C++ Strings

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In this video we learn about the sthe std::string class in C++

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English [Auto] In this video we'll learn about C++ strings standard string as a class in the C plus plus standard template library or as T.L. we could do an entire course on GCSE and that course would be very long and complex. So in this video only talk about the major elements of the C plus plus string class in order to use C++ strings you must include the string header file. Strings are in the standard namespace. So in order to use them without using namespace standard you must prefix them with standard and the scope resolution operator. This is also true for the standard string methods that work with C++ strings like C-style strings C++ strings are stored continuously in memory. However unlike C-style strings which are fixed in size C++ strings are dynamic and can grow and shrink as needed at runtime C++ strings work with the stream insertion and extraction operators just like most other types and C++ the C++ string class provides a rich set of methods or functions that allow us to manipulate strings easily. Chances are that if you need to do something with a string that functionality is already there for you without having to rewrite it from scratch C++ traits also work with most of the operators that were used to for assigning comparing and so forth. This is a huge advantage over C-style strings since C-style strings don't work well with those operators even though C++ strings are preferred in most cases. Sometimes you need to use C-style strings. Maybe you're interfacing with a library that's been optimized for C-style strings. Well in this use case you can still use C++ strings and take advantage of them and when you need to you can easily convert the C++ string into a C-style string and back again like vectors C++ strings are safer since they provide methods that can bounds check and allow you to find errors in your code so you can fix them before your program goes into production. Let's see how we can declare an initialized C++ strings in all the examples in this video. I'm assuming that the string header file has been included and that we're using the standard namespace. Here you can see six examples of declaration and initialization of C++ strings. There are other ways as well using constructor and assignment syntax but I'm mainly using the initializer syntax in this video in the first example we declare s one is a string. Notice that the string type is lowercase unlike C-style strings C++ strings are always initialized. In this case s one is initialized to an empty string. No garbage in memory to have to worry about in the second example. I'm declaring an initialising as to the string Frank noticed that Frank is a C-style literal. That's ok it will be converted to a C++ string in the third example S-3 is initialized from as to. So a copy of s to is created as two and three will both be frank but different francs in different areas of memory in the fourth example. I'm declaring an initializing S-4 from Frank but I'm only using the first three characters of the string Frank. So as for will be initialized to the string. F R A in the fifth example I'm initializing S-5 from S-3 which is Frank. But notice the two integers that follow the three in the initializer. The first integer is the starting index and the second is the length. So in this case we initialize S-5 with the two characters at index 0 and 1 from S 3. So S-5 will be initialized to f r. Finally we can initialize a string to a specific number of a specific character. In this case three x's. Note that this uses constructor syntax with the parentheses instead of the Curley's. Now that we've declared some strange That's how we can assign the values to them with C++ strings. You can use the assignment operator. This feels much more natural than having to use the string copy function like we would have to in C-style strings. In this example I've declared as one and it's empty. Then I can assign the C-style literal C++ rocks to s one. Pretty cool and pretty easy as one will grow dynamically as needed in the second example of declared as to an initialized the two hello. Then I assign s one to as to. In this case as to will no longer contain hello It will contain a copy of S1 C++ rocks. Let's see how we can concatenate strings together concatenation of strings just means building up a string from two other strings. We can use the plus operator to concatenate C++ strings. In this example I created two strings part 1 which is C++ and part 2 which is powerful. Then I have an empty string sentence. Notice that I'm assigning two sentences the concatenated result of part 1 plus a space plus part 2 plus a space plus language. If I displayed sentence now it would display C++ is a powerful language. Notice that the last example in the slide will not compile. This is because we have two C-style literals and you can't concatenate C-style literals. It only works with C++ strings. A combination of C++ strings and C-style strings is ok though as we saw in the previous assignments just like we did with vectors we can use the same operators to access string elements. In this case the elements of a string are characters so we can use the subscript operator as well as the method. Remember the method performs bounds checking. So if you go over bounds you'll get an exception which you can fix. Let's see how we can display screen characters one at a time in this example we have a string as one initialized to Frank we can use the range based for loop to display the string characters. In this case f r a and k and the null character will be displayed pretty much what you expected right. Notice that the type of the loop variable is char in this case. What do you think will happen if we change that to integer. In this case I've changed it to integer. Is this what you expected. We told the compiler to use an integer and that's exactly what it's doing. So instead of displaying the character value of each element in the string it's displaying the integer value that represents those characters. So in this case 70 114 97 110 1 0 7 and 0 which represent f r a and k. And of course the null character. These are the ASCII codes for those characters. Comparing C++ strings couldn't be any easier or more intuitive. We use the same equality and relational operators that we've been using all along. We're comparing two string objects so they'll be compared. Character by character and their character values will be compared lexically. So a capital A is less than a capital Z and a capital A is less than a lower case a. That's because the capital letters come before the lower case letters in the ASCII table. We can't use these operators on two C-style literals but we can use them in the following cases. If we have to see postponed strings if we have one C++ string and a C-style literal or if we have one C++ string and one C-style string. Let's see some examples. Here we're defining five C++ string variables as one through as five and then we perform some comparison operations and see the results. Of course you would normally use these boolean expressions in an IF statement or looping conditional expressions in the first example. We check to see if s one is equal to as 5. This is true since they both contain the string. Apple s s to is false since as one is Apple and as to his banana. How about s one not equal to as two. This is true since apples are not equal to banana in the case of s one less than as two. This is also true since Apple comes before banana lexically in the ASCII table as to greater than s one is also true since banana comes before Apple lexically. Notice that banana has an upper case B whereas Apple has a lowercase. As for less than S-5 asphalt since Apple with a lower case does not come before Apple with an upper case. And then finally a one equal Apple is true because they're the same. Notice in this case Apple is a C-style string literal the C++ string class has a rich set of very useful methods. Many methods to cover in detail in this video. I encourage you to study the C++ string class since it's going to be a class that you'll use often and it's important to know what it provides so you don't reinvent the wheel when you need to solve a problem. The substring method extracts a substring from a C++ string. It doesn't change the strain. It simply returns the substring. And you can do whatever you want with it. In this case I'm simply displaying it. But you could easily assign it to a string variable. Here I've initialized as one too. This is a string. The first example takes a substring of the string starting at index 0 and including exactly four characters. If there are less than four characters left in the original string then all the remaining characters are included in this case the substring is the first word in the string. This in the second example we returned the substring starting at index 5 and include two characters. That's the substring is yes. Finally the last example starts in index 10 and includes four characters. This will return the substring test. Let's see how we can search a string for another. The C++ string class has a very handy method named find find works with characters and strings. It expects a string or character and returns the index or position of the beginning of that string or character in the original string. So if you have a string as one that's initialized to. This is a test. And we want to find the string this we'd get back in zero since this starts and index 0 in the second example we're looking for the string is in this case it would return to since the first is starts at index 2 into 30 sample we're finding this string test and we get back a 10 in the fourth example we're searching for a single character the lowercase E which is found at index 11 in the example we use a variant of the method that also allows the index where you want to start to search from. In this case I want to find the is substring again but I want to start an index for. So this time it finds the is that's located at index 5. Finally what happens if the string or character we want to find just isn't there. Well in this case the method returns the end position which means no position information available. You can check for this value in an if statement and if true. You know what you were searching for wasn't there. Very easy very powerful. There's also an R find method that starts searching from the end of the stream to the beginning of the string. We can also remove characters from a C++ string using the erase and clear methods. For the erase method you provide the starting index and how many characters to delete the clear method deletes all the characters in the strings of the string becomes the empty string. We've seen a lot of string methods and you can see how powerful this class is let's look at one more useful method and one more useful OPERATOR They're commonly used. The method is the length method. It returns the number of characters currently in the string object. In this example as one is Frank. So it's one dot length will return to five. This is so easy and something that's impossible to do is see substrings. Since they don't contain size information the operator I want to cover is the compound concatenation assignment operator. In this case s one is Frank and I can say s one plus equals James. And James will be concatenated to Frank and the entire result string will be assigned back to s one. This is really handy and works very much the same way that the compound assignment operators worked with integers and doubles and so forth. There are also many more methods in the C++ string class for you to discover as you study C++. OK there's one more thing I'd like to talk about before we end this video input with C++ strings. C++ strings worked great with input and output streams. As you've seen inserting C++ variables to an output stream like C out is pretty easy and works just like we've been doing all along. Extracting a C++ tree from an input stream like C in also works the same way we expect. However there's one issue that's also true for C-style strings. Suppose we've defined as one as a C++ string and we extract a stream from C in as usual. Now suppose I type in Hello space there. When I display S1 I will only see Hello. There was not extracted. This is because the extraction operator stops when it sees whitespace. In many cases we want to read an entire line of text up to when the user presses enter. For example I want the stream to be hello there. Suppose I asked you to enter your full name. I want to be able to read William Smith not just William in this case we can use the getline function to getline function has a couple of variants. The first variant expects two elements inside the parentheses. The first element is the input stream. In this case we're using C in which defaults to the keyboard. The second element is the name of the C++ string where you want the text that the user enters. Stuart gets it very easy. In the example I'm saying get line c in S one. Now everything the user types is stored into s one. Getline stops reading when it sees the new line and doesn't include the newline in the string it just discards it the other variant of getline has another element in the parentheses. The first two are the same as before the input stream and the C++ string variable name. The third is called the delimiter. This is the character that you want getline to stop reading input at. So as long as the user doesn't enter this character everything will be stored in the string variable. Once the delimiter is seen it's not included in the stream variable and it's discarded in the last example. I'm using a lowercase X as the delimiter. So if I type this is X then the strings stored in s one will be this is and the X is discarded. Well we've covered a lot of material in this video and there is much more in the String class to learn but this gives you a good starting point so you can use the C++ string class effectively. Also you've now been introduced to object oriented programming with both vectors and strings. Pretty soon we'll be developing our own classes which is pretty cool. That completes this video. Please play with the string class create examples assigned to lead display and try out some of the methods in this video. It won't take long before you're really comfortable working with C++ strings.