1. ECE 264 Object-Oriented Software Development
Instructor: Dr. Honggang Wang
Fall 2012
Lecture 18: Arrays and vectors

2. Lecture outline Announcements / reminders Today Lab 5 due next Monday
Project groups: Dr. Wang by Fri., 10/19
Groups of 3 or 4 students
Those who don’t choose a group will be randomly assigned
Can me with “sub-group”; I’ll fill rest of group
Today
Arrays
Vectors
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ECE 264: Lecture 11

3. Arrays Contain related items of same type Array size is constant
Declaring array allocates space in memory
e.g., int c[12] 
Array with n elements has indices between 0 and n-1
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ECE 264: Lecture 11

4. Using arrays Can initialize using comma-separated list:
int n[] = {10, 20, 30, 40, 50};
Array size is optional if initialized at declaration
Compiler will determine array size
Can access individual elements using []
cout << n[1] would print 20
Can pass arrays to functions
Array size is ignored
Must at least indicate that parameter is array
void printArray(int arr[], int size);
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ECE 264: Lecture 11

5. Example 1: Array use Output: Array a1: 1 3 5 7 9
#include <iostream> using std::cout; using std::endl; void printArray(int arr[], int size) { for (int i = 0; i < size; i++) cout << arr[i] << " "; cout << endl; } int main() { int a1[] = {1, 3, 5, 7, 9}; int a2[8]; for (int i = 0; i < 8; i++) a2[i] = i * 4; cout << "Array a1: "; printArray(a1, 5); cout << "Array a2: "; printArray(a2, 8); return 0;
Output:
Array a1:
Array a2:
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ECE 264: Lecture 11

6. Example 2: Common pitfalls
// Assume printArray, includes from previous slide int sumArray(int arr[], int size) { for (int i = 0; i < size; i++) arr[i+1] = arr[i] + arr[i+1]; return arr[size]; } int main() { int a1[] = {1, 3, 5, 7, 9}; cout << "Array a1: "; printArray(a1, 5); cout << "Sum of elements in a1: " << sumArray(a1, 5) << endl; cout << "Reprinting a1: "; return 0;
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ECE 264: Lecture 11

7. Example 2 (cont.) Program outputs (before crashing):
Array a1:
Sum of elements in a1:
Reprinting a1:
What’s wrong with the sum?
sumArray function returns arr[size], not arr[size-1]
Body of function will actually try to access arr[size] when summing elements
Why are values in a1 different?
Array names are pointers  always passed by reference
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ECE 264: Lecture 11

8. Vectors Array shortcomings
Fixed size
Boundary checking difficult
Can’t check array equality
Can’t simply copy using assignment operators
C++ class template vector addresses all of these issues (must #include <vector>
Examples:
vector <double> list; //empty vector
vector<string> wordList(n); //capacity:n strings
//vector of 8 integers, each initialized to 0
vector<int> intList(8,0);
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ECE 264: Lecture 11

9. Operators
The square bracket operator ( [ ] ) is defined for vector objects.
The assignment operator ( = )is defined for vectors of the same type.
Example:
vector<int> list(10);
vector<int>newList;
list[0]=10; …
newList = list;
cout << newList.size() << endl; //method
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ECE 264: Lecture 12

10. Vector methods Common member functions:
empty()returns true if vector contains no values
clear() removes all elements from the vector
pop_back()deletes last element in vector
push_back(element) add element to end of vector
resize(int) changes the size of vector
If new size > old size, elements are initialized to default value
size() returns the size of vector
at(int)allows you to insert element in vector, but also provides boundary checking
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ECE 264: Lecture 12

11. Example 3: Vector functions
#include <iostream> #include <vector> using std::cout; using std::endl; using std::vector; int main() { vector <int> example; //Vector to store integers example.push_back(3); //Add 3 onto the vector example.push_back(10); //Add 10 to the end example.push_back(33); //Add 33 to the end for (int x = 0; x < example.size(); x++) cout << example[x] << " "; cout << endl;
Output:
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ECE 264: Lecture 11

12. Example 3 (cont.)
if (!example.empty()) //Checks if empty example.clear(); //Clears vector vector <int> another_vector; another_vector.push_back(10); //10->end of vector example.push_back(10); //Same if (example==another_vector) //To show testing equality example.push_back(20); for (int y=0; y<example.size(); y++) cout<<example[y]<<" "; cout << endl;
Output (cont.):
10 20
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ECE 264: Lecture 11

13. Example 3 (cont.)
vector <int> one_more_vector(10); // Another vector one_more_vector.push_back(1); one_more_vector.push_back(2); one_more_vector.push_back(3); one_more_vector.pop_back(); // Remove last element one_more_vector.pop_back(); one_more_vector.resize(9); cout << "Vector size = " << one_more_vector.size() << endl; cout << one_more_vector[0] << " " << one_more_vector[1] << endl; return 0; }
Output (cont.):
Vector size = 9
0 0
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ECE 264: Lecture 11

14. Final notes
Next time
In-class vector programming example
Acknowledgements: this lecture borrows heavily from lecture slides provided with the following texts:
Deitel & Deitel, C++ How to Program, 8th ed.
Etter & Ingber, Engineering Problem Solving with C++, 2nd ed.
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ECE 264: Lecture 11