Translator – Multiple String Assignments (Release)

Upon contemplating the string flags in the table entries, I realized that care must be taken to make sure the string flag is set in the table entries when an operator or function code has a string operand. A better design is to set the string flag automatically during table initialization if any of the operands of the string data type. Therefore, the string flags were removed from the table entries and the Table constructor was modified set the flags in the table entries automatically.

To correct the issue of saving all the operands for string list assignments, a temporary simple stack is used to save the operands as they are popped off of the done stack and processed. Since the last two operands (the value being assigned and the last item in list) have already been popped and processed by the find code routine, these are pushed to the simple stack before processing the rest of the operands. Each additional operand processed is also pushed to this stack.

After the list operands are processed for a string list assignment a new array needs is allocated and filled from the temporary simple stack. In order to determine the size of this array, either the operands need to be counted as they are processed, or the number of items in the stack needs to be accessed. Since the simple stack already knows how many items it has, a new access function was added to the SimpleStack class to return the number of items.

It had been decided previously that mixed strings and sub-strings would be allowed in a multiple list assignment statement. Having a mix string list assignment replaces the need for a separate sub-string list assignment code, the mix-string list assignment will handle this case. So, a new AssignListMixStr associated code was added to AssignList (this new code has a sub-string as the first operand, the value being assigned). The list assignment handling code was modified to detect if the list contains both reference strings and sub-strings. If it does, then the token is changed to this new AssignListMixStr code.

Several new sub-string and mix-string assignments were added to test inputs. This completes string handling in the Translator. The code now handles expressions and assignment statements and ibcp_0.1.11-src.zip has been uploaded at Sourceforge IBCP Project along with the binary for the program. Next the real meat of the Translator begins, translating actual BASIC commands...

Translator – Sub-String Assignments (Release)

After testing the Table initialization code that checks maximum number of operands and maximum number of associated codes, several sub-string assignments statements were added and tested.  Then some sub-string assignments with error were added.

One of the types of errors being tested were the assignment of temporary strings like MID$(A$+B$,2)=C$. There error was correctly detected, but the error was pointing to the MID$, which could be confusing. Therefore, a change was made that for an assignment, if the expected reference flag is not set, then if the token without the reference flag is a sub-string function (its data type is sub-string), then the error token is set to the token if the first operand of the sub-string function. So for this statement, the error will be pointing to the + of the A$+B$.

During testing of the sub-string assignments, one of tests tried was the mixing of regular strings and sub-string, for example A$,LEFT$(B$,1),C$=D$. This did not work correctly (this type of statement will be allowed). Also noticed that for lists, the assignment operator output list item only had two operands (the last variable being assigned and the operand being assigned). It should contain all of the operands being assigned. So these are two more issues that need to corrected.

In any case, sub-string assignments are working so another developmental release is being made and ibcp_0.1.11-dev-3-src.zip has been uploaded at Sourceforge IBCP Project along with the binary for the program. Now to fix assignment list  operands and mixing of string types...

Translator – Sub-String Assignments (Testing)

The changes to support sub-string assignment were implemented, which included adding the SubStr data type entries to the conversion code array in the match code routine; changing the data type to SubStr for the LEFT$, MID$ and RIGHT$ table entries; and added the sub-string reference checking. Initial testing started with the existing test inputs – no problems were discovered.

Upon trying a sub-string assignment, discovered that it didn't work because there was no AssignSubStr associated code, so this code was added along with its table entry. Now, many of the existing test inputs were failing. Next discovered that the maximum associated codes needed to be changed from 2 to 3 because Assign now had three associated codes. The wrong value was causing the find code routine to malfunction.

I thought it would be best to calculate both the maximum operands and maximum associated codes automatically during the Table initialization, so moving these constants to members of the Table class seemed to be the best solution. Unfortunately, these values need to be constants because they are used to define the sizes of several arrays.

It would still be prudent to have the the Table initialization at least check to make sure these constants agree with what was in the table entries. So code was added to the Table initialization to find the maximum operands and associated codes as it was scanning the entries for code checking. Two new table error types were added for these errors, which are reported by exceptions from the Table constructor. Testing continues...

Translator – Sub-String Assignments

There will be several associated codes for assigning strings from a reference string or temporary string to a reference string or to a sub-string. The Encoder will determine which code to use once the string type for each operand is determined. For the Encoder to know if a sub-string is present, a new SubStr data type is needed, which only the LEFT$, MID$ and RIGHT$ functions return.

The Translator will check to make sure that sub-string assignments are valid by making sure the string argument is a reference string, at least if it is a Paren or NoParen token type. If it turns out to be a user function, then the Encoder will report the error. The Translator will also make sure that a compound sub-string assignment like LEFT$(RIGHT$(B$,3),2)="AB" is not entered.

The find code routine needs to handle sub-strings where the operands are popped off of the done stack and the reference flag is checked. For the first operand (last operand popped), if the data type of the token is SubStr (indicating a sub-string function) and the operand is a String, then the token's reference flag is set to operand's reference flag (the reference flag is transferred).  The operand's reference flag is then cleared as it is currently implemented. This assumes that the first argument of the sub-string function is the string being assigned.

If the operand's data type is anything but String (SubStr or TmpStr), then the reference flag is not transferred. For the invalid compound sub-string assignment, since the data type of the operand will be SubStr, the reference flag of the function's token will not be set. When the reference flag is checked for an assignment operator, the reference flag will only be set if the string operand was reference, otherwise if won't be set and an error will be returned.

Sub-String Assignments

Many BASICs support sub-string assignments, the syntax that will be supported is the same used as in GW-Basic, QuickBASIC, FreeBASIC, etc.. Here is an example sub-string assignment along with it's translation:

MID$(A$,5,2) = "AB"       A$ 5 2 MID3$ "AB" AssignSubStr

Notice the new code AssignSubStr and the lack of <ref> on A$. When the MID$ is processed by the Translator, the reference flags of its operands are cleared, hence no A$<ref>. However, the MID3$ needs to have its token reference flag set when the assign operator checks for a reference, since that MID3$ token will be on the done stack. Consider how this statement is processed at run-time.

At the A$, a reference string is pushed on the evaluation stack, that is, its pointer and length are copied to the stack. When the MID3$ is processed, it will pop the 2 and 5 off of the stack. The pointer on top of the stack will be changed to point to the fifth character in A$ and the length is set to 2 (assuming that A$ is at least 6 characters long).

When the AssignSubStr is processed, the "AB" string constant will be popped off of the stack. Two characters of this value are copied directly to the pointer that is on top of the stack (which is pointing to the fifth character in A$). A different AssignSubStr vs. AssignStr code is required because no allocation occurs, only a copy to an existing character array. An AssignSubStrTmp code is also required for the case that the value being assigned is a temporary string, which needs to be deleted after the characters are copied.

Since LEFT$ and RIGHT$ are also sub-string functions, there is no reason these functions can't also be used to assign sub-strings – and as it turns out, no extra code is required. Next, how sub-string assignments will be handled by the Translator...

Translator – Sub-Strings

The processing of operators and internal functions with string operands needs to be delayed until encoding because in some cases, the Translator does not know if a string is a reference (from a variable or array element) or a temporary (from a user string function) because the Translator does not know if identifier is a variable, array element or a user function.

At run-time, if the string argument to a sub-string function is a reference string, then it's result is also a reference string in that the string does not need to be deleted. If the string argument is a temporary string, then it's result is also a temporary string in that the string needs to be deleted.

Each sub-string function will have an associated code. The main code will have a reference string operand and will return a reference string. The associated code will have a temporary string operand and will return a temporary string.

It would appear that sub-strings have no impact on the Translator. However, there is one more capability that needs to be considered, and that is sub-string assignments where a portion of a string is assigned without affecting the rest of the string...

Translator – Saved Operand Correction (Release)

The problem where the saved operand pointers were not pointing to a conversion code that was inserted (was pointer to the original operand), was a simply correction. All that was needed was to set the operand array element to the output of the output list append call where the conversion code token was inserted. The ibcp_0.1.11-dev-2-src.zip file, another developmental release, has been uploaded at Sourceforge IBCP Project along with the binary for the program.

Sub-Strings – Details

The results of sub-string functions (LEFT$, MID$ and RIGHT$) will use the same character array as the argument string. Exactly how this is accomplished depends on whether the argument string is a reference string or a temporary string.

For a reference string, the pointer and length of the character array are copied to the evaluation stack. This means that the pointer and length on the stack can be modified without affecting the actual pointer and length of the variable or array element. For LEFT$, only the length on the stack needs to be changes. For MID$ and RIGHT$, both the pointer and length need to changed depending on the integer arguments.

For a temporary string, the sub-string operation is a little more involved because the pointer to the character array on the stack cannot be modified – it is needed to delete the character array when the temporary string is no longer needed. The length is not needed to delete the character array, so it can be modified. Since the pointer cannot be modified, the portion of the sub-string needs to be moved to the beginning of the character array. Again for LEFT$, only the length needs to be changed, the sub-string is already at the beginning of the array. But for MID$ and RIGHT$, the sub-string result needs to be moved to the beginning of the array.

For reference strings, there is no allocation, copying or deletion required for the sub-string operation. For temporary strings, there is still a moving of characters, but there is no allocation of a new character array or deletion of the old character array. After the expression is evaluated, the temporary string will be deleted. There could be some unused space in the character array from a sub-string operation when the temporary string is assigned and used as it. Enough of discussing how sub-strings will work at run-time, next, what impact sub-strings have on the Translator...

Sub-Strings

Because strings are variable length, they are dynamically allocated as needed during run-time. It is therefore beneficial to reduce as much as possible the amount of allocating, copying and deleting of the character arrays. This is why reference strings, the values of string variables and array elements, are used as-is so a new character array does not need to be allocated and copied to in order to put the reference string on the evaluation stack. But this will require extra code to know when to delete a temporary string and not to delete a reference string on the stack, which will be accomplished with additional associated codes.

There is another way to reduce some allocation and deleting of temporary strings for the sub-string internal functions, aka LEFT$, MID$, and RIGHT$. These functions can have a reference string or a temporary string as an argument. The obvious way to implement these functions is to create a new temporary character array of the appropriate size for the resulting string, copying the characters from the argument string to the new array and if the argument string is a temporary string, to delete it.

There is a simpler way to handle sub-strings that will eliminate the allocation of a new character array, deleting the temporary string argument if present and the copying for a reference string. Consider how a string is stored, there is a character array, there is a pointer to the character array and there is the length of the character array. The pointer and length make up the members of the String class along with the allocated array. A resulting sub-string will never be larger than the string argument. so why not use the same character array, since it has already been allocated. Next, details on how this will work with reference strings and temporary strings...