Some C++ Tricks from Codeforces

Here, In folder C++.

I choose some of them which is useful for me and note them in these blog.

all(x)

#define all(x) x.begin(), x.end

before:

vector<int> a;
// some operation...
sort(a.begin(), a.end());

now:

vector<int> a;
// some operation...
sort(all(a));

before:

vector<int> a;
// some operation...
sort(a.begin() + 1, a.end())

now:

vector<int> a;
// some operation...
sort(1 + all(x));

std::unique

We know that std::unique won’t delete the rest number, but put them in the end.For example, vec = {1, 1, 2, 2, 3, 2, 1} may become {1, 2, 3, 2, 1, 2, 1}. It not good.We can do like this:

sort(all(vec));
vec.resize(unique(all(vec)) - vec.begin());

read()

inline int read() {
	int x = 0, f = 0; char c = 0;
	while (!isdigit(c)) f |= c == '-', c = getchar();
	while (isdigit(c)) x = (x << 3) + (x << 1) + (c & 15), c = getchar();
	return f ? -x : x;
}

Faster and Shorter.

before:

int n;
cin >> n; // or scanf("%d", &n);

now:

int n = read();

---

我为什么要用阴文。。。写不下去了。。。

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std::generate

将给定的区间用给定的函数赋值。举个例子：

int x = 0;
int up() { return ++ x; }

vector<int> a(4);
generate(all(a), up);
// a = {1, 2, 3, 4}

配合 read 可以快速读入数组。

for (int i = 1; i <= n; i ++) cin >> a[i];

现在变成了

generate(a + 1, a + n + 1, read);

使用 generate_n 还可以写成：

generate_n(a + 1, n, read);

输出一个数的二进制

使用 std::bitset 可以这样：

int n;
// ...
cout << bitset<31>(n) << endl;

lambda 表达式

见我的另一篇 blog。

std::max

int r = max({a, b, c});

{}

pair<int, int> x = {1, 2} // 等价于 make_pair(1, 2)
vector<int> y = {1, 1, 4, 5, 1, 4} // 对 vector 直接赋值

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