Multidimensional Arrays, and STL containers: vectors and maps

Multidimensional Arrays, and STL containers: vectors and maps Computer programming

Objectives • Learn how to declare • multidimensional arrays • Vectors • Maps • Illustrate how and when to use • multidimensional arrays • Vectors • Maps Computer programming

Motivations • Write a program that, given two cities, will tell us the distance between the two cities • How to represent an image? • How to represent a matrix? • How to represent a two or a three dimensional space? • Use multidimensional arrays Computer programming

Multidimensional arrays • Declaration typeName name[SIZE_1][SIZE_2]…[SIZE_n] • Examples double distances[5][5]; double surface[100][100]; int threeDimension[10][10][10]; • Initialization double distances[5][5]={{0,1,2,3,4,5},{2,3}}; double myArray[][2]={{2,3},{4,5}};//a 2x2 array Computer programming

Multidimensional arrays • Initialize and reference/access a multidimensional array using for loops • nested for loops • Outer loop counting through the first dimension • The next Inner loop counting through the second dimension • And so on. double surface[10][10]; //initialize to 0 for (int i=0; i<10; i++){ cout <<endl; for (int j=0; j<10; j++){ surface[i][j]=1/(i+j+1.); cout << ++surface[i][j]<< “”; } } Computer programming

Visualizing 2D arrays • A table with columns (vertical) and rows (horizontal) const int distances[6][6] Computer programming

STL containers and algorithms • STL containers • A collection of data structures that can hold any kind of data • Vectors • Maps • sets • Queues • Lists • They are classes. So, several methods are already defined on them • sort • find • STL algorithms • A collection of algorithms that can be used to manipulate STL containers. • E.g. copy, sort, find, min, max, etc. Computer programming

STL containers: vectors • Arrays that we dealt with so far have a size that must be known before the compiling time • Arrays do not check whether the index is out of bounds • When we implement a function we must pass • The name of the array • The bounds on each dimension • To manipulate arrays, you need to implement the functions to do so • Arrays are not classes; they are not self-contained objects • Vectors allows us to solve all these problems Computer programming

Vectors • A vector is a class template similar to a dynamic array. • Examples of a vector declaration vector<typeName> vectorName; vector<typeName> vectorName(size); • Inserting an element at the end of a vector vectorName.push_back(…) • Access an element you can either use [] or at vectorName[…];//does not check for “out of bound” vectorName.at(…);//checks for “out of bound” • The method size returns the number of elements in the vector; you can use it in a for loop vectorName.size() Computer programming

Vectors: example #include <iostream> #include <vector> using namespace std; int main() { int n; cin >> n; //read the size from the keyboard vector<double> vec(n); //declare a vector of n elements for(int i=0;i<vec.size();i++) //size is n vec[i]=1./(i+1); vec.push_back(1.0); for(int i=0;i<vec.size();i++) //size is n+1 cout << vec[i]<<“ “; cout << endl; return 0; } Computer programming

Maps • Arrays and vectors use integer indices. • Can we use other types, such as strings, as indices? • Yes; use maps! • Maps: a set of pairs (key, value) • Keys cannot be duplicated • Values can be duplicated • Recall the concept of map/function from math! • Declaring a map Map<typeName1,typeName2> mapName Computer programming

Maps • Initialize and access a map using [] or insert • To go through a map use a for loop and iterators • Iterators allow us to go through STL containers • To declare an iterator STLContainer::iterator itr • To access the first element in the container use itr.begin() • To increment an iterator use the operator ++ • To detect the end of the user itr.end() (the location after the last element) • itr->first is the key • itr->second is the value Computer programming

Problem Write a program that counts the number of occurrences of words that the users enters. Compute the probability of each word. Computer programming

Maps: Example #include <iostream> #include <vector> #include <map> using namespace std; int main() { map<string, int> dictionary; string str; cin >> str; while (str != "q") { dictionary[str]++; cin >> str; } dictionary.insert(pair<string,int>("bel",++dictionary["bel"])); map<string,int>::iterator itr; //iterator for (itr=dictionary.begin();itr!=dictionary.end();++itr) cout << itr->first << "--->"<<itr->second<<endl; return 0; } Computer programming

Multidimensional Arrays, and STL containers: vectors and maps