From QB64 Wiki
Jump to navigation Jump to search

The _PRESERVE REDIM action preserves the current contents of dynamic arrays, when resizing or changing indices.


REDIM _PRESERVE array(newLowerIndex& [TO newUpperIndex&]) [AS variabletype]


  • REDIM or the $DYNAMIC metacommand must be used when the array is first created to be able to resize and preserve.
  • If _PRESERVE is not used, the current contents of the array are cleared by REDIM.
    • All element values of an array are preserved if the array size is increased.
    • The remaining elements of the array are preserved if the array size is decreased.
    • If the new index range is different from the original, all values will be moved to the new corresponding indices.
  • REDIM _PRESERVE cannot change the number of array dimensions, but can change the number of elements.
  • Always use the same array TYPE suffix (AS type) or a new array type with the same name may be created.


  • SUB or FUNCTION arrays created using REDIM require that they be recreated to be used after arrays are ERASEd.
  • Warning: Do not use negative upper array index values as an "Out of Memory" error (or global Operating System errors) will occur.
  • Use _PRESERVE before SHARED or an "invalid variable name" error will occur.


Example 1: Changing the upper and lower array bounds

REDIM a(5 TO 10) ' create array as dynamic using REDIM a(5) = 123 REDIM _PRESERVE a(20 TO 40) PRINT a(20)

Explanation: a(20) is now the 123 value a(5) was. The upper or lower bounds of arrays can be changed, but the type cannot. New array indices like a(40) are null(0) or empty strings. If the array element count is not reduced, all of the data will be preserved.

Example 2: Sizing an array while storing file data.

REDIM Array$(1) 'create a dynamic string array filename$ = "Readme.txt" 'Qb64 information text file OPEN filename$ FOR INPUT AS #1 DO UNTIL EOF(1) count = count + 1 IF count > UBOUND(Array$) THEN REDIM _PRESERVE Array$(count * 3 / 2)'increase array's size by 50% without losing data END IF LINE INPUT #1, textline$ Array$(count) = textline$ LOOP CLOSE #1 FOR c = 1 TO count PRINT Array$(c) IF c MOD 20 = 0 THEN k$ = INPUT$(1) NEXT END

See also

Keyword Reference - Alphabetical
Keyword Reference - By Usage
Main Wiki Page