public: True class: center, middle # 循环结构 Fancy 2020/1/22 --- # for 语句 ```cpp for (初始化表达式; 条件表达式; 增量表达式) { 语句1; 语句2; ... } ``` 例:计算 10! ```cpp for (int i = 1; i <= 10; ++i) { fac *= i; } ``` --- # for 语句 ```cpp for (初始化表达式; 条件表达式; 增量表达式) { 语句1; 语句2; ... } ``` 执行过程: 1. 执行“初始化表达式”,使控制变量获得一个初值。 2. 判断控制变量是否满足“条件表达式”,若满足条件则执行一遍循环体(语句1, 2, ...),否则结束循环。 3. 根据增量表达式,计算出控制变量所得到的新值。 4. 转到第2步。 --- # for 语句 - 将控制变量从 1 加到 100,增量为 1 ``` for (i = 1; i <= 100; ++i) ``` - 将控制变量从 20 减到 2,增量为 -2 ``` for (i = 20; i >= 2; i -= 2) ``` - 将控制变量的值以数列 55, 44, 22, 11 变化 ``` for (i = 55; i > 0; i -= 11) { if (i != 33) { ...... } } ``` - 将控制变量 i 和 j 共同进行循环,1 从 1 加到 50,j 从 5 加到 54,增量为 2 ``` for (i = 1, j = 5; i <= 50 && j <= 54; ++i, ++j) ``` --- # for 语句 例:计算 1~100 中奇数的和、偶数的和。 ```cpp int main() { int sum1 = 0, sum2 = 0; for (int i = 1; i <= 100; ++i) if (i % 2 == 1) sum1 += i; else sum2 += i; cout << sum1 << " " << sum2; return 0; } ``` ```cpp int main() { int sum1 = 0, sum2 = 0; for (int i = 1, j = 2; i <= 100; i += 2, j += 2) { sum1 += i; sum2 += j; } cout << sum1 << " " << sum2; return 0; } ``` --- # while 语句 ```cpp while (条件表达式) { 语句1; 语句2; ... } ``` 例:计算 10! ```cpp int i = 0, fac = 1; while (i <= 10) { ++i; fac *= i; } ``` 执行过程: 1. 计算作为循环控制条件表达式的值 ,若为真,则执行循环体(语句1, 2, ...);否则结束循环。 2. 转到第2步。 --- # while 语句 例:求 s=1+2+3+...+n,当加到第几项时,s 的值会超过 1000? ```cpp int main() { int n = 0, s = 0; while (s <= 1000) { ++n; s += n; } cout << n; return 0; } ``` ```cpp int main() { int n, s = 0; for (n = 1; s <= 1000; ++n) { s += n; } cout << n - 1; return 0; } ``` --- # while 语句 例:输入一些整数(值不超过 1000),求出它们的最小值和、最大值和平均值(保留 3 位小数)。 输入样例:2 8 3 5 1 7 3 6 ^Z 输出样例:1 8 4.375 ```cpp const int INF = 1e8; int main() { int x, mini = INF, maxi = -INF, num = 0, sum = 0; while (scanf("%d", &x) != EOF) { mini = x < mini ? x : mini; maxi = x > maxi ? x : maxi; ++num; sum += x; } printf("%d %d %.3lf\n", mini, maxi, (double)sum / num); return 0; } ``` --- # do-while 语句 ```cpp do { 语句1; 语句2; ... } while (条件表达式); ``` 例:计算 10! ```cpp int i = 0, fac = 1; do { ++i; fac *= i; } while (i < 10); ``` 执行过程: 1. 执行循环体(语句1, 2, ...)。 2. 计算作为循环控制条件表达式的值 ,若为真,则转到第1步;否则结束循环。 --- # 循环嵌套 例:计算 1! + 2! + 3! + ... + 10! ```cpp int sum = 0; for (int i = 1; i <= 10; ++i) { int fac = 1; for (int j = 1; j <= i; ++j) fac *= j; sum += fac; } cout << sum; ``` ```cpp int sum = 0, fac = 1; for (int i = 1; i <= 10; ++i) { fac *= i; sum += fac; } cout << sum; ``` --- # 循环嵌套 例:计算 100~999 之间的水仙花数。若三位数 ABC = A
^3
+ B
^3
+ C
^3
则称 ABC 为水仙花数。如 153 =
1^3 + 5^3 + 3^3
。 ```cpp for (int a = 1; a <= 9; ++a) for (int b = 0; b <= 9; ++b) for (int c = 0; c <= 9; ++c) if (a * a * a + b * b * b + c * c * c == a * 100 + b * 10 + c) cout << a * 100 + b * 10 + c << endl; ``` ```cpp for (int i = 100; i <= 999; ++i) { int a = i / 100; int b = (i % 100) / 10; int c = i % 10; if (a * a * a + b * b * b + c * c * c == i) cout << i << endl; } ``` --- # 循环嵌套 例:输出 2~100 之间的所有素数。 ```cpp for (int i = 2; i <= 100; ++i) { int x = 2; while (x * x <= i && i % x != 0) ++x; if (x * x > i) cout << i << endl; } ``` ```cpp for (int i = 2; i <= 100; ++i) { bool prime = 1; for (int x = 2; x * x <= i; ++x) if (i % x == 0) { prime = 0; break; } if (prime == 1) cout << i << endl; } ```