WEBVTT 00:01.450 --> 00:07.930 Hello again! In this video, we are going to look at basic string operations. So this section is 00:07.930 --> 00:12.190 going to look at the C++ library standard string class. 00:12.880 --> 00:13.930 It's a little bit unusual. 00:13.930 --> 00:17.950 It doesn't quite fit in with the other containers in C++. 00:18.640 --> 00:25.630 The reason for that is that the string class was originally designed in isolation based on various classes 00:25.630 --> 00:28.450 that people had hacked up, because there wasn't one in C++. 00:29.740 --> 00:34.300 And then later on, the standard template library came along and they decided it would be a good idea 00:34.720 --> 00:41.350 to make the interface for the standard string consistent with the interface for containers in the standard 00:41.350 --> 00:42.190 template library. 00:42.610 --> 00:47.140 So the string actually has two interfaces: its own native interface and the STL container. 00:47.770 --> 00:50.950 So the result of this is that it has a very big interface. 00:51.550 --> 00:55.690 I think there's over 100 member functions, and don not worry, we are not going to cover them all! 00:56.440 --> 00:59.920 But we are going to look at some of the most important and the most useful ones. 01:00.430 --> 01:04.210 So in this video, we are going to start off with the basic string operations. 01:06.610 --> 01:09.330 So, some of the things you can do with strings. You can assign them. 01:09.340 --> 01:12.550 So you can say that one string is equal to another string. 01:13.570 --> 01:19.060 You can append strings so you can add one string onto the back of another string. 01:20.470 --> 01:26.740 You can concatenate strings, which means that you take one string and another string and then return 01:28.030 --> 01:30.490 the first string, followed by the second string, without modifying them. 01:31.090 --> 01:32.920 And you can also do a comparison. 01:33.250 --> 01:39.730 You can see whether one string is less than another, or equal to, or greater than equal to, and so on. 01:41.500 --> 01:48.190 Obviously, these all work with standard strings, but they can also work with C-style strings on the 01:48.190 --> 01:49.270 right hand side as well. 01:50.680 --> 01:56.200 So you can assign the string to a C-style string, you can append and so on. 01:58.730 --> 02:03.620 Concatenation works if either of the strings is a C-style string, because you are not modifying them. 02:03.620 --> 02:05.000 So that is pretty straightforward. 02:05.780 --> 02:10.370 Modifying a C-style string is generally a bit of a pain, especially if it is on the stack. 02:11.810 --> 02:14.240 And these - except for assignment. 02:14.540 --> 02:20.060 So these three operations also work if there is a char on the right hand side. So you can append a single 02:20.060 --> 02:24.980 character to a string, you can compare a string against a single character and so on. 02:25.670 --> 02:29.750 And you can also have a single character for either one of these arguments in a concatenation. 02:33.630 --> 02:39.360 Talking of C-style strings, there is a lot of codes around that still uses C-style strings. 02:39.360 --> 02:45.300 It's either interfaces in C, to perhaps an operating system or a database or some third party library. 02:45.870 --> 02:49.260 Or old code, or code that has been written in an old way. 02:50.250 --> 02:54.750 And the good news is that you don't have to use C-style strings in your code all [over] the place. 02:54.990 --> 02:58.590 You can actually convert a standard string to a C-style string. 02:59.280 --> 03:06.360 There is a member function called c underscore str, and this will return a copy of the data in the string 03:07.140 --> 03:08.910 as a C-style string. 03:08.910 --> 03:13.920 So that's an array of const char with a null character terminating it. 03:15.610 --> 03:18.630 And this will actually return a pointer to the const char. 03:19.330 --> 03:24.280 So this is useful for working with code that is designed to take C-style strings. 03:27.690 --> 03:31.140 And the last operation we are going to look at is getting a substring. 03:31.470 --> 03:33.840 So that is just the substr member function. 03:34.590 --> 03:38.400 The argument is the index where you want to start the substring. 