Upside-down numbers: Difference between revisions

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 ;* [https://www.numbersaplenty.com/set/upside-down_number/ Numbers-A-Plenty: Upside-down number]
 ;* [[oeis:A299539|OEIS:A299539 - Upside-down numbers]]
+=={{header|Python}}==
+<syntaxhighlight lang="python">""" rosettacode.org task Upside-down_numbers """
+def gen_upside_down_number():
+    """ generate upside-down numbers (OEIS A299539) """
+    wrappings = [[1, 9], [2, 8], [3, 7], [4, 6],
+                 [5, 5], [6, 4], [7, 3], [8, 2], [9, 1]]
+    evens = [19, 28, 37, 46, 55, 64, 73, 82, 91]
+    odds = [5]
+    odd_index, even_index = 0, 0
+    ndigits = 1
+    while True:
+        if ndigits % 2 == 1:
+            if len(odds) > odd_index:
+                yield odds[odd_index]
+                odd_index += 1
+            else:
+                # build next odds, but switch to evens
+                odds = sorted([hi * 10**(ndigits + 1) + 10 *
+                              i + lo for i in odds for hi, lo in wrappings])
+                ndigits += 1
+                odd_index = 0
+        else:
+            if len(evens) > even_index:
+                yield evens[even_index]
+                even_index += 1
+            else:
+                # build next evens, but switch to odds
+                evens = sorted([hi * 10**(ndigits + 1) + 10 *
+                               i + lo for i in evens for hi, lo in wrappings])
+                ndigits += 1
+                even_index = 0
+print('First fifty upside-downs:')
+for (udcount, udnumber) in enumerate(gen_upside_down_number()):
+    if udcount < 50:
+        print(f'{udnumber : 5}', end='\n' if (udcount + 1) % 10 == 0 else '')
+    elif udcount == 499:
+        print(f'\nFive hundredth: {udnumber: ,}')
+    elif udcount == 4999:
+        print(f'\nFive thousandth: {udnumber: ,}')
+    elif udcount == 49_999:
+        print(f'\nFifty thousandth: {udnumber: ,}')
+    elif udcount == 499_999:
+        print(f'\nFive hundred thousandth: {udnumber: ,}')
+    elif udcount == 4_999_999:
+        print(f'\nFive millionth: {udnumber: ,}')
+        break
+</syntaxhighlight>{{out}}
+<pre>
+First fifty upside-downs:
+19   28   37   46   55   64   73   82   91
+258  357  456  555  654  753  852  951 1199
+1379 1469 1559 1649 1739 1829 1919 2198 2288
+2468 2558 2648 2738 2828 2918 3197 3287 3377
+3557 3647 3737 3827 3917 4196 4286 4376 4466
+Five hundredth:  494,616
+Five thousandth:  56,546,545
+Fifty thousandth:  6,441,469,664
+Five hundred thousandth:  729,664,644,183
+Five millionth:  82,485,246,852,682
+</pre>