Understanding Vectors in C++ Programming

1. CMSC 202 Lesson 3 C++ Primer: Vectors

2. There are several compiler errors and logic errors in the following code. Identify them! #include <iostream>; using namespace std; int main() { cout >> "How many items?" >> endl; int size; cin << size; int array[size]; for (int i = 0; i <= size; ++i) { cout >> "Next Item?" >> endl; cin << array[i]; } return 0; } Warmup

3. Announcements • Labs start Today! • ENGR 104-A • Project 0 is due on Friday (9/15) • Ensure that the submit system is setup for you so that you can flawlessly submit Hw1 • Homework 1 is due on Sunday (9/17) • Testing – test the programs for bugs • Note: errors that all of them share are not considered bugs • Submit – makefile, testing files and README

4. Arrays, Good? Arrays, Bad? Arrays are Evil, Why?

5. Vectors, Gooooooood! • Similar to Arrays • Direct index-based access • Compact • Different than Arrays • Completely dynamic • Occasional memory re-allocation • Bounds-safe • Methods • #include <vector>

6. Vectors • Syntax vector<type> vecName; vector<type> vecName ( size ); vector<type> vecName ( size , initVal ); • Example vector<int> integers; vector<Car> cars; vector<string> messages; vector<Student> students(100); // 100 Students vector<double> grades(100, 0.0); // 100 doubles, all 0.0

7. Vector Access • Two methods • [index] – just like arrays • Syntax vecName[index] = newValue; temp = vecName[index]; • .at(index) – different from arrays • Syntax vecName.at(index) = newValue; temp = vecName.at(index); • NEW: dot operator • Calls a method associated with that object • vecName is the object, at() is the method

8. Vector Access • Examples vector<double> grades(100, 0.0); grades[0] = 97.6; grades.at(1) = 86.4; grades[2] = grades.at(1) / 2.0; • Why use .at(i)? • Boundary checking! • Throws an exception if out-of-bounds If there are 10 items in a vector, what is the index of the last item?

9. Vectors – Bigger is Better! • .push_back(item) • Add an item to back of vector vector<int> numbers(9); for (int i = 0; i < 9; ++i) numbers.at(i) = i + 1; numbers.push_back(10); What’s going on here? .push_back(10) 1 2 3 4 5 6 7 8 9 10

10. Vectors – Other Methods • .size() • Number of elements • .capacity() • Number of elements it “can” hold (reserved…) • .erase( iterator ) • Erase the item at the iterator position • .reserve( size ) • Reserves space for size number of elements • .resize( size ) • Shrinks/grows the vector to size • .empty() • Returns true if vector is empty, false otherwise • .clear() • Erases all items in the vector

11. Vectors - Iterators • Erase? Iterators? How does that work? • Iterator • Special “pointer” that points into the vector • NOT the index of the item! • Using an index WITH an Iterator • vecName.begin() is an iterator • Add the index to the iterator • Example numbers.erase( numbers.begin() + 3 ); • This will erase the 4th item (with index 3)

12. Vectors - Multidimensional • Multidimensional arrays • 10 x 20 matrix, 10 rows, 20 columns int scores[10][20] • 3 x 3 x 3 matrix int cube[3][3][3] • Multidimensional vectors • A little different! • For each dimension (except last) • Push_back a collection of vectors!

13. Vectors - Multidimensional Extra space – VERY IMPORTANT, why? • Example vector<vector<int> > integers; for (int i = 0; i < 10; ++i) { integers.push_back( vector<int>() ); for (int j = 0; j < 20; ++j) { integers[i].push_back(i * j); } } Push_back() whole vector Push_back() integer

14. Practice • Create a vector of strings • Hint: vector<…> … • Add your first, middle and last name to the vector • Hint: …push_back(…)

15. Challenge • Use vectors and strings • Ask the user to enter in text • Store each word in the vector • When they enter a period (as a word by itself), terminate the input • Print the text, one word on a line