WEBVTT 00:00.840 --> 00:06.900 Hello again! In this video, we are going to look at stream iterators. The iterators we have used so far are 00:06.900 --> 00:12.480 designed for use with containers. They are so-called random access iterators, which means you can 00:12.480 --> 00:17.130 jump anywhere in a container. And it takes the same speed, whether you are going to the first element or the 00:17.130 --> 00:17.680 one millionth element. 00:20.190 --> 00:23.680 The standard library provides these iterators, which work on streams. 00:24.130 --> 00:25.950 They are defined in the iterator header. 00:26.640 --> 00:29.310 They can only work if all the data is of the same type. 00:30.330 --> 00:31.680 There is an input stream 00:31.680 --> 00:32.300 iterator, 00:32.400 --> 00:35.060 istream_iterator, and an output stream 00:35.070 --> 00:37.140 iterator, ostream_iterator. 00:37.920 --> 00:43.470 These are templated types, so we need to instantiate them. And we need to give a parameter, which is 00:43.470 --> 00:46.140 the type of the data that is going to be used with the stream. 00:47.730 --> 00:50.790 These stream iterators have a very limited interface. 00:51.270 --> 00:55.140 The only thing you can do with an output stream operator is to assign to it. 00:55.680 --> 01:02.040 And that will cause the argument of the operation to be pushed onto the stream. With an input stream 01:02.040 --> 01:03.080 [iterator], you can dereference it. 01:03.870 --> 01:07.350 And that will give you the object, which is at the current position in the stream. 01:08.280 --> 01:14.760 Or you can increment the iterator, and that will move the position market of the stream to the next object. 01:16.110 --> 01:18.840 So it will be ready to read the next object. 01:20.640 --> 01:26.070 The output stream operator does define these dereferencing and increment operators, but they do not 01:26.070 --> 01:26.820 actually do anything. 01:27.720 --> 01:32.970 They are just there, so you can write code that looks the same as any other code that uses iterators. 01:37.940 --> 01:43.550 To create an output stream iterator, we need to bind it to an output stream, and we do that by 01:43.550 --> 01:46.370 passing a stream object as the argument to the constructor. 01:47.030 --> 01:53.150 So here we have an output stream iterator, which will work with ints, and it is bound to cout, to 01:53.150 --> 01:54.260 the standard output. 01:54.680 --> 01:58.430 Every time that we assign to this iterator, data is pushed onto the stream. 02:01.540 --> 02:04.330 This is done using the left shift operator for the type. 02:04.570 --> 02:09.730 So if you have a data type that does not support the left shift operator, then this will not compile. 02:11.570 --> 02:15.050 So here we are assigning 7. Star oi equals 7. 02:15.740 --> 02:16.930 The star is optional here, 02:17.210 --> 02:23.110 but I have written it, so it looks the same as other iterator code. And that will actually cause the '7' 02:23.120 --> 02:24.770 to appear on the output. 02:29.040 --> 02:33.380 We can also provide a second argument, which is a delimiter, this is optional. Here, 02:33.390 --> 02:36.930 I have provided a backslash 'n', the newline character. 02:37.650 --> 02:39.330 This has to be a C string. 02:41.620 --> 02:46.750 And then when we assign to this iterator, the argument of the assignment, the variable i, will be sent 02:46.750 --> 02:49.090 to the stream, followed by a new line. 02:49.990 --> 02:57.610 So in this loop: each iteration, the value of i will be sent to the output stream, followed by a 02:57.610 --> 02:58.060 new line. 02:58.600 --> 03:02.740 So we should get the numbers from 0 to 9, each displayed on a separate line. 03:04.780 --> 03:06.220 So let's try this out. 03:07.600 --> 03:13.540 Here's the code with the iterator header. The output stream iterator. And then the loop, which 03:13.540 --> 03:15.490 assigns to the iterator. 03:15.850 --> 03:18.640 I have also included the increment as well, which is - 03:19.800 --> 03:22.800 Again, it does nothing, but it looks more consistent with iterator code. 03:25.320 --> 03:29.580 So there we are, we get the numbers from 0 to 9, each followed by a newline character. 03:32.160 --> 03:34.650 Creating an input stream operator is very similar. 03:35.070 --> 03:39.240 We give the data type as the parameter, and we bind it to an input stream. 03:39.720 --> 03:44.670 So this is an iterator, ii, which will read ints from the keyboard. 03:46.860 --> 03:51.390 And then, when we dereference this, it will read an integer from the keyboard. 03:56.120 --> 03:57.200 So let's try that. 03:58.830 --> 04:06.120 There is our stream iterator, than we dereference it. And then we store the value that it gets in a int and then 04:06.120 --> 04:07.470 we print out the value of that. 04:09.120 --> 04:09.870 So we should get 04:09.870 --> 04:11.070 whatever I type in here. 04:11.970 --> 04:12.870 There we are, 42. 04:17.810 --> 04:21.290 We can use a loop with an iterator to read multiple input. 04:22.610 --> 04:25.610 The usual question arises. How do we know when to stop? 04:26.510 --> 04:28.910 The way this works is actually rather clumsy. 04:30.350 --> 04:34.250 When there is no more input to read, the iterator will be empty. 04:34.790 --> 04:39.020 And then we have to compare the iteartor against an empty iterator. 04:40.250 --> 04:41.900 So how do we get an empty iterator? 04:42.320 --> 04:44.810 We do that by not binding it to any stream. 04:45.950 --> 04:51.800 So here we have an iterator, which can read strings but is not bound to any input stream. 04:56.300 --> 04:58.550 So in fact, we need two input iterators. 04:58.940 --> 05:05.600 One to read the data and an empty one to compare against. And then we have a loop where we compare 05:05.600 --> 05:06.740 the two iterators. 05:08.910 --> 05:12.060 If there is some data to read, then this will not be empty. 05:12.510 --> 05:15.990 So they will be not equal, and we go into the loop. 05:16.920 --> 05:23.340 We can dereference the iterator. That will give us the data that we read from the keyboard, and then we 05:23.340 --> 05:29.190 can store that somewhere. And then we can increment the iterator, so that it will move the stream marker 05:29.190 --> 05:30.270 to the next position. 05:30.660 --> 05:33.420 So that is now ready to read the next int from the keyboard. 05:34.200 --> 05:35.370 And then this goes on. 05:35.850 --> 05:39.210 And then when we read the end of input, this iterator will be empty. 05:39.600 --> 05:42.210 And it will compare equal to this one, and the loop will end. 05:45.000 --> 05:46.200 So let's try this out. 05:46.560 --> 05:50.310 We have our two iterators, the one to read data and the empty one. 05:50.880 --> 05:53.790 We have a vector where we are going to store all the data that we read. 05:54.660 --> 05:55.710 Then we have our loop. 05:56.730 --> 06:00.600 Is there any data in this iterator, or is it empty? 06:01.440 --> 06:07.770 And then in the loop, we dereference the iterator to get the data that we have read, and we increment 06:07.770 --> 06:08.640 the iterator. 06:08.940 --> 06:13.170 So we are ready to read the next int. And this will continue, until we read the end of input. 06:13.890 --> 06:18.630 And then, at the end, we have a range-for loop. And this will print out all the elements of the vector, 06:18.900 --> 06:21.060 which should contain all the data that we have just read. 06:26.580 --> 06:28.380 So I need to enter some numbers here... 06:32.140 --> 06:33.160 End of input... 06:33.640 --> 06:34.210 So there we are. 06:34.450 --> 06:37.750 We do get all the numbers that I entered, in the order that I entered them. 06:38.320 --> 06:44.380 So the control Z is of input and then the input iterator is equal to this empty iterator. 06:47.790 --> 06:50.820 So what is the point of having these stream iterators? 06:51.300 --> 06:54.740 Well, we can actually make the input-output statement a lot more concise. 06:54.750 --> 06:55.560 We could actually do 06:56.210 --> 06:56.550 this... 07:00.700 --> 07:04.330 Now we have just a single line which will read in data. 07:04.720 --> 07:08.590 So that is very concise. And I hope it still works... 07:12.010 --> 07:12.340 Yes! 07:13.480 --> 07:19.750 Always a worry when doing live coding! The other advantage is we can use these with standard algorithms, 07:19.750 --> 07:20.950 which we are going to look at later. 07:21.310 --> 07:28.030 (Sorry, more teasers!) These algorithms can do various things with containers such as transforming them, 07:28.570 --> 07:32.200 and you can copy the results into another container. 07:32.200 --> 07:34.510 So you just given an iteraator to the start of that container. 07:35.110 --> 07:40.900 If instead you give an output stream operator, then the result of that operation will be displayed 07:40.900 --> 07:41.500 on the screen. 07:42.130 --> 07:47.050 So if you want to check that your program is doing the right thing, you just need to change an 07:47.050 --> 07:47.440 iterator. 07:47.740 --> 07:49.450 You do not need to write lots more code. 07:51.040 --> 07:52.480 Okay, so that is it for this video. 07:52.900 --> 07:53.740 I will see you next time. 07:53.740 --> 07:56.050 But meanwhile, keep coding!