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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])]

  • If there are no parameters passed or they are SHARED the parameters and parenthesis are not required in the procedure.
  • Variable names within the procedure do not have to match the names used in the CALL, just the value types.
  • 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. Code lines before or after the procedure are not permitted except for a DEF default variable TYPE declaration before the SUB line.
  • 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.
  • EXIT SUB can be used to exit early or to exit before GOSUB procedures using RETURN.
  • SUB procedures can save program memory as all memory used in a SUB is released on procedure exit except for STATIC values.
  • $INCLUDE text library files with needed SUB and FUNCTION procedures can be included in programs after other sub-procedures.
  • SUB procedures cannot have ON events, DATA or ON ERROR statements inside of them. Procedure code can use GOSUB and GOTO linenumbers and linelabels inside of the procedure only. QB64 allows TYPEs and DECLARE LIBRARY declarations.
  • 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.
  • 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.
  • 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.
  • WARNING! Do not place DATA fields after SUB or FUNCTION procedures! QB64 will FAIL to compile properly!
Qbasic allowed programmers to add DATA fields anywhere because the IDE separated the main code from other procedures.

Example: Text PRINT screen centering using PEEK to find the SCREEN mode width. Call and SUB procedure code:

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!

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