03:38.790 --> 03:39.720 So if we have 03:41.410 --> 03:47.260 argument six, that means the element with index six, six and in "Hello world", that's going to be the 03:47.360 --> 03:47.680 'w'. 03:48.400 --> 03:52.030 So this will return everything from that element up to the end of the string. 03:52.840 --> 03:54.250 So that is going to return "world". 03:56.650 --> 04:01.420 We can all so put a second argument, which is just the number of characters in the substring. 04:01.960 --> 04:05.890 So in this case, we're going to have the same substring, but only get the first two characters. 04:06.550 --> 04:08.440 So that is going to be the 'w' and the 'o'. 04:14.050 --> 04:15.670 So let's try this out. 04:16.150 --> 04:18.700 We have got a standard string, "Hello world". 04:20.140 --> 04:20.560 We are 04:21.580 --> 04:23.670 practicing our use of index notation. 04:23.740 --> 04:27.740 So we are going to change the second element to an 'a'. So it is now going to be 04:27.760 --> 04:28.720 "Hallo world". 04:30.150 --> 04:34.500 And then we are going to get the sub strings that we had in the slide and then we are going to print all these 04:34.500 --> 04:35.400 strings out. 04:36.970 --> 04:38.200 So what do we get? 04:40.920 --> 04:41.780 So we get 04:41.910 --> 04:42.270 "Hallo 04:42.300 --> 04:47.790 world", because we changed the second element. We get the substring from index six, which is "world". 04:48.450 --> 04:54.450 And then we get the same substring, but only the first two characters, which is "wo". And then 04:54.450 --> 04:58.050 finally, constructors, which perhaps I should have mentioned before. 04:58.050 --> 05:00.090 But there are ones which take substring. 05:00.090 --> 05:03.930 So... I mean, it is difficult to get started in C++. 05:03.930 --> 05:07.020 You cannot really do everything step by step. 05:07.020 --> 05:11.340 You have to sort of approach from the air and circle around and get some idea. 05:11.340 --> 05:14.130 Then you can go in and land, and look at things more closely. 05:16.200 --> 05:19.320 The default constructor, which takes no arguments. 05:20.530 --> 05:22.920 That will create an empty string with no data in it. 05:23.760 --> 05:30.300 So zero length. We can construct a string from a string literal. As we have been doing so far, without 05:30.900 --> 05:32.010 really mentioning it. 05:33.780 --> 05:38.250 There is another form which takes a number and a character. 05:38.820 --> 05:45.210 So this first number is the number of elements, and this character is the value of the elements. 05:45.690 --> 05:52.230 So three and 'x'x is going to give us a string with three elements which are 'x'. So we get the string 05:52.230 --> 05:52.900 "xxx". 05:54.570 --> 05:58.380 We can also use an initializer list, which is possibly not that useful. 05:59.840 --> 06:01.940 And we can also use a substring. 06:02.180 --> 06:05.600 We do not actually need to call the substring member function directly. 06:06.020 --> 06:08.720 We can just give the string and the index. 06:09.470 --> 06:12.650 So this will get the substring starting with index one, 06:12.680 --> 06:14.240 so that is going to be "ello". 06:15.530 --> 06:19.610 And if we add another argument, that will be the number of elements in the substring. 06:20.300 --> 06:23.870 So that is going to be the first three characters, which is just "ell". 06:28.150 --> 06:30.460 So this is all coded up here. 06:30.670 --> 06:35.290 We are also going to print out the number of characters in each string and then the data. 06:35.770 --> 06:36.850 So cout, left 06:36.850 --> 06:42.910 shift string will just print out the data in the string. As we have been doing, again without really saying. 06:45.030 --> 06:49.860 So we have our default constructor with an empty string, so expect zero length and no data. 06:50.990 --> 06:58.310 C-style string, the repeated value, the initializer list and then the two versions with substrings. 07:00.740 --> 07:04.310 And there we are, we get an empty string with nothing in it. 07:06.260 --> 07:09.770 And the others are all what we would expect from the slide. 07:11.150 --> 07:13.100 Okay, so that is it for this video. 07:13.580 --> 07:14.420 I'll see you next time. 07:14.420 --> 07:17.570 But meanwhile, keep coding!