Programmieren lernen mit Python

1. 00:00Welcome back.
2. 00:01In the last video we have already looked at, how we create functions without parameters, so for example you can draw a tree.
3. 00:08But we have always used some with parameters, such as the print function.
4. 00:15Let's take a closer look at how it works.
5. 00:17Exactly, so here we see the function definition, which looks very familiar to you from the last video.
6. 00:25But what has been added now is a list of parameters, which we can give in parenthesis.
7. 00:31These can also be several parameters, which we then separate with a comma.
8. 00:37Parameters are therefore variables, that we can use in the function definition.
9. 00:43We then use arguments when calling the function, which are actual values for these parameters.
10. 00:50The example that Sebastian has just mentioned, print, we have used it several times before.
11. 00:56For example, we have given "Hello" as an argument here.
12. 00:59But we didn't just give you "hello." but many different arguments.
13. 01:03And depending on the arguments we give we also get different results from the function.
14. 01:11Let us first look at this in an example, again with the tree, on.
15. 01:14You already know the example, we want to draw a tree, but this time anywhere.
16. 01:19Therefore we write two additional parameters for the function definition in line 2, so we can pass the values for x and y.
17. 01:29We then use this to run to the corresponding position, pick up our pencils, go there and put our pen back down.
18. 01:38At the beginning these values are then correspondingly initialized by the x and y values as we pass them.
19. 01:47We must then specify them when using them, that means we draw a tree here at the position 0, 0 and another one just up on the right in 70 and 70.
20. 01:58If we run the program now, we see the tree first starting in the center and then just executed in the upper right-hand corner.
21. 02:05We can also make this function call within a loop.
22. 02:11For example, if we want to call the Tree function four times, we can do this on a loop.
23. 02:18To do this, we pass a list of values back to the for loop, so these four values you see in line 2, and this then takes the function tree as i-value.
24. 02:31So the function tree is called four times here, the y-value remains the same, i.e. always 0, so we always draw the tree at the same height.
25. 02:41However, the x value changes, so for example at the beginning is the -120.
26. 02:48So we must be careful, that we always pass the same number of arguments, as defined in the function definition.
27. 02:57And also the order remains the same, so the first value here is the x-value and the second is always the y-value.
28. 03:03And so we now see the four trees, which are drawn on one level.
29. 03:08Fancy. But now we move on and actually want to go to London, so we finally get there.
30. 03:15For this purpose, everyone should create a list of the attractions, which have a certain initial letter.
31. 03:22Simon receives the initial letter B and I'm supposed to pick out all the attractions from the list.
32. 03:28That is, next to Tower Bridge, for example, he looks mainly to Big Ben, since Big Ben starts with a B.
33. 03:35To solve this problem, we can now create more subtasks.
34. 03:42The first subtask would be to determine all the attractions, that begin with the letter B.
35. 03:50It's a bit like that, as we explained in one of the list videos, where we wanted to check all the words to see if they had an E in the second position.
36. 04:00So with the help of the square brackets. access the first letter, above the 0, because we are 0-indexed.
37. 04:09And then we can check using this double equal sign, whether the first letter is a B.
38. 04:17And in the comparisons we get, as we have already learned, always returns a truth value, so either True or False.
39. 04:25In the code it now looks like this: So we define a function starts_with_b and hand over an attraction.
40. 04:34So first we check, whether the first letter is the same as the B.
41. 04:40This truth value that we receive, is then stored in the variable result and returned.
42. 04:46So far we have not used this function, but only defined.
43. 04:52This is similar to functions without parameters, so it makes no difference.
44. 04:56If we go through with this, then it looks like this:
45. 05:00So we use the function starts_with_b and indicate a value to be checked in the round brackets, so the string "Big Ben."
46. 05:08We then store this value in the variable big_ben_with_b and try to spend it.
47. 05:13We get True back, because Big Ben is known to start with the letter B.
