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Multidimensional Arrays, and STL containers: vectors and maps

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Objectives

Learn how to declare– multidimensional arrays– Vectors– Maps

Illustrate how and when to use– multidimensional arrays– Vectors– Maps

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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

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Multidimensional arrays

DeclarationtypeName name[SIZE_1][SIZE_2]…[SIZE_n]

Examplesdouble distances[5][5];

double surface[100][100];

int threeDimension[10][10][10];

Initializationdouble distances[5][5]={{0,1,2,3,4,5},{2,3}};

double myArray[][2]={{2,3},{4,5}};//a 2x2 array

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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]<< “”;

}

}

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Visualizing 2D arrays

A table with columns (vertical) and rows (horizontal) const int distances[6][6]

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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.

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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

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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 atvectorName[…];//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 loopvectorName.size()

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Vectors: example

#include <iostream>#include <vector>

using namespace std;

int main() {int n;cin >> n; //read the size from the keyboardvector<double> vec(n); //declare a vector of n elementsfor(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;

}

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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 mapMap<typeName1,typeName2> mapName

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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

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Problem

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

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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; //iteratorfor (itr=dictionary.begin();itr!=dictionary.end();++itr) cout << itr->first << "--->"<<itr->second<<endl;return 0;

}