Lambda Variable Capture
A lambda can use parameters and other variables in its function body
To use variables from its surrounding scope, captures are required
Variables placed within
[]before the parameter list are captured (by value or reference with&)
1 int main() {
2 vector<int> vec = {45, 63, 28, 21, 80, 91, 88, 72, 34, 57, 50, 69};
3 int factor;
4
5 cout << "Enter a number between 1 and 25: " << endl;
6 cin >> factor;
7 cout << "The first number that is a multiple of " << factor << " is: ";
8 cout << *find_if(begin(vec), end(vec), [factor](int n) {
9 return n % factor == 0;
10 }); // find_if()
11 } // main()
There are several ways to capture variables in a lambda:
[]captures no variables fromt he surrounding scope[=]captures all varibales in the surrounding scope by value (i.e. a copy of each variable is made)[&]captures all variables in the surrounding scope by reference[foo]captures only the variablefoo, by value[&foo]captures only the variablefoo, by reference[foo, bar]captures only the variablesfooandbar, by value[=, &foo]captures all variables in the surrounding scope by value except forfoo, which is captured by reference[&, foo]captures all variables in the surrounding scope by reference except forfoo, which is captured by value[this]captures the current object - needed if a lambda defined in an object's member function needs access to is member variables
Using the STL¶
i. Learning STL
Online tutorials and examples
-
Discusses possible implementations of STL functions
You can copy / modify examples and run them online
Practice with small tester programs
- Try algorithms with different data structures / input
Detailed coverage in books
- Josuttis, Stroustrup, recent edition algorithm books
Ways to learn a library
Skim through documentation
Study what seems interesting
Learn how to use those pieces
Come back when you need something
ii. Debugging STL-heavy code
Compiler Errors
Compiler often complains about STL headers, not your code - induced errors
You will need to shift through many lines of messages, to find line reference to your code
Good understanding of type conversions in C++ is often required to fix problems
Double-check function signatures
Runtime Debugging
Crashes can occur in STL code, started by an error in your code
Debugging needed with ANY library
Ing gdb, use "where" or "bt" commands to find code that calls STL
90% of STL-related crashes are due to user's dangling pointers or references going out og scope
iii. Randomization
C++ 11 introduced the
<random>library to support random number generation:Generators: generate random numbers
- Ex:
std::random_device,std::mt19937,std::mt19937_64
- Ex:
Distributions: convert generator output into staistical distributions
- Ex:
std::normal_distribution,std::uniform_int_distribution,std::uniform_real_distribution
- Ex:
Generally preferred over
rand()Randomization is great for testing code
Example 1:
1 #include <iostream>
2 #include <vector>
3 #include <algorithm> // Needed for shuffle()
4 #include <numeric> // Needed for iota()
5 #include <random> // Needed for random_device and mt19937_64
6 using namespace std;
7
8 int main() {
9 random_device rd; // Create a device to start random # generation
10 mt19937_64 mt(rd()); // Create a Mersenne Twister to generate random #
11 int size = 20; // Could also read size from cin
12 vector<int> values(size);
13
14 iota(begin(values), end(values), 0);
15 shuffle(begin(values), end(values), mt);
16
17 for (auto v : values)
18 cout << v << " ";
19
20 cout << endl;
21 return 0;
22 }
Example 2:
#include <iostream>
#include <random> // Needed for random_device and mt19937_64
using namespace std;
int main() {
random_device rd; // Create a device to start random # generation
mt19937_64 mt(rd()); // Create a Mersenne Twister to generate random #
int size = 10; // Could also read size from cin
cout << size << "\n";
// Run a loop, generating random x / y coordinates (important for project 4)
for (int i = 0; i < size; i++) {
// mt() generates a large random number, % limits the range, - makes
// about half the numbers negative; the range is -10 to 10 (inclusive)
int x = static_cast<int>(mt() % 21) - 10;
int y = static_case<int>(mt() % 21) - 10;
cout << x << ' ' << y << '\n';
}
cout << endl;
return 0;
}
STL Container Performance¶
Filling an empty vector with different values. (vector_pre used vector::resize(), which is an single allocation)
Fill the container with numbers $[0, N)$ shuffle at random; search for each value using std::find().
Fill the container with numbers $[0, N)$ shuffle at random; pick a random position by linear search; insert 1000 values.
Fill the container with numbers $[0, N)$ shuffle at random; pick a random position by linear search; remove 1000 values.
Insert new values at the front. A vector needs to move all prior elements, but a list does not.