48. 05:18We can of course do this with another attraction, such as the London Eye.
49. 05:25Here we use a nested call.
50. 05:28These are always processed from the inside, that is, first we pass the string "London Eye" to the function start_with_b
51. 05:36and then we hand over the result obtained there, which we get by return in line 3, to the function print and spend it accordingly.
52. 05:47So we get false.
53. 05:49So we have already solved our first subtask.
54. 05:53The second subtask is now, to make a list of all the attractions that begin with B.
55. 05:59For this we first create a new empty list result with just two square brackets that are empty.
56. 06:07And to add a new element to this empty list, we know the append function.
57. 06:14That means we can on our score, call the append function on our empty list and then add the new value in round brackets.
58. 06:23So we are now defining ourselves a new definition, attractions_with_b and pass it a list, for example, as you can see here on the right side a short list of three attractions.
59. 06:37Then we create ourselves within this function, therefore also indented, an empty list result.
60. 06:44This is followed by a for loop, in which we go through all the attractions on this list one by one.
61. 06:53And for each of these attractions we call the function, which we have already defined on the previous slides, starts_with_b at and so they give this one attraction.
62. 07:07Before that, as you can see, there is an if, that is, we receive a truth value, so true or false.
63. 07:14So if this execution is true and we get a True back, then the value, i.e. this attraction, should be added to the list, because we know that the attraction starts with B.
64. 07:26And so we call here result .append with our attraction.
65. 07:31Since we are doing this with the for loop, we now add all the attractions, that appear in the previous list, that begin with a B
66. 07:41and can therefore with the return our result, so return our new list, which contains only attractions with B.
67. 07:49Let's combine this with the actual list of attractions, that they received on the trip to London.
68. 07:56The attractions are relatively many, so we've grouped these together in small print, but they're all on line 1.
69. 08:03Now we add the function start_with_b again, which we already know and which just checks whether a word begins with B
70. 08:11and the just introduced function attractions_with_b, which, as is well known, calls the function from above, starts_with_b again.
71. 08:21So now we just have to call the function attractions_with_b with our list of attractions and then output the result.
72. 08:30To do this, we call both functions again nested.
73. 08:34That is, we pass the list of attractions to the function attractions_with_b and the result we get there, which is a list, we pass this to the print function to output it.
74. 08:47And this long list suddenly becomes a rather short list, which then includes Big Ben, Buckingham Palace and British Library.
75. 08:56Yeah, that's pretty cool, but what about all the other contestants?
76. 09:00They are supposed to create lists with different initial letters.
77. 09:04Surely we can reuse it.
78. 09:06Right. But then we don't check if we have a B here, but simply whether we have a letter, where all attractions start with this letter.
79. 09:18Of course we have to include this in the function definition.
80. 09:23So we add the letter here and at the same time change the name of the function to start_with,
81. 09:30because we still don't know which letter is being passed and how. and thus cannot call the function starts_with_b.
82. 09:38Here below in line 10 the function is called again, so we have to give the letter again.
83. 09:45And since the function is called from within another function, we have to pass the list also up here at attractions_with_b, where up to now we only pass the list,
84. 09:57also pass the letter with and change the name here as well.
85. 10:00We call this function at the bottom of the print, so it needs to be adapted here as well.
86. 10:08And in addition, we are now also showing off here, with which letter all our attractions should actually begin.
87. 10:15And thus we receive the "London Eye" issue as here.
88. 10:18We can now easily adapt this and, for example, an N here and thus receive a new output with a new list of attractions that start with N.
89. 10:31So much for functions with parameters.
90. 10:35That's how we conceived you, how you can create your own functions.
91. 10:39In the function definition, you specify the following within the parameters to specify a list of parameters that you want to use accordingly.
92. 10:48If these are called by a function call, you then pass the corresponding values to her,
93. 10:55which you then use as variables, called arguments, are available within the function call and can be used similarly to variables.

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