DO...LOOP - QB64 Wiki

DO...LOOP

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DO...LOOP statements are used in programs to repeat code or return to the start of a procedure.


Syntax:

DO [{WHILE|UNTIL} condition]
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.
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LOOP
DO
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.
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LOOP [{WHILE|UNTIL} condition]


Description:

  • DO UNTIL or DO WHILE used with LOOP. The DO will evaluate the condition before running the loop code:
UNTIL checks if the condition is False each time before running code.
WHILE checks if the condition is True each time before running code.
  • DO used with LOOP UNTIL or LOOP WHILE. The loop code will run at least once:
UNTIL checks if the condition is False before running loop code again.
WHILE checks if the condition is True before running loop code again.
  • NOTE: You cannot use a condition after both the DO and LOOP statements at the same time!
  • Use EXIT DO to exit a DO loop within the loop code.
  • If a loop never meets an exit condition requirement, it will never stop!
  • Condition expressions are required after WHILE or UNTIL or QB64 will return a compiler error!


Relational Operators:
Symbol Condition Example Usage
<  Less than  IF a < b THEN
>  Greater than  IF a > b THEN
=  Equal  IF a = b THEN
<=  Less than or equal  IF a <= b THEN
>=  Greater than or equal  IF a >= b THEN
<>  NOT equal  IF a <> b THEN


Example 1: Using WHILE to clear the keyboard buffer.

DO WHILE INKEY$ <> "": LOOP ' checks evaluation before running loop code DO: LOOP WHILE INKEY$ <> "" ' checks evaluation after one run of loop code


Example 2: Using UNTIL to clear the keyboard buffer.

DO UNTIL INKEY$ = "": LOOP ' checks evaluation before running loop code DO: LOOP UNTIL INKEY$ = "" ' checks evaluation after one run of loop code


Example 3: Using a one time DO loop to exit ANY of several FOR LOOPs, without using GOTO.

SUB reads header contents of a BSAVE file that may include embedded RGB color settings before the image.

DEFINT A-Z INPUT "Enter a BSAVE file name to read the file for screen mode:"', filenm$ CheckScreen filenm$ END DEFINT A-Z SUB CheckScreen (Filename$) 'find Screen mode (12 or 13) and image dimensions DIM Bsv AS STRING * 1 DIM Header AS STRING * 6 Scr = 0: MaxColors = 0 OPEN Filename$ FOR BINARY AS #1 GET #1, , Bsv '1 check for small 2 character GET #1, , Header '2 - 7 rest of file header IF Bsv <> CHR$(253) THEN ' small 2 character denotes a BSAVE file COLOR 12: LOCATE 15, 33: PRINT "Not a BSAVE file!": SLEEP 3: EXIT SUB END IF GET #1, , widN '8 no color info bmp sizes GET #1, , depN '9 " " " DO IF widN > 63 OR depN > 63 THEN EXIT DO ' width and depth already found FOR i = 10 TO 55 'check for Screen 12 embedded colors GET #1, , RGB tot12& = tot12& + RGB 'PRINT i; RGB; : SOUND 300, 1 'test sound slows loop in QB IF RGB > 63 OR RGB < 0 THEN EXIT DO IF i = 55 AND tot12& = 0 THEN EXIT DO NEXT GET #1, , wid12 '56 GET #1, , dep12 '57 IF wid12 > 63 OR dep12 > 63 THEN EXIT DO FOR i = 58 TO 775 'check for Screen 13 embedded colors GET #1, , RGB tot13& = tot13& + RGB 'PRINT i; RGB; : SOUND 300, 1 'test IF RGB > 63 OR RGB < 0 THEN EXIT DO IF i = 775 AND tot13& = 0 THEN EXIT DO NEXT GET #1, , wid13 '776 GET #1, , dep13 '777 LOOP UNTIL 1 = 1 'TRUE statement exits one-time LOOP CLOSE #1 COLOR 14: LOCATE 10, 25 SELECT CASE i CASE IS < 56: IF widN > 640 THEN Scr = 13: MaxColors = 0 PRINT "Default Screen 13:"; widN \ 8; "X"; depN ELSE LOCATE 10, 15 PRINT "Screen 12 ("; widN; "X"; depN; ") OR 13 ("; widN \ 8; "X"; depN; ")" DO: SOUND 600, 4 COLOR 13: LOCATE 12, 23 'ask if no data found. Prevents ERROR opening in wrong mode INPUT "Enter a Screen mode 12 or 13 : ", Scrn$ Scr = VAL(Scrn$) LOOP UNTIL Scr = 12 OR Scr = 13 END IF IF Scr = 12 THEN MaxColors = 0: PWidth = widN: PDepth = depN IF Scr = 13 THEN MaxColors = 0: PWidth = widN \ 8: PDepth = depN CASE 56 TO 775 PRINT "Custom Screen 12:"; wid12; "X"; dep12 Scr = 12: MaxColors = 16: PWidth = wid12: PDepth = dep12 CASE 776: PRINT "Custom Screen 13:"; wid13 \ 8; "X"; dep13 Scr = 13: MaxColors = 256: PWidth = wid13 \ 8: PDepth = dep13 END SELECT END SUB

Code by Ted Weissgerber
Explanation: The SUB procedure reads a file that was BSAVEd previously. If the RGB colors are stored before the image, the values can only be between 0 and 63. Higher values indicate that the image width and height are located there and that there are no stored color values to be read. SUB later displays the dimensions of the file image that GET placed in the file array. The loop is set to only run once by creating a TRUE UNTIL statement such as 1 = 1. When a screen mode cannot be determined, the user must select one.
Dimensions and location of width and height information indicates the screen mode as SCREEN 13 if it has 768 RGB values and dimensions of 320 X 200 max. If the file only holds 64 settings and/or is larger than 320 X 200, it uses SCREEN 12 or 9. The procedure EXITs the DO LOOP early when the image size is found with or without custom color settings.
Divide SCREEN 13 GET widths by 8.


See also:



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