Python – schnell und intensiv Programmieren lernen

1. 00:00Welcome to the last lesson this week. Dictionaries, also called dictionaries,
2. 00:05but mostly called a dictionary in German as well. It stores data in the form of
3. 00:13key-value. This is optimized to retrieve values very quickly when the key is known,
4. 00:20and fast here means in terms of computing. If the dictionary is very large and contains a lot of
5. 00:26things in it, it's still very fast. Keys are unique and the data type of the values
6. 00:33doesn't matter. It could also be a list or another dictionary or whatever. A few
7. 00:39usage examples. You would use this for a product catalog. Or in a hospital for
8. 00:45patient IDs, if additional information is linked to the ID. Or HTTP status codes,
9. 00:53the code and the meaning of that status. You can create a phone book or even
10. 00:59an actual dictionary. Or in online shops, the shopping cart. They look like this. I have my
11. 01:08object name and an equal sign to create the object. And then curly brackets.
12. 01:13The entries are again separated by commas, as we also know from lists. Now
13. 01:19we have a slightly different syntax. We have a key name, a colon
14. 01:24and then the content. I have two examples here with a string and with an integer.
15. 01:30And don't worry, that's not my real age, but I might like to be that old. I can
16. 01:36use len() to see how many key-value pairs there are in this dictionary. Two pieces. I
17. 01:44can access a tag with square brackets, as with lists. But now I'm not using
18. 01:50the position. We had the third position or everything from the fourth to the end or
19. 01:54or whatever. Now I take the name of the key. In this case, that's Name. And
20. 02:00it's also case-sensitive. So if I did it with a capital N at the top,
21. 02:06I have to do it that way now. I can also overwrite an entry if I want to change my age
22. 02:11because it was my birthday, then I can do it that way. And from now on that's just
23. 02:16changed. I can also add an entry, exactly the same syntax. If the key
24. 02:22does not yet exist, it will be added. A few conventions for the names. I use these
25. 02:30apostrophes, so a single quotation mark, in the dictionaries because I can work with them in f-strings
26. 02:37well. I can't mix double and single quotation marks in the f-string
27. 02:44We've already had an f-string. Here it is again as a reminder. f and then the
28. 02:50quotation marks for the character string. And we can use the curly brackets here to
29. 02:54do calculations or reference objects. And if I now want to access my dictionary
30. 03:01I can do that with the quotation marks. I'll show you in the demo what it
31. 03:06looks when you mix that. I've already prepared a bit here, a few words and
32. 03:13then I can call it up and see that the value of A fits. It is 17. If I now also
33. 03:22would work, but it doesn't work within
34. 03:27the f-string and I get a syntax error because I'm working with double here. It would
35. 03:33work again if I replace the double quotes with single ones, but since I usually use double quotes in the strings
36. 03:38and therefore also do so in the f-string, I have simply got into the habit of
37. 03:43to show the key name in the dictionaries with a single quotation mark from the outset and
38. 03:54I use that consistently. I can access dictionaries with these square brackets,
39. 04:02but I can also do it with another method that is error-proof. If I
40. 04:07use the get method that is inherent in every dictionary, I can say, get the value for
41. 04:17the key name and if the key is missing, then it returns a different value. In this case
42. 04:23the key name is in there and so I get Berry. Now you may briefly consider what
43. 04:27comes out if I write names in capital letters here and value if key is missing. That
44. 04:35means that my program does not throw an error, but just does something alternative. I can have the
45. 04:42keys, I can also output the keys once, it's more or less a list, it's not quite like
46. 04:48but it's similar and I can do the same with the values, dozent.values and if
47. 04:55I want to iterate, we'll do next week, then with items. I can see if the
48. 05:03key has the entry age and that also works. I can see that this is true,
49. 05:09but in large dictionaries I have the problem that the keys have to be calculated first and in
50. 05:18the memory story have to be cached and then it is checked whether age is in there
51. 05:23And I said yes, dictionaries are very fast at looking up data,
52. 05:28so we can just do that, is there an age in the lecturer and that is both shorter in the code,
53. 05:34and also faster. This speed only becomes important when you have large amounts of data,
54. 05:38but that happens sometimes. I can also remove elements from a dictionary. I
55. 05:45I inserted a new lecturer dictionary here and with del for delete I can
56. 05:52delete a complete entry. It's just gone. I can also use pop()
57. 05:58as with lists and here, too, I can specify which element is to be removed with
58. 06:04the key name and that is also, as with pop, in the case of lists, the result or that,
59. 06:11what is removed is returned. This means that I can save this in the “studie” object
60. 06:18and then the lecturer no longer has the element in it. If I now try to remove the age
61. 06:26we have already done that above, so the key is no longer there. I get
62. 06:31an error. And here, too, I can avoid the error by saying, well, give a
63. 06:36alternative back. For example, you could do non if I want to indicate, well, if it
64. 06:42exists, then I'll delete it, but if not, then yes, that's okay too. Then
65. 06:48just basically skip the rest. And if I want to remove all the entries, I can use clear.
66. 06:54Then I see, Lecturer is now an empty dictionary. Again, a reminder that
67. 07:01we have memory pointers, so pointers and we need copies, if we want to have copies.
68. 07:06A dictionary 1 with a few things in it, a dictionary 2, which points to dictionary 1 and
69. 07:13is now not an object itself, but just a pointer to the other, and Dict 3, which is an
70. 07:21actual independent copy using the copy function, just like with lists. If I
71. 07:26now change Dict 1, Dict 2 also changes, as with all other mutable objects.
72. 07:33I change Dict 1, I write something different in the C entry and now I see that Dict 2 has also
73. 07:38changed and dictation 3 is not. So, the same thing as we have already seen with lists, but it
74. 07:44is important, that's why we repeated it again. Dictionaries have a great
75. 07:49function, they can be updated if I have a dictionary with some things
76. 07:53in it and a second one with maybe new values in it or new ones for the existing keys
77. 08:00values. Here, for example, I see that I now increase the key A to 11 and add the key D
78. 08:05I do that with Dict.Update. This updates the values that already
79. 08:13for the existing keys and for new keys it adds them. That means that if I
80. 08:17I now look at Dict 1, I can see that A is 11 and that D has been added. The
81. 08:24others remain untouched. I can output a dictionary formatted and that is
82. 08:33a great transition into next week. It's worth staying tuned to the course. Don't
83. 08:38be surprised if we do a preliminary loop without explaining it in detail. It will all come next
84. 08:41week. But we have a dictionary again and now I say I take these items and run
85. 08:48a loop through them for the keys and values, hence K and V, and I now
86. 08:54just a little bit in each iteration and then I can do a lot with very little code
87. 08:59output. The entry has the value and I can format that and it's great. So
88. 09:05just a sneak peek at some of the cool stuff we'll be doing next week.
89. 09:09A recap at the end. So we saw that dictionaries are key-value pairs
90. 09:17by indicating this with curly brackets and a colon or a colon. We
91. 09:26have square brackets to indicate by name. Dictionaries are not ordered,
92. 09:32so the key is not necessarily the first or second entry, but it is
93. 09:38somewhere there. With get, I can get a replacement or a fallback if the
94. 09:45key is not present and I can check if a key is present with in. I can
95. 09:50remove things, there is also a replacement option there, and finally, as usual, dict.copy.
96. 09:57Finally, the update, which is a great way to connect several data sources
97. 10:04As always, have fun with the tasks, let me know if anything is unclear,
98. 10:10that happens, it tends to be more our fault than yours, and we're happy to change it.
99. 10:16And otherwise, have fun highlighting in your Revcard. Ciao, see you next week.

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