84 lines
3.4 KiB
Markdown
84 lines
3.4 KiB
Markdown
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---
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title: Loops
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localeTitle: 循环
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---
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# 循环
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## 介绍
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现在让我们讨论称为循环的东西。假设您要在屏幕上打印1到1000的偶数。单程 这样做是为了写下面这几行
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\`\`\`c ++ cout << 0 << endl; cout << 2 << endl; cout << 4 << endl; ...。 ...。 ...。 cout << 1000 << endl;
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```
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But the problem with this approach is that you have to write the same line again and again. And if suppose you have to print
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prime numbers from 1 to 1000 then this will be more hectic.
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Therefore, in order to solve such problems loops are introduced.
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There are different types of loop functions:
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### While and do while loops
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While and do while loops allow you to make the loop until a condition finishes.
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The difference between While and Do while is that Do while always executes once.
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Here you can see an example:
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```
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C ++ while（condition）{ //条件为true时执行的代码 } 做{ //将执行一次，直到条件为假 } while（condition）;
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```
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### For loops
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For loops are usually used when you know how many times the code will execute.
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The flow can be seen in this [graph](https://www.tutorialspoint.com/cplusplus/images/cpp_for_loop.jpg).
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They are declared this way:
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```
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C ++ for（初始化变量;检查条件;增加初始化变量）{ //要执行的代码 }
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```
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Lets write a program which will print numbers from 0 to 1000 including 1000 on the screen using a for loop.
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```
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C ++ for（int i = 0; i <= 1000; i ++） { cout << i << endl; }
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```
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When you execute this code in a c++ program numbers from 1 to 1000 will be printed.
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Now lets discuss how the for loop works.
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* You start a for loop by typing the keyword 'for'. It means you are starting a for loop
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` for `
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* Next you open and close a round bracket. In this brackets you write some conditions which will be discussed later
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` for()`
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* Inside the brackets first you write the initial condition ie the value from where the loop will start. Like in the
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above program we write int i = 0
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` for(int i = 0)`
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* Then you write the semicolon and then condition until when the loop will be executed. In the above code you define
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i < 1000. It means until value of i is less then 1000 execuete the loop.
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` for(int i=0;i<=1000) `
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* Then you define the incremented value that is how much i has to be incremented in each iteration. In the above code
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we write i++. It means value of i will be incremented by 1 every time.
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` for(int i=0;i<=1000;i++) `
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* If there is only one statement inside the loop then the curly bracket is optional but its better to write loop code
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within brackets so that you don't get confused.
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``` c++
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for(int i=0;i<=1000;i++)
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{
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}
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```
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* Then inside the loop you write what do you want to do. In the above program we output the value of i.
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So, in this way the for loop works
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If you want to print even numbers from 1 to 1000 then your program will look like this
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```
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C ++ for（int i = 0; i <= 1000; i = i + 2） { cout << i << endl; }
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\`\`\`
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* 第一个程序和第二个程序的区别是增量部分。其余代码是一样的。该程序将打印0和 然后添加2并在控制台上打印2，依此类推，i的值等于1000。
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我们打印偶数0到1000的最终程序将如下所示。
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\`\`\`c ++
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# 包括
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使用命名空间std; int main（） { for（int i = 0; i <= 1000; i = i + 2） { cout << i << endl; } 返回0; } \`\`\`
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