Level 1, lesson 8

# 1.8 Copying a string variable

You might want to copy one string variable to another. There is a special word for that, named 'CMOVE'. It takes the two strings and copies a given number of characters from the source to the destination. Let's take a look at this example:

         32 string one                   \ define the first string
         32 string two                   \ define the second string

         " Greetings!" one copy          \ initialize string one
         count                           \ get the length of string one
         1+                              \ account for the terminator!
         two swap                        \ get the order right
         cmove                           \ copy the string
         two count type  cr              \ print string two

The most difficult part to understand is probably why and how to set up the data for 'CMOVE'. Well, 'CMOVE' wants to see these values on the stack:

         source destination #chars

With the expression:

         one count

We get these data on the stack:

         source length

But the terminator hasn't been accounted for so far. That's why we add:

         1+

So now this parameter has the right value. Now we have to tell 'CMOVE' where to copy the contents of string one to. If we simply add:

         two

The data is not presented in the right order:

         source #chars destination

So we add the extra 'SWAP' in order to get it right. Of course you may define a word that takes care of all that:

         : copy$ swap count 1+ rot swap cmove ;
         32 string one
         32 string two
         " Greetings!" one copy
         two copy$

You may wonder why we keep on defining words to make your life easier. Why didn't we simply define these words in the compiler instead of using these hard to understand words?

Well, 4tH is derived from Forth and we wanted to stay as compatible as possible. That means if you have a good knowledge of both 4tH and Forth you can write programs that can be ported to full-fledged Forth compiler. Much later we will give you tips on how to do that.