Arrays and Vectors

Objectives

• Be able to describe the differences between C-style arrays and Vectors
• Use built-in methods to manipulate C++ Vectors

C-Style Arrays

• A C-style array is simply a block of contiguous memory.
• First element is always 0.
• Note the < in the for loop. If you put <= you will certainly have touble!
  • If you get a “runtime error” from the judge, check for that.
• Access an element by index with brackets: \({\cal O}(1)\) time. Very fast!
  • If you have to “look for” an element, it’s \({\cal O}(n)\) time. Use with caution.

int arr[100];
int i;

for(i=0; i<100; ++i) {
   arr[i] = i * 10;
 }

C-Style Arrays, initialization

• You can initialize arrays inline in C if you need to.
• Note that uninitialized items are undefined!
  • You don’t have to initialize right away, but you have to before you use it!

int foo[10] = {8,6,7,5,3,0,9};
char suits[4] = "SHCD"; // Spades, Hearts, Clubs, Diamonds

Vectors

• Vectors in C++ are awesome. Use them unless you have good reason not to.
  • They can grow dynamically! No need to determine the proper size in advance.
  • Many iterators to provide traversals.
  • Reasonable default initialization.
  • Use push_back to insert an element at the end.
    • Inserting at the beginning is slow! Don’t do it!

vector<int> foo;

for(int i=0; i<N; ++i) {
 cin >> data;
 foo.push_back(data);
}

Vector initializations

• The constructor can initialize the vector for you.
  • One argument \(n\): \(n\) instances of the default.
  • Two arguments \(n\) and \(x\): \(n\) copies of \(x\).

vector<int> foo(100);
vector<int> foo(500,123);

• You can use foo.reserve(1000) to pre-allocate space.

Looping

• C Style

int sum=0;
for(i = 0; i<foo.size(); ++i)
  sum += foo[i]:

• Iterator Style

int sum=0;
for(auto i = foo.begin(); i != foo.end(); ++i)
  sum += *i;

• Reverse Iterator Style

int sum=0;
for(auto i = foo.rbegin(); i != foo.rend(); ++i)
  sum += *i;

Style

• Certain types come up a lot, so some standard typedefs have evolved:

typedef vector<int> vi;
typedef vector<vi> vvi;

Pairs

• It is often convenient to define tuples as well.

pair<int,int> coord;

coord.first = 10;
coord.second = 999;

• We have standard typedefs for them too.

typedef pair<int,int> ii;
typedef vector<ii> vii;