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CS253 Vector

Old School

• Old-style C arrays still work just fine.
• They have no methods.
• They are NOT stretchy.

int a[10];
for (int i=0; i<10; i++)
    a[i] = i*i;
for (int i=0; i<10; i++)
    cout << a[i] << ' ';

0 1 4 9 16 25 36 49 64 81 

int a[10];
for (int i=0; i<10; i++)
    a[i] = i*i;
for (auto val : a)
    cout << val << ' ';

0 1 4 9 16 25 36 49 64 81 

vector

• A vector is a stretchy array
• Need to #include <vector>

vector<int> v;
for (int i=0; i<10; i++)
    v.push_back(i*i*i);
for (size_t i=0; i<v.size(); i++)
    cout << v[i] << ' ';

0 1 8 27 64 125 216 343 512 729 

vector<int> v(10);
for (int i=0; i<10; i++)
    v[i] = i*i*i;
for (auto val : v)
    cout << val << ' ';

0 1 8 27 64 125 216 343 512 729 

Useful vector methods

For a vector named v:

• v.size(): returns the current length, as a size_t
• v.push_back(): add an element to the end
• v.pop_back(): remove an element from the end
• v.erase(): remove an element from anywhere
• v[i]: indexing
• and many more methods

vector constructors

Vectors have several constructors, including:

// default size of zero elements
vector<float> a;

// initialize with these values
vector<float> b = {1.2, 3.4};

// resize to this many zeroes
vector<float> c(170);

if (a != b)
    cout << "unequal\n";

unequal

vector of anything

You can have a vector of nearly any type, but you have to pick one.

vector<string> some_strings;
vector<double> some_doubles(42);
vector<bool> some_bools = {true, false};

cout << some_strings.size() << '\n';
cout << some_doubles.size() << '\n';
cout << some_bools.size() << '\n';

0
42
2