REDIM variable-name,integer-expression

Changes the capacity of an existing array to the new size given by integer-expression.

Arrays are zero-indexed and range-checked: valid indices are 0 to size - 1.
The array’s element type is determined by variable-name and does not change (A integer, A# real, A$ string).
Calling REDIM with the current size performs no operation.

Remarks: REDIM preserves existing elements from index 0 up to min(old_size, new_size) - 1.; When growing, new slots are initialised to 0 (integer or real arrays) or "" (string arrays).

Note: When shrinking, elements at indices new_size to old_size - 1 are permanently lost.; REDIM changes the size only. It does not change type, and it does not create a new array name.; Declare the array first with DIM.

Grow an integer array

DIM A,3
A(0) = 10
A(1) = 20
A(2) = 30
 
REDIM A,5
PRINT A(0)
PRINT A(1)
PRINT A(2)
REM prints 0
PRINT A(3)
REM prints 0   
PRINT A(4)

Shrink and lose tail elements

DIM B,5
B(0) = 1
B(1) = 2
B(2) = 3
B(3) = 4
B(4) = 5
 
REDIM B,3
PRINT B(0)
PRINT B(1)
PRINT B(2)
REM B(3) and B(4) no longer exist

Make room before inserting with PUSH

DIM Q,3
Q(0) = 100
Q(1) = 200
Q(2) = 300
 
REDIM Q,4
PUSH Q,1
Q(1) = 150

Strings

DIM NAMES$,2
NAMES$(0) = "ALPHA"
NAMES$(1) = "BETA"
 
REDIM NAMES$,3
REM prints empty string
PRINT NAMES$(2)

The new size must be large enough for any indices you intend to access (0..new_size-1).
After shrinking, accessing an index >= new_size raises a runtime error.
Use REDIM together with PUSH or POP to manage contents while changing capacity.

See also:
DIM · PUSH · POP · Array