Lecture 2 --- STL Strings & Vectors
- STL Strings, char arrays (C-style Strings), & converting between these two types
- L-values vs. R-values
- STL Vectors as “smart arrays”
2.1 Character Arrays and String Literals
- In the line below "Hello!" is a string literal and it is also an array of characters (with no associated variable name).
cout << "Hello!" << endl;
- A char array can be initialized as:
char h[] = {'H', 'e', 'l', 'l', 'o', '!', '\0'};
or as:
char h[] = "Hello!";
In either case, array h has 7 characters, the last one being the null character.
- The C language provides many functions for manipulating these “C-style strings”. We don’t study them much anymore because the “C++ style” STL string library is much more logical and easier to use. If you want to find out more about functions for C-style strings look at the cstdlib library http://www.cplusplus.com/ reference/cstdlib/.
- One place we do use them is in file names and command-line arguments, which you will use in Homework 1.
2.2 Conversion Between Standard Strings and C-Style String Literals
- We regularly convert/cast between C-style & C++-style (STL) strings. For example:
std::string s1( "Hello!" );
std::string s2( h );
where h is as defined above.
- You can obtain the C-style string from a standard string using the member function c_str, as in s1.c_str().
2.3 STL Vectors: a.k.a. “C++-Style”, “Smart” Arrays
- Standard library “container class” to hold sequences.
- A vector acts like a dynamically-sized, one-dimensional array.
Capabilities:
– Holds objects of any type.
– Starts empty unless otherwise specified.
– Any number of objects may be added to the end — there is no limit on size.
– It can be treated like an ordinary array using the subscripting operator.
– A vector knows how many elements it stores! (unlike C arrays)
– There is NO automatic checking of subscript bounds.
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):
std::vector<int> scores;
- 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.
- push back is a vector function to append a value to the end of the vector, increasing its size by one. – There is NO corresponding push front operation for vectors.
- size is a function defined by the vector type (the vector class) that returns the number of items stored in the vector.
- After vectors are initialized and filled in, they may be treated just like arrays. – In the line
sum += scores[i];
scores[i] is an “r-value”, accessing the value stored at location i of the vector. – We could also write statements like
scores[4] = 100;
to change a score. Here scores[4] is an “l-value”, providing the means of storing 100 at location 4 of the vector. – 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().
2.4 Initializing a Vector — The Use of Constructors
Here are several different ways to initialize a vector:
- This “constructs” an empty vector of integers. Values must be placed in the vector using push_back.
std::vector<int> a;
or,
std::vector<int> a{};
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:
return {};
- This constructs a vector of 100 doubles, each entry storing the value 3.14. New entries can be created using push_back, but these will create entries 100, 101, 102, etc.
int n = 100;
std::vector<double> b( 100, 3.14 );
- This constructs a vector of 10,000 ints, but provides no initial values for these integers. Again, new entries can be created for the vector using push_back. These will create entries 10000, 10001, etc.
std::vector<int> c( n*n );
- This constructs a vector that is an exact copy of vector b.
std::vector<double> d( b );
- This is a compiler error because no constructor exists to create an int vector from a double vector. These are different types.
std::vector<int> e( b );
2.5 Passing Vectors (and Strings) As Parameters
The following outlines rules for passing vectors as parameters. The same rules apply to passing strings.
- If you are passing a vector as a parameter to a function and you want to make a (permanent) change to the vector, then you should pass it by reference.
- What if you don’t want to make changes to the vector or don’t want these changes to be permanent? – The answer we’ve learned so far is to pass by value. – The problem is that the entire vector is copied when this happens! Depending on the size of the vector, this can be a considerable waste of memory.
- The solution is to pass by constant reference: pass it by reference, but make it a constant so that it can not be changed. median grade.
- As a general rule, you should not pass a container object, such as a vector or a string, by value because of the cost of copying.