Introduction
This is the list of modern CPP tricks often used in Coding Interviews and Competitive Programming.
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Contents:
- No more nested
min(x, min(y, ...)) - JavaScript like Destructuring using Structured Binding in C++
- Powerful Logging and Debugging
- Generic Reader and Writer for multiple variables and containers
- Decorators in C++ and Multiple Parameters
No more nested min(x, min(y, ...))
Use initializer list and std::min and std::max to make life easy
small = min(x, min(y, min(z, k))); // the old way
small = min({x, y, z, k}); // life is easyJavaScript like Destructuring using Structured Binding in C++
pair<int, int> cur = {1, 2};
auto [x, y] = cur;
// x is now 1, y is now 2
// no need of cur.first and cur.second
array<int, 3> arr = {1, 0, -1};
auto [a, b, c] = arr;
// a is now 1, b is now 0, c is now -1Powerful Logging and Debugging
How debug macros work?
Straight to the point, I have often used the debug macro which stringifies the variable names and their values.
#define deb(x) cout << #x << " " << x
int ten = 10;
deb(ten); // prints "ten = 10"This is often useful in debugging.
The Problem with this macro - its not scalable
However, when you have multiple variables to log, you end up with more deb2 and deb3 macros.
#define deb(x) cout << #x << " " << x
#define deb2(x) cout << #x << " " << x << " " << #y << " " << y
#define deb3(x, y, z) cout << #x << " " << x << " " << #y << " " << y << " " << #z << " " << z This is not scalable.
Solution using a powerful macro
Here is the solution using variadic macros and fold expressions,
#define deb(...) logger(#__VA_ARGS__, __VA_ARGS__)
template<typename ...Args>
void logger(string vars, Args&&... values) {
cout << vars << " = ";
string delim = "";
(..., (cout << delim << values, delim = ", "));
}
int xx = 3, yy = 10, xxyy = 103;
deb(xx); // prints "xx = 3"
deb(xx, yy, xxyy); // prints "xx, yy, xxyy = 3, 10, 103"Generic Reader and Writer for multiple variables and containers
template <typename... T>
void read(T &...args) {
((cin >> args), ...);
}
template <typename... T>
void write(string delimiter, T &&...args) {
((cout << args << delimiter), ...);
}
template <typename T>
void readContainer(T &t) {
for (auto &e : t) {
read(e);
}
}
template <typename T>
void writeContainer(string delimiter, T &t) {
for (const auto &e : t) {
write(delimiter, e);
}
write("\n");
}Usage
// Question: read three space seprated integers and print them in different lines.
int x, y, z;
read(x, y, z);
write("\n", x, y, z);
// even works with variable data types :)
int n;
string s;
read(n, s);
write(" ", s, "has length", n, "\n");
// Question: read an array of `N` integers and print it to the output console.
int N;
read(N);
vector<int> arr(N);
readContainer(arr);
writeContainer(" ", arr); // output: arr[0] arr[1] arr[2] ... arr[N - 1]
writeContainer("\n", arr);
/**
* output:
* arr[0]
* arr[1]
* arr[2]
* ...
* ...
* ...
* arr[N - 1]
*/Decorators in C++ and Multiple Parameters
Live Demo on YouTube
- Check the online demo on YouTube here on Rachit's channel.
Printing as many variables in one line
template<typename ...T>
void printer(T&&... args) {
((cout << args << " "), ...);
}
int age = 25;
string name = "Rachit";
printer("I am", name, ',', age, "years old");
// ^ This prints the following
// I am Rachit, 25 years oldPowerful decorator functions in C++
template<typename F>
auto debug_func(const F& func) {
return [func](auto &&...args) { // forward reference
cout << "input = ";
printer(args...);
auto res = func(forward<decltype(args)>(args)...);
cout << "res = " << res << endl;
return res;
};
}
debug_func(pow)(2, 3);
// ^ this automatically prints
// input = 2 3 res = 8Exploiting decorators by nesting them
Lets define another decorator beautify as follows.
template<typename F>
auto beautify(const F& func) {
return [func](auto &&...args) { // forward reference
cout << "========" << endl;
func(forward<decltype(args)>(args)...);
cout << "========" << endl;
};
}
beautify(debug_func(pow(2, 3)));
// ^ this now prints
// ========
// input = 2 3 res = 8
// ========Its amazing how much you can do by writing such generic decorators and nest them.
Think about decorators like log_time that calculates the time taken for a given function.