WEBVTT 00:01.500 --> 00:06.600 Hello again! In this video, we are going to look at converting between strings and numbers. 00:08.480 --> 00:14.900 In C, there were the functions atoi, atod and so on, for ints, doubles, and these worked with 00:14.900 --> 00:19.400 C-style strings of character. For going the other way, 00:19.400 --> 00:22.640 C does not provide anything, although you can use sprintf. 00:23.630 --> 00:24.520 Or sprintf? 00:24.900 --> 00:25.940 However you want to pronounce it! 00:26.570 --> 00:32.840 That is a bit clumsy, so many component vendors provide non-standard functions like itoa for converting 00:33.110 --> 00:34.670 ints to strings. 00:36.350 --> 00:38.210 C++ does have these functions, 00:38.580 --> 00:45.230 but now it also provides functions which work with standard strings, and these are much better. For 00:45.230 --> 00:47.140 converting numbers to strings, 00:47.150 --> 00:50.770 There's to underscore string, which is part of the standard library. 00:51.410 --> 00:57.860 There are overloads which will take any numeric built-in type. And they will return the argument as a standard 00:57.920 --> 00:58.340 string. 00:59.690 --> 01:06.380 So, for example, if we have a floating-point number, which represents the digits of pi, this will 01:06.380 --> 01:11.150 be converted to a string containing the characters '3', '.', '1', '4', '5', 9. 01:11.360 --> 01:17.680 So those are actually character digits and not a numeric bit pattern. So we can try that out. 01:18.020 --> 01:22.520 We are going to do that conversion and use that to initialize a string called pi. 01:23.180 --> 01:27.560 Then we are going to append this string to another string, just to prove that we really have got a string 01:27.560 --> 01:27.980 object. 01:30.940 --> 01:31.600 So there we are. 01:31.630 --> 01:36.040 We now have the digits of PI appended to this string, "hello". 01:36.520 --> 01:43.390 And I notice that it's added on a zero, so obviously there is some sort of a rounding or extension going 01:43.390 --> 01:43.570 on. 01:50.260 --> 01:56.050 For going the other way, there's a different function for each built-in numeric type. As an example, 01:56.050 --> 02:02.080 stoi will take a string arguments and return the int which has the same digits. 02:03.190 --> 02:09.040 So if we have a string with the digits '4' and '2', this will convert it to an int with the value 02:09.040 --> 02:09.700 42. 02:11.230 --> 02:14.690 The way this works, is that it ignores leading whitespace. 02:14.710 --> 02:21.670 So if we have some space characters at the front, those are ignored. Then it will try to convert the 02:21.670 --> 02:24.070 following characters into a number. 02:25.670 --> 02:29.390 If it reaches something which is not a numeric character, it will stop. 02:29.870 --> 02:33.020 So in this case, the result would be 314. 02:34.880 --> 02:39.140 So the space at the beginning is ignored, but this space in the middle causes the conversion to 02:39.140 --> 02:39.470 stop. 02:44.740 --> 02:49.840 If we want to know where the conversion stopped, then we can pass an optional second argument. 02:50.320 --> 02:52.060 This is passed by address. 02:53.220 --> 02:58.680 And when the function returns, it will contain the number of characters that were successfully processed. 02:59.700 --> 03:05.280 And if the conversion was completely successful, this will be equal to the number of characters in 03:05.280 --> 03:05.730 the string. 03:06.960 --> 03:13.050 If it was partially successful, this will give us the index of the first character, which is not numeric. 03:14.350 --> 03:18.940 And if the conversion is completely unsuccessful, if it can't even get started, then it will throw an 03:18.940 --> 03:19.450 exception. 03:20.320 --> 03:22.900 And I am afraid I have got to throw another teaser in here. 03:22.930 --> 03:25.150 We are going to look at exceptions later on in the course. 03:25.990 --> 03:28.810 But for now, just imagine that it means an unrecoverable error. 03:30.300 --> 03:36.270 The function will assume that the string represents a decimal number, base 10, unless we tell it otherwise. 03:36.660 --> 03:41.040 And if we want to use a different base, then there's another optional argument we can give. 03:42.210 --> 03:47.130 So that is the third argument. If we are not using the second argument - if we do not care how many characters 03:47.130 --> 03:49.500 were processed - we can just set that to null pointer. 03:50.550 --> 03:52.110 And then this argument will be the base. 03:52.410 --> 03:59.010 So here we are using base 16, hexadecimal, and then we are converting this hexadecimal number. 03:59.850 --> 04:02.580 So that is 2a, which will be 42. 04:04.080 --> 04:07.080 The base can be anything from 0 to 36. 04:07.620 --> 04:11.510 I am not quite sure what base 0 is useful for, but it is there. 04:12.420 --> 04:17.520 (I am sure someone must neede it) and the digits in here can be numeric digits or they can be A to Z. 04:18.510 --> 04:23.160 So you know that in hexadecimal, we can use ABCDEF as well as the numbers. 04:23.820 --> 04:27.330 So if you have a number in base 36, you can go all the way up to Z! 04:30.690 --> 04:32.430 So let's try out some code. 04:32.820 --> 04:38.250 So first of all, we're converting a string with the characters four and two into the number 42. 04:39.210 --> 04:45.060 Then we have the string we saw in the slide, where the conversion may not succeed. 04:45.060 --> 04:46.140 So let's see how that goes. 04:47.790 --> 04:53.190 And then finally, we have an example of where we are converting from base 16. I have commented out the 04:53.670 --> 04:55.560 example where it fails, for the moment. 04:59.040 --> 04:59.880 So there we are. 05:00.330 --> 05:09.900 We get 42, the first non-numeric character in here is at index five. So it has processed the first 05:09.900 --> 05:12.300 two space characters and the number. 05:13.430 --> 05:14.400 And it has got the result 05:14.470 --> 05:15.230 314. 05:15.920 --> 05:23.450 And then the hexadecimal conversion gave us 42. If I now uncomment this one, which is going to fail. 05:26.210 --> 05:33.110 And, right. That has actually called abort(), so - well, we will go into that later on, but obviously the conversion 05:33.110 --> 05:33.620 has failed! 05:39.470 --> 05:45.470 And there are also other functions like stoll, which will convert to long long, and other 05:45.470 --> 05:50.750 ones for unsigned long and any other built-in integer type. They all work exactly the same way. 05:52.380 --> 05:57.450 We can also do this with floating point numbers. There is stod for double, and functions for float 05:57.450 --> 05:58.470 and long double. 05:59.630 --> 06:03.950 This one is almost the same, except you cannot use different bases. It all has to be decimal. 06:06.300 --> 06:11.790 And just to illustrate the difference, if we imagine we have the string with the first few digits 06:11.790 --> 06:18.210 of pi, "3.14159". If we use stoi, that is just going to get the integer part. It will 06:18.210 --> 06:21.630 see the decimal point and it will say, "That is not a character." 06:22.080 --> 06:26.730 "I am going to stop processing." With the stod, the floating-point version, 06:27.120 --> 06:33.180 it will recognize that a dot is part of a floating point number, or it can be part of the representation 06:33.180 --> 06:34.410 of a floating point number. 06:35.070 --> 06:36.750 And you could have exponents as well. 06:37.320 --> 06:40.530 And so it's going to keep on trying until it gets to the end. 06:40.530 --> 06:44.100 Or finds a non-numeric character that cannot be part of a floating number. 06:47.530 --> 06:49.840 So we have got our string with the digits of pi. 06:50.740 --> 06:55.390 We are going to find out how many characters we processed, just to see how that works. 06:55.960 --> 06:58.750 And then we have the integer version and the floating point version. 07:01.990 --> 07:08.980 So with stoi, we get the answer three, and it has processed one character. So it processed the digit '3'. 07:09.310 --> 07:14.860 And then it stopped, because it does not understand that dot is part of a floating-point number. 07:15.070 --> 07:19.990 It only knows about integers. With stod, we got the full result. 07:20.530 --> 07:25.330 So it does recognize that as a floating-point number. And it possessed all seven characters. 07:26.620 --> 07:28.330 Okay, so that's it for this video. 07:28.660 --> 07:29.610 I'll see you next time. 07:29.620 --> 07:31.780 But meanwhile, keep coding!