Thursday, August 08, 2013

Test SRM #2 : Revenge of the C++11

So, there is another test SRM scheduled for Saturday. This is great news. Whilst TC will be testing new compiler versions for c++ and python, I will be testing a new code environment.

It looks like TopCoder is getting gcc-4.8.1. This is quite serious. Because the c++11 support will be enabled. Take a look to the list of c++11 features available in gcc-4.8.1: Compare it with the puny gcc-4.4 list: Of course, some things like threads will not be supported in SRMs, but otherwise the list of syntax candy will be increased greatly. gcc-4.8.1 is so new, that ubuntu 12.04 repositories don't support it. The list of new features is humongous. If you thought auto and tuples were a big deal. Brace yourselves...

range-based for loops

string canTransform(string init, string goal)
    string x = "";
    for (auto ch : goal) {            
        if ( ch != 'z') {
            x += ch;
    return (x == init) ? "Yes" : "No";

You could use char ch too if that's your taste.

Lambda expressions

Remember sorting in c++? When the ordering is not a trivial one, you often had to declare a whole struct with strange syntax inside and then use the struct in the call to sort. This is an alternative to all that:

// How to sort elements in a vector by their absolute value:
vector<int> v = {5, 0,-1,-5, -3,3,2, -5};
sort(v.begin(), v.end(), [](int a, int b) {
    return abs(a) < abs(b);
// shows: 0 -1 2 -3 3 5 -5 -5
for (auto &y : v ) {
    cout << y << ", ";
cout <<endl;

Lambdas are anonymous functions that you can create inside code blocks. In the example we use the [](int a, int b) { return boolean } syntax to create a function pointer that is then used and called by sort. The world of lambdas is a huge one. And their usefulness is understated the first time you hear about them. Here is a page that explains them throughly.


One of the good things about lambda syntax is that it enables closures. Aaaand things just got crazy...

For example, what if you want to sort some numbers `x` by the values `y[x]` in some array?

vector<int> y = {10, 7, 8, 3, 5, 1 ,1};    

vector<int> x = {0, 1, 2, 3, 4, 5 ,6};
// sort x[i]s by y[x[i]]:
sort(x.begin(), x.end(), [=](int a, int b) {
    return (y[a] < y[b]);
// note the [=] syntax, we are making copies of vector<int> y.

// display 5, 6, 3, 4, 1, 2, 0,
for (int z : x ) {
    cout << z << ", ";
cout <<endl;
// we can use int z : x
// If we did int &z: x, they would be references
//   (modifiable and maybe faster)


It's hard to overstate my satisfaction.

If only the python version was python 3, everything would be 110% perfect

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