CS100 Lecture #4 20220223

Author： Bi_Gu
发布时间：February 23, 2022
232 views
No comments
5110 words
Categories：课堂笔记计算机编程

# CS100 Lecture#4 20220223

## Control Flow

### The `if` Statement

```C
if (expression)
    statement;
/* simple or compound statement enclosed with brackets */
```

The boundary of `if` statement is decided by brackets `{}`. If there are no brackets, the statement executed when the expression is `True` would be and only be **one line below**.

> Unlike python, c language won't interpret $\textcolor{blue}{\text{indentations}}$.
> 
> In fact, the boundary of **most statements** is decided by `{}`. For comparison, in `python` they are all decided by indentations.

### The