Reversing an Integer's Digits

[Eddie W. Shore]

What Should I Add To Reverse the Digits?

Let A, B, C, D, and E be individual digits (0-9) of an integer. AB would represent a two digit integer with the value of 10 * A + B. ABC would represent a three digit integer with the value of 100 * A + 10 * B + C.

Reversing a Two Digit Integer

AB + # = BA
10 * A + B + # = 10 * B + A
# = 9 * (B - A)

Example: Let AB = 76.
A = 7, B = 6
# = 9 * (6 - 7) = -9
76 - 9 = 67

Reversing a Three Digit Integer

ABC + # = CBA
100 * A + 10* B + C + # = 100 * C + 10 * B + A
# = 99 * (C - A)

Example: ABC = 469
# = 99 * (9 - 4) = 495
469 + 495 = 964

Reversing a Four Digit Integer

ABCD + # = DCBA
1000 * A + 100 * B + 10 * C + D + # = 1000 * D + 100 * C + 10 * B + A
# = 999 * (D - A) + 90 * (C - B)

Example: ABCD = 7219
# = 999 * (9 - 7) + 90 * (1 - 2) = 1908
7219 + 1908 = 9127

Reversing a Five Digit Integer

ABCDE + # = EDBCA
10000 * A + 1000 * B + 100 * C + 10 * D + E + # =
10000 * E + 1000 * D + 100 * C + 10 * B + A
# = 9999 * (E - A) + 990 * (D - B)

Example: ABCDE = 52693
# = 9999 * (3 - 5) + 990 * (9 - 2) = -13068
52693 - 13068 = 39625

Having the Calculator Do It

The program REVINT reverses the digits of an integer, up to 11 digits. The program does not allow numbers that have non-zero fractional parts or integers more than 11 digits. Instead of solving for # (see above), the program splits the integers into a list in reverse order, and uses list processing to get the final answer.

HP Prime Program: REVINT

Caution: Integers that end or begin with zero may not return accurate results. My suggestion is not use 0s with this program. See examples below for more details.

Code:

EXPORT REVINT(N)
BEGIN
// 2022-09-18 EWS
// reverse the integer N
// up to 12 digits
LOCAL D,P,A,I,M,L;
L:={};
P:=XPON(N);

// check size 
  IF P>11 THEN
  RETURN "TOO BIG";
  KILL;
  END;
 
// check type
  IF FP(N) THEN
  RETURN "NOT AN INTEGER";
  KILL;
  END;
   
D:=N;

// loop
  FOR I FROM P DOWNTO 0 DO
  A:=D/ALOG(I);
  L:=CONCAT({IP(A)},L);
  D:=D-IP(A)*ALOG(I); 
  END;
  
// rebuild 
M:=ΣLIST(MAKELIST(ALOG(X),X,P,0,−1)*L);
RETURN M; 
END;

Examples:

REVINT(4321) returns 1234

REVINT(56765) returns 56765 (56765 is a palindrome, reversing the digits results in the same number)

REVINT(42910) returns 1924 (01924 - be aware about integers ending or beginning with 0)

REVINT(67.28) returns "NOT AN INTEGER" (error)