分支和循环

使用 if 语句做出决定

传统的if语句，格式见下面代码

int a = 5;
int b = 6;
if (a + b > 10)
    Console.WriteLine("The answer is greeater than 10.");

The answer is greeater than 10.

让 if 和 else 完美配合

int a = 5;
int b = 3;
if (a + b > 10)
{
    Console.WriteLine("The answer is greater than 10");
}
else
{
    Console.WriteLine("The answer is not greater than 10");
}

The answer is not greater than 10

&&表示且

int a = 5;
int b = 3;
int c = 4;
if ((a + b + c > 10) && (a > b))
{
    Console.WriteLine("The answer is greater than 10");
    Console.WriteLine("And the first number is greater than the second");
}
else
{
    Console.WriteLine("The answer is not greater than 10");
    Console.WriteLine("Or the first number is not greater than the second");
}

The answer is greater than 10
And the first number is greater than the second

||表示或

int a = 5;
int b = 3;
int c = 4;
if ((a + b + c > 10) || (a > b))
{
    Console.WriteLine("The answer is greater than 10");
    Console.WriteLine("Or the first number is greater than the second");
}
else
{
    Console.WriteLine("The answer is not greater than 10");
    Console.WriteLine("And the first number is not greater than the second");
}

The answer is greater than 10
Or the first number is greater than the second

使用循环重复执行运算

while循环
++运算符：自加1

int counter = 0;
while (counter < 10)
{
    Console.WriteLine($"Hello World! The counter is {counter}");
    counter++;
}

Hello World! The counter is 0
Hello World! The counter is 1
Hello World! The counter is 2
Hello World! The counter is 3
Hello World! The counter is 4
Hello World! The counter is 5
Hello World! The counter is 6
Hello World! The counter is 7
Hello World! The counter is 8
Hello World! The counter is 9

do-while循环：先执行，后判断

int counter = 0;
do
{
    Console.WriteLine($"Hello World! The counter is {counter}");
    counter++;
}while (counter < 10);

Hello World! The counter is 0
Hello World! The counter is 1
Hello World! The counter is 2
Hello World! The counter is 3
Hello World! The counter is 4
Hello World! The counter is 5
Hello World! The counter is 6
Hello World! The counter is 7
Hello World! The counter is 8
Hello World! The counter is 9

使用 for 循环

for(初始值;循环条件;循环后操作)

for(int counter = 0; counter < 10; counter++)
{
    Console.WriteLine($"Hello World! The counter is {counter}");
}

Hello World! The counter is 0
Hello World! The counter is 1
Hello World! The counter is 2
Hello World! The counter is 3
Hello World! The counter is 4
Hello World! The counter is 5
Hello World! The counter is 6
Hello World! The counter is 7
Hello World! The counter is 8
Hello World! The counter is 9

注意3个语句执行顺序，经测试，如果把for语句写成如下形式
for(①;②;③)，那么执行顺序为

①②-代码块
③②-代码块
③②-代码块
③②-代码块
…

练习：

//我的答案
int sum = 0;
for(int counter=0;counter < 21; counter++)
{
    if(counter%3 == 0)
    {
        sum += counter;
    }
}
Console.WriteLine(sum)

//官网答案
int sum = 0;
for (int number = 1; number < 21; number++)
{
  if (number % 3 == 0)
  {
    sum = sum + number;
  }
}
Console.WriteLine($"The sum is {sum}");

The sum is 63

总结

1个判断语句，2个循环语句

1个判断语句if-if-else
2个循环语句for，while-do-while
使用范式
if

if(条件）
{
    条件为True代码块
}

if-else

if(条件）
{
    条件为True代码块
}
else
{
    条件为False代码块
}

for

for(初始条件；条件；循环完成后的操作）
{
    代码块
}

注意3个语句执行顺序，经测试，如果把for语句写成如下形式
for(①;②;③)，那么执行顺序为

①②-代码块
③②-代码块
③②-代码块
③②-代码块
…
while

while(条件)
{
    条件为True代码块
}

while-do-while

do
{
    代码块
}while(条件);

杂项

&&：与
||：或
++：自加1

C# 官方快速入门学习笔记（三）—— 分支和循环