Digits of a number

For a non-negative integer, the last decimal digit is the remainder on division by 10. Integer division by 10 removes that digit. Repeating these operations visits digits from right to left. Review DIV and MOD first.

Sum of digits#

1n = 40722x = n3sum = 04WHILE x > 05  digit = x MOD 106  sum = sum + digit7  x = x DIV 108END9PRINT sum10PRINT n

The output is 13, then 4072. The algorithm consumes the copy x, preserving n. When n = 0, the loop never runs, but the initial sum 0 is already correct.

Reverse a number#

Append a digit to the right by multiplying the current result by 10 and adding that digit.

1n = 12042x = n3reversed = 04WHILE x > 05  reversed = reversed * 10 + x MOD 106  x = x DIV 107END8PRINT reversed

The result is 4021. Reversing 1200 gives the number 21: numbers do not preserve leading zeros. If zeros belong to an identifier, use a string.

Digit count and edge cases#

Start a counter at zero and increment it whenever you remove a digit. Handle n = 0 separately: its decimal representation contains one digit, though WHILE x > 0 runs zero times. For negatives, decide whether to process ABS(n); a minus sign is not a digit.

Every step removes a digit, so k digits take O(k) steps. Keep calculations within the language's exact integer range; a reversed number may be larger than its input.

Practice#

1. Count even digits in 4072: there are 3, including zero.
2. Test whether a number equals its reversal; try 0, 7, 1221, 120.
3. Find the product of the digits of 205: it is 0, not 10.