string from the list when it encounters it


Describe what you have tried to solve this problem
tried tracing it on paper, got very confused

Turing:

%my type being used: type cell :     record         data : string         link : pointer to cell     end record % procedure in question proc insert_cell (test : string)     p := first     var p2 : pointer to cell := first -> link     var pn, temp : ^cell := nil         loop             put "p1= ", p -> data             put "p2= ", p2 -> data             if test <= p2 -> data then                 put "condition met: test >= p2  ... exiting"                 put "                  ", test, " <= ", p2 -> data                 new pn                 temp := p2 -> link                 pn -> data := test                 pn -> link := p2                 p -> link := temp                 exit             end if             p := p -> link             p2 := p2 -> link         end loop     getch (ch)     len += 1 end insert_cell

Please specify what version of Turing you are using
4.1

[jbking]

code:

new pn                 temp := p2 -> link                 pn -> data := test                 pn -> link := p2                 p -> link := temp

Let's look at this sequence of steps.

Temp is assigned to whatever is after p2 in the list.
Pn is given the test data and is linked to p2.
p's link is assigned to put at temp which is after p2 in the list, which I don't think is what you want. This is basically saying that after p in the list should be what came after p2 which is like deleting it from the list.
Consider just taking the temp assignment and the p->link line and I think you should see the problem here.

Another point is that if the test string came before the start of the list or at the end of the list you aren't handling these cases I think.

[paulm]

Thanks alot jbking! you're the first to respond to any of my threads here, ever

I revised the code and got it working thanks to your help:

Turing:

proc insert_cell (test : string)     findlast                                                   %finds the last node in the list     p := first     var p2 : pointer to cell := first -> link     var pn : ^cell := nil     if len < 7 then         if test <= first -> data then             new pn             first -> link := pn             pn -> link := p2             pn -> data := test         elsif test >= last -> data then             new pn             last -> link := pn             pn -> data := test             pn -> link := nil         else                                            p := first             loop                 if test <= p2 -> data then                     new pn                     p -> link := pn                     pn -> link := p2                     pn -> data := test                     exit                 end if                 p := p -> link                 p2 := p2 -> link             end loop         end if         len += 1     else         put "max linked list length reached ...exiting"         getch (ch)     end if end insert_cell

thanks again