A question about creating temporary functions in C++

Question

I have a very basic C++ question. But I didn't find an appropriate answer after my own search. I saw some C++ code written like this:

sort(a.begin(), a.end(), [](auto &a, auto &b){return a[0] < b[0];});

1. a is a variable of type vector<vector<int>> .

2. a[i][0] means the key of the i th element

3. the purpose of those code is to sort a by key

4. So we need to pass a self defined compare function when calling "sort()"

5. the self defined compare function is defined by these codes:

    [](auto &a,auto &b){return a[0] < b[0];}

I've never seen this way of writing (just like a lambda function in C++,)? could you tell me how to understand these code?

Answer 1

As per your question:

    sort(a.begin(), a.end(), [](auto &a, auto &b){return a[0] < b[0];});

a.begin()[First], a.end()[Last] - range of elements to sort

for the 3rd parameter, there are two options:

1. Lambda expression

    [](auto &x, auto &x){return x[0] < y[0];}

2. Comparison function

    bool comp(vector<int>& x, vector<int>& y) { return x[0] < y[0]; } sort(a.begin(), a.end(), comp);

a is a vector<vector<int>>

x and y are elements of a ie vector<int>

the lambda expression or the comparison function takes a pair of elements from a and sorts them based on the first value( x[0] ) of each element in a .

If you see, both are very similar.

a. Both accept an argument list. ( x and y in this case)

b. Both have a body return x[0] < y[0];

I hope this answers all of your questions.