     Next: Some Useful Shortcuts Up: More on Generating Previous: Combining Several Lists

## Modifying Data

So far when I have asked you to type in a list of numbers I have been assuming that you will type the list correctly. If you made an error you had to retype the entire def expression. Since you can use cut--and--paste this is really not too serious. However it would be nice to be able to replace the values in a list after you have typed it in. The setf   special form is used for this. Suppose you would like to change the 12 in the list x used in the Section 4.3 to 11. The expression

```(setf (select x 4) 11)
```
will make this replacement:
```> (setf (select x 4) 11)
11
> x
(3 7 5 9 11 3 14 2)
```

The general form of setf is

```(setf form value)
```
where form is the expression you would use to select a single element or a group of elements from x and value is the value you would like that element to have, or the list of the values for the elements in the group. Thus the expression
```(setf (select x (list 0 2)) (list 15 16))
```
changes the values of elements 0 and 2 to 15 and 16:
```> (setf (select x (list 0 2)) (list 15 16))
(15 16)
> x
(15 7 16 9 11 3 14 2)
```

A note of caution is needed here. Lisp symbols are merely labels for different items. When you assign a name to an item with the def command you are not producing a new item. Thus

```(def x (list 1 2 3 4))
(def y x)
```
means that x and y are two different names for the same thing. As a result, if we change an element of (the item referred to by) x with setf then we are also changing the element of (the item referred to by) y, since both x and y refer to the same item. If you want to make a copy of x and store it in y before you make changes to x then you must do so explicitly using, say, the copy-list   function. The expression
```(def y (copy-list x))
```
will make a copy of x and set the value of y to that copy. Now x and y refer to different items and changes to x will not affect y.     Next: Some Useful Shortcuts Up: More on Generating Previous: Combining Several Lists

Luke Tierney
Tue Jan 21 15:04:48 CST 1997