关于STL黑科技

加上头文件 #include <numeric>

加上头文件 #include <functional>

废话不多说，直接上代码：

数组求和

加和

sum = 0;
for(int i = 0;i < n;++i)
    sum += a[i];
printf("%d", sum);

等价于

printf("%d", accumulate(a, a + n, 0));

乘积

sum = 1;
for(int i = 0;i < n;++i)
    sum *= a[i];
printf("%d", sum);

等价于

printf("%d", accumulate(a, a + n, 1, multiplies<int>()));

异或和

for(int i = 0;i < n;++i)
    sum ^= a[i];
printf("%d", sum);

等价于

printf("%d", accumulate(a, a + n, 0, [](int a, int b){return a ^ b;}));

差分

减法

for(int i = 1;i <= n;++i)
    d[i] = a[i] - a[i - 1];

等价于

adjacent_difference(a + 1, a + n + 1, d + 1);

除法

for(int i = 1;i <= n;++i)
    d[i] = a[i] * inv[a[i - 1]] % M;

等价于

adjacent_difference(a + 1, a + n + 1, d + 1, [](int a, int b){return a * inv[b] % M;});

异或

for(int i = 1;i <= n;++i)
    d[i] = a[i] ^ a[i - 1];

等价于

adjacent_difference(a + 1, a + n + 1, d + 1, [](int a, int b){return a ^ b;});

奇怪的式子

考虑计算 $\sum\limits_{i=1}^na_ib_i$

sum = 0;
for(int i = 1;i <= n;++i)
    sum += a[i] * b[i];
printf("%d", sum);

等价于

printf("%d", inner_product(a + 1, a + n + 1, b + 1, 0));

前缀和

加和

for(int i = 1;i <= n;++i)
    s[i] = s[i - 1] + a[i];

等价于

partial_sum(a + 1, a + n + 1, s + 1);

乘积

for(int i = 1;i <= n;++i)
    s[i] = s[i - 1] * a[i];

等价于

partial_sum(a + 1, a + n + 1, s + 1, multiplies<int>());

异或

for(int i = 1;i <= n;++i)
    s[i] = s[i - 1] ^ a[i];

等价于

partial_sum(a + 1, a + n + 1, s + 1, [](int a, int b){return a ^ b;});

数组初始化

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

等价于

iota(a + 1, a + n + 1, 1);