STL’s vector and going through it (for loop)¶

STL’s vector¶

The STL (Standard Template Library) data type vector resembles closely the Python type list. They can both include several data elements you can access if you know their index. The difference from Python is that in C++, all of the elements of vector must be of the same type.

Let us have a look at a small example program able to optimize the route found through a (loopless) labyrinth. The route is expressed as a string where the character N means a movement of the length of one square to the north, S means one square to the south and so on. For example, if find the route NWENENWEEWSEWWSE by randomly searching through the labyrinth, the optimized route is NE, because you can remove the second and third character, WE, and many others that are useless.

#include <iostream>
#include <vector>
#include <string>

using namespace std;

const char NORTH  = 'N';
const char EAST   = 'E';
const char SOUTH = 'S';
const char WEST = 'L';

void print_route(const vector<char>& routevector) {
    vector<char>::size_type index = 0;
    while ( index < routevector.size() ) {
        cout << routevector.at(index);
        ++index;
    }
    cout << endl;
}

int main() {
    string route = "";
    cout << "Input route: ";
    getline(cin, route);

    vector<char> optimized_route = { };
    string::size_type i = 0;
    while ( i < route.length() ) {
        char next_direction = route.at(i);

        if ( optimized_route.size() == 0 ) {
            optimized_route.push_back(next_direction);

        } else {
            char previous_direction = optimized_route.back();
            if ( previous_direction == EAST
                    and next_direction == WEST ) {
                optimized_route.pop_back();
            } else if ( previous_direction == WEST
                           and next_direction == EAST ) {
                optimized_route.pop_back();
            } else if ( previous_direction == NORTH
                           and next_direction == SOUTH ) {
                optimized_route.pop_back();
            } else if ( previous_direction == SOUTH
                           and next_direction == NORTH ) {
                optimized_route.pop_back();
            } else {
                optimized_route.push_back(next_direction);
            }
        }

        ++i;
    }
    print_route(optimized_route);
}

In order to use the vector type, you need to insert the following line at the beginning of the code:

#include <vector>

The definition of a vector type variable goes like this:

vector<element_type> variable_name;

for example

vector<int> scores;

as a result of which you can store several integers into the variable scores.

You can add new elements one by one at the end of the vector:

scores.push_back(21);

and as a result, the size of the vector (that is, the amount of elements saved in it) grows by one.

The last element can be removed:

scores.pop_back();

and as a result, the size of the vector is decreased by one.

If the vector is empty (there are no elements in it) when pop_back is executed, an exception occurs. We will get to know exceptions at a later stage (not on this course).

The right way to call the method pop_back is to first make sure that there is something left to be removed in the vector, as below:

if ( scores.size() != 0 ) {
    scores.pop_back();
} else {
    // Error, you cannot remove anything from an empty vector.
}

where size reveals the amount of elements saved in the vector.

If we want to save the return value of the method size in the variable, its type should be

vector<element_type>::size_type

For example

vector<int>::size_type number_of_scores = scores.size();
...
number_of_scores = scores.size();

You can access the first and the last value of an element:

cout << "first: " << scores.front() << endl
     << "last:  " << scores.back() << endl;

The individual values of a vector can be accessed the same way as the single characters of a string:

cout << scores.at(3) << endl;
...
scores.at(index) = new_score;

Indexing starts at zero, and the index of the last element is the number of the elements minus one.

If you try using the method at to index an element that does not exist from the vector, you will get an exception that will crash the program if it is not handled.

If you use the vector as a parameter of a function, it can be either a value or a reference parameter as usual:

void function1(vector<double> measurements);
void function2(vector<double>& measurements);

Large data structures should usually not be passed to the function as a value parameter if it can be avoided. (Think about what happens in passing a value. You can draw a picture, or you can go back to the section 3.2 of this material.) For efficiency reasons, you should use const references instead of value parameters:

void function3(const vector<double>& measurements);

The keyword const together with the reference makes it impossible to change the target of the reference. This makes passing by a reference parameter safer: The caller of the function knows that the function will not make any changes to the variable passed as a parameter.

If you want, you can set the amount of elements in the vector during the vector definition:

// A vector with room for 20 integers at the start.
// Integers themselves not initialized.
vector<int> numbersA(20);

// A vector with room for 10 real numbers,
// each of them initialized to have the value 5.3.
vector<double> numbersB(10, 5.3);

Please note that there are multiple different initializing notations for vectors (and other STL structures). Below you can see some examples:

vector<string> namesA(5);
vector<string> namesB(10, "unknown");
vector<string> namesC = { "Matti", "Maija" };

The STL vector is a data structure that keeps the elements in the order they were stored. In STL terminology, structures like these are called sequences. There are several different sequences in STL:

• The vector is great for storing multiple data elements for later processing, especially if you are not going to search elements from the stored data, or if the speed of the search is not significant. Naturally, vector is a good choice when you have to be able to process the elements in the same order they were added into it.
• We have earlier mentioned deque that is very similar to vector, but less efficient. However, deque has the benefit that you can add elements to its beginning with the method push_front and remove them from the beginning with the method pop_front.
• There is also list which, despite the name, has nothing in common with the Python list structure, and you should not use list if you are not familiar with dynamic memory management.

You will learn more about the differences of containers on the course COMP.CS.300 Data structures and algorithms 1, where their efficiency, among other things, will be discussed.

For troubleshooting¶

When using STL containers in the program code, you should add the following line into your Qt Creator project file:

QMAKE_CXXFLAGS += -D_GLIBCXX_DEBUG -D_GLIBCXX_DEBUG_PEDANTIC

Having added it, you will get more warnings from the compiler about suspicious code on STL containers (such as out-of-bounds indexing), which usually helps in detecting errors.

The personnel of the course have added the line in question into the project files of the assignments that use STL containers.

C++’s for loop¶

There are basically two ways to use the for loop in C++:

1. Going through the elements in a container:

   vector<int> numeric_vector;
   ...
   for ( int vector_element : numeric_vector ) {
       cout << vector_element << endl;
   }
   

   You should basically know this usage already. It is an equivalent of the Python structure:

   for element in list:
       print(element)
   

2. Going through the interval of chosen integer values:

   for ( int number = 5; number < 10; ++number ) {
       cout << 9 * number << endl;
   }
   

   which is a more complex equivalent of the Python structure:

   for number in range(5, 10):
       print(9 * number)
   

for ( int vector_element : numeric_vector ) {
    cout << vector_element << endl;
}

for ( int& vector_element : numerical_vector ) {
    vector_element = vector_element * 2;
}

int i = 42;

auto i = 42;

vector<int> numerical_vector;
...
for ( auto vector_element : numerical_vector ) {
    cout << vector_element << endl;
}

for ( int number = 5; number < 10; ++number ) {
    cout << 9 * number << endl;
}

for ( initialization; condition; increment ) {
    body
}

for ( ;; ) {
    ...
}

while ( true ) {
    ...
}

for( int i : { 2, 3, 5, 7, 11, 13, 17 } ) {
    ...
}

More examples¶

From the material of this round you can find an example program for printing prime numbers up to a certain limit (examples/04/primes). The program uses for loop and a vector, the element type of which is bool.