135 lines
5.9 KiB
Markdown
135 lines
5.9 KiB
Markdown
# Lecture 2 --- STL Strings & Vectors
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- STL Strings, char arrays (C-style Strings), & converting between these two types
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- L-values vs. R-values
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- STL Vectors as “smart arrays”
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## 2.1 Character Arrays and String Literals
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- In the line below "Hello!" is a string literal and it is also an array of characters (with no associated variable
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name).
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```cpp
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cout << "Hello!" << endl;
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```
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- A char array can be initialized as:
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```c
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char h[] = {'H', 'e', 'l', 'l', 'o', '!', '\0'};
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```
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or as:
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```c
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char h[] = "Hello!";
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```
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In either case, array h has 7 characters, the last one being the null character.
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- The C language provides many functions for manipulating these “C-style strings”. We don’t study them much
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anymore because the “C++ style” STL string library is much more logical and easier to use. If you want
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to find out more about functions for C-style strings look at the cstdlib library http://www.cplusplus.com/
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reference/cstdlib/.
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- One place we do use them is in file names and command-line arguments, which you will use in Homework 1.
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## 2.2 Conversion Between Standard Strings and C-Style String Literals
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- We regularly convert/cast between C-style & C++-style (STL) strings. For example:
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```cpp
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std::string s1( "Hello!" );
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std::string s2( h );
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```
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where h is as defined above.
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- You can obtain the C-style string from a standard string using the member function c_str, as in s1.c_str().
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## 2.3 STL Vectors: a.k.a. “C++-Style”, “Smart” Arrays
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- Standard library “container class” to hold sequences.
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- A vector acts like a dynamically-sized, one-dimensional array.
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Capabilities:
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– Holds objects of any type.
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– Starts empty unless otherwise specified.
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– Any number of objects may be added to the end — there is no limit on size.
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– It can be treated like an ordinary array using the subscripting operator.
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– A vector knows how many elements it stores! (unlike C arrays)
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– There is NO automatic checking of subscript bounds.
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Here’s how we create an empty vector of integers (let's say we create a vector called *scores* which will be used to store students' scores):
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```cpp
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std::vector<int> scores;
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```
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- Vectors are an example of a templated container class. The angle brackets < > are used to specify the type of object (the “template type”) that will be stored in the vector.
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- push_back is a vector function to append a value to the end of the vector, increasing its size by one. <!--This is an O(1) operation (on average).-->
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- There is NO corresponding push_front operation for vectors.
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- size is a function defined by the vector type (the vector class) that returns the number of items stored in the vector.
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- After vectors are initialized and filled in, they may be treated just like arrays.
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- In the line
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```cpp
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sum += scores[i];
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```
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scores[i] is an “r-value”, accessing the value stored at location i of the vector.
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- We could also write statements like
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```cpp
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scores[4] = 100;
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```
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to change a score. Here scores[4] is an “l-value”, providing the means of storing 100 at location 4 of the vector.
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- It is the job of the programmer to ensure that any subscript value i that is used is legal —- at least 0 and strictly less than scores.size().
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## 2.4 Initializing a Vector — The Use of Constructors
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Here are several different ways to initialize a vector:
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- This “constructs” an empty vector of integers. Values must be placed in the vector using push_back.
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```cpp
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std::vector<int> a;
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```
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or,
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```cpp
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std::vector<int> a{};
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```
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Here, in C++, a pair of curly braces is used to initialize an empty container. Vector is a type of container, and we will learn other containers later this semester. Interesting trick: if you have a function which has a return type of vector, and in your function body, you want to return an empty vector without specifying a vector name, you can do:
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```cpp
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return {};
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```
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- This constructs a vector of 100 doubles, each entry storing the value 3.14. New entries can be created using
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push_back, but these will create entries 100, 101, 102, etc.
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```cpp
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int n = 100;
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std::vector<double> b( 100, 3.14 );
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```
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- This constructs a vector of 10,000 ints, but provides no initial values for these integers. Again, new entries can
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be created for the vector using push_back. These will create entries 10000, 10001, etc.
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```cpp
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std::vector<int> c( n*n );
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```
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- This constructs a vector that is an exact copy of vector b.
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```cpp
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std::vector<double> d( b );
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```
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- This is a compiler error because no constructor exists to create an int vector from a double vector. These are
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different types.
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```cpp
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std::vector<int> e( b );
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```
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## 2.5 Passing Vectors (and Strings) As Parameters
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The following outlines rules for passing vectors as parameters. The same rules apply to passing strings.
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- If you are passing a vector as a parameter to a function and you want to make a (permanent) change to the
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vector, then you should pass it by reference.
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- What if you don’t want to make changes to the vector or don’t want these changes to be permanent?
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– The answer we’ve learned so far is to pass by value.
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– The problem is that the entire vector is copied when this happens! Depending on the size of the vector,
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this can be a considerable waste of memory.
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- The solution is to pass by constant reference: pass it by reference, but make it a constant so that it can not
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be changed.
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median grade.
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- As a general rule, you should not pass a container object, such as a vector or a string, by value because of the
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cost of copying.
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## 2.6 Exercises
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- [Leetcode problem 8: String to Integer (atoi)](https://leetcode.com/problems/string-to-integer-atoi/)
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- [Leetcode problem 211: Design Add and Search Words Data Structure](https://leetcode.com/problems/design-add-and-search-words-data-structure/)
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- [Leetcode problem 1662: Check If Two String Arrays are Equivalent](https://leetcode.com/problems/check-if-two-string-arrays-are-equivalent/) Solution: [p1662_check_two_strings.cpp](../../leetcode/p1662_check_two_strings.cpp)
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