A SUB procedure is a procedure within a program that can calculate and return multiple parameter values just like a full program.
- SUB Procedure_name [(parameter[, list...])]
- ... 'procedure variable definitions and statements
- END SUB
- Parameters passed after the procedure call must match the variable types in he SUB parameters in order.
- If there are no parameters passed or they are SHARED the parameters and parenthesis are not required in the procedure.
- Parameter Variable names in the procedure do not have to match the names used in the CALL, just the value types.
- All dynamic variable values return to 0 or null strings when the procedure is exited except for STATIC variable values.
- SUB procedures can return multiple values through the parameters unlike functions.
- SUB procedures return to the next code statement after the call in the main or other procedures.
- EXIT SUB can be used to exit early or to exit before GOSUB procedures using RETURN.
- TYPE and DECLARE LIBRARY declarations can be made inside of SUB procedures in QB64 only.
- SUB procedures can save program memory as all memory used in a SUB is released on procedure exit except for STATIC values.
- _DEFINE can be used to define all new or old QB64 variable TYPE definitions instead of DEF***.
- $INCLUDE text library files with needed SUB and FUNCTION procedures can be included in programs after all sub-procedures.
- QB64 ignores all procedural DECLARE statements! Define all parameter TYPEs in the SUB procedure.
- Images are not deallocated when the SUB or FUNCTION they are created in ends. Free them with _FREEIMAGE.
- The IDE can create the SUB and END SUB lines for you. Use the Make SUB option in the Edit Menu. A box will come up for you to enter a name for the SUB. Enter the procedure code between the SUB and END SUB lines created.
- Qbasic SUB procedures could not have ON events, DATA, TYPE or ON ERROR statements inside of them.
- Procedure code can use GOSUB and GOTO linenumbers and linelabels inside of the procedure only.
- The Qbasic IDE will DECLARE a SUB at the top of the module code after it has been referenced and the BAS file is saved.
- Only DEFINT, DEFSNG, DEFLNG or DEFDBL default variable type declarations are allowed before the SUB block.
- Qbasic's IDE may place a DEFINT, DEFSNG, DEFLNG, DEFDBL or DEFSTR statement before the SUB line if it is used in the main module. It may even be the wrong variable type needed. It can be removed or changed if necessary.
- An uncalled SUB will not be DECLAREd in Qbasic if it is not used.
- Qbasic allowed programmers to add DATA fields anywhere because the IDE separated the main code from other sub-procedures.
DEFINT A-Z SCREEN 13 Center 10, 15, "This text is centered." ' example module sub call END DEFINT A-Z ' only code allowed before SUB line is a DEF statement or a comment SUB Center (Tclr, Trow, Text$) DEF SEG = &H40 Columns = PEEK(&H4A) DEF SEG Middle = (Columns \ 2) + 1 ' reads any screen mode width Tcol = Middle - (LEN(Text$) \ 2) COLOR Tclr: LOCATE Trow, Tcol: PRINT Text$; ' end semicolon prevents screen roll END SUB
- Explanation: The procedure centers text printed to the screen. The parameters are the text color, row and the text itself as a string or string variable. The maximum width of the screenmode is found and divided in half to find the center point. The text string's length is also divided in half and subtracted from the screen's center position. The procedure will also work when the WIDTH statement has been used. When adding variables to Text$ use the + concatenation operator. Not semicolons!
Example 2: SUB and FUNCTION procedures always return to the place they were called in the main or other sub-procedures:
exit 2 exit 1 13
- Note: Parameter a is used to call the sub-procedures even though parameters n and m are used internally.