Add reference to std::string (#686)

* Add reference to std::string

Added reference to std::string in about.md, introduction.md and hints.md
add hints

* Resolve suggestions of #686

- Change the mention of 'string' to 'std::string'
- Add reference to 'std::string'
- improved the wordings

* [hints] change integer to number

Closes #643

concepts/strings/about.md

@@ -1,3 +1,65 @@
 # About
 
-TODO: add content from introduction.md, when finished.
+A `string` in C++ is a mutable object that represents text as a sequence of characters (letters, digits, punctuation, etc.). 
+Strings are manipulated by calling the string's methods.
+
+## Strings Library
+
+In C++ the string type and the associated functions have to be included from the strings library before usage.
+You can do so by adding  `#include <string>` to the top of your file.
+They will then populate the `std` namespace.
+The string literal uses the double quote character: `"`.
+
+```cpp
+#include <string>
+std::string w_berry_quote{"A well-made sentence, I think, is a thing of beauty."};
+```
+
+## Common String Operations
+
+You can use the `+` operator to concatenate strings:
+
+```cpp
+std::string original_title{"The School of Rock"};
+std::string sequel_indicator{"Electric Boogaloo"};
+std::string next_movie_title = original_title + " 2: " + sequel_indicator;
+```
+
+To use the strings library, you need to know that it is possible to call a function that belongs to an object.
+These are called member functions.
+Later in the syllabus, you will learn more about member functions and the connected class concept.
+
+```cpp
+std::string qualification{"awesome"};
+// 1st argument: from the index to the end of the string:
+std::string who_is_awesome = qualification.substr(5); 
+// => "me"
+
+// 2nd optional argument for the length:
+std::string material{"haunted books"};
+std::string ghost = material.substr(8, 3);
+// => "boo"
+```
+
+
+The `find` function is also very useful.
+It is called as a member function on the string and takes a string as the argument.
+`find` returns the zero-indexed position of the first occurrence in the string.
+
+```cpp
+std::string new_release{"apple released a new app!"};
+new_release.find("app");
+// => 0
+new_release.find("!");
+// => 24
+```
+
+There is also the `to_string` function, which can be used to convert integers and float/double values to string.
+`to_string` returns a string.
+
+```cpp
+int num{92};
+double exponent{0.92e2};
+std::string msg{ std::to_string(num) + " can also be written as " + std::to_string(exponent)};
+// => 92 can also be written as 92.0000000
+```

concepts/strings/introduction.md

@@ -54,3 +54,12 @@ new_release.find("app");
 new_release.find("e");
 // => 4
 ```
+
+There is also the `std::to_string` function, which can be used to convert integers and float/double values to string.
+
+```cpp
+int num{92};
+double exponent{0.92e2};
+std::string msg{ std::to_string(num) + " can also be written as " + std::to_string(exponent)};
+// => 92 can also be written as 92.0000000
+```

exercises/concept/making-the-grade/.docs/hints.md

@@ -5,11 +5,12 @@
 - `while` loops are used for _indefinite_ (uncounted) iteration
 - `for` loops are used for _definite_, (counted) iteration.
 - The keywords `break` and `continue` help customize loop behavior.
+- `std::to_string()` from `string` library can be used to convert a integer to string
 
 Also being familiar with the following can help with completing the tasks:
 
 - [`vectors`][vectors]: indexing, size, [`<vector>.emplace_back`][emplace], [`<vector>.pop_back()`][pop].
-- [`string`][string]: using the `+` to concatenate strings.
+- [`string`][string]: using the `+` to concatenate strings, integer to string conversion, [`to_string`][to\_string].
 
 ## 1. Rounding Scores
 
@@ -34,6 +35,8 @@ Also being familiar with the following can help with completing the tasks:
 ## 5. Matching Names to Scores
 
 - If both containers are the same length and sorted the same way, could you use the `index` from one to retrieve a `value` from the other?
+- `std::to_string(int)` can be used to convert a number to string.
+- Don't forget the follow the format of the example's output.
 
 ## 6. A "Perfect" Score
 
@@ -42,5 +45,6 @@ Also being familiar with the following can help with completing the tasks:
 
 [vectors]: https://en.cppreference.com/w/cpp/container/vector
 [string]: https://en.cppreference.com/w/cpp/string/basic_string
+[to\_string]: https://en.cppreference.com/w/cpp/string/basic_string/to_string
 [emplace]: https://en.cppreference.com/w/cpp/container/vector/emplace_back
-[pop]: https://en.cppreference.com/w/cpp/container/vector/pop_back
+[pop]: https://en.cppreference.com/w/cpp/container/vector/pop_back

exercises/concept/making-the-grade/.meta/design.md

@@ -38,5 +38,6 @@ This concept exercise should also teach how to interrupt or change the normal fl
 
 - [vectors](https://en.cppreference.com/w/cpp/container/vector-)
 - [strings](https://en.cppreference.com/w/cpp/string/basic_string)
+- [to\_string](https://en.cppreference.com/w/cpp/string/basic_string/to_string)
 - [for](https://en.cppreference.com/w/cpp/language/for)
 - [while](https://en.cppreference.com/w/cpp/language/while)