c++ 教程：执行分支控制

分支结构(Conditional Branching)

if 语句块

if (condition1) {
  // do this if condition1 is true
}
else if (condition2) {
  // else this if condition2 is true
}
else {
  // otherwise do this
}

---

如果条件满足则执行，否则不执行

条件表达式为 bool 或可以转化成 bool 类型

从上到下依次比较每个if

---

if (true)  { cout << "yes\n"; }  // yes
if (false) { cout << "yes\n"; }  // –
if (23)    { cout << "yes\n"; }  // yes (23 → true)
if (0)     { cout << "yes\n"; }  // – (0 → false)


int i = 0;
cin >> i;
if (i < 0) { 
  cout << "negative\n"; 
} if else (i == 0) {
  cout << "zero\n"; 
} else {
  cout << "positive\n"; 
}

---

三元条件操作符

Result = Condition ? If-Expression : Else-Expression

如果条件满足结果为 If-Expression 的值， 否则为 Else-Expression 的值

---

int i = 8;
int j = i > 10 ? 1 : 2;         j: 2
 
int k = 20;
int l = (k > 10) ? 1 : 2;       l: 1
 
int b = true;
double d =  b ? 2.0 : 0.5;      d: 2.0
double e = !b ? 2.0 : 0.5;      e: 0.5

switch 语句块

支持整数类型 (char, int, long, enums, …)

从上到下依次检查并执行

如果某个 case 满足则从这个case开始执行代码直到 break 语句

---

int i = 0;
int m = i % 5;
switch (m) {
  case 0:   // do this if m is 0
    break;
  case 1:   // do this if m is 1
  case 3:   // do this (also) if m is 1 or 3
    break;
  default:  // do this if m is not 0, 1 or 3
}

循环结构(Loop)

for 循环

for (initialization; condition; step) { … }

---

// prints 0 1 2 3 4
for (int i = 0; i < 5; ++i) {
  std::cout << i << ' ';
}

---

for Range

for (variable : range) { … }

遍历容器中的元素, e.g., std::vector

---

std::vector<int> v {1,2,3,4,5};
// print all elements of vector to console
for (int x : v)  { std::cout << x << ' '; }

while 循环

while (condition) { … }

条件满足才会执行首次循环

---

//prints 5 6 7 8 9
int j = 5;  
while (j < 10) {
  std::cout << j << ' ';
  ++j;
}

---

do while 循环

do { … } while (condition);

至少执行一次循环，条件判断前就会执行一次循环

---

//prints 10 9 8 … 1 executed until j ≤ 0
int j = 10;
do { 
  std::cout << j << ' ';
  --j;
} while (j > 0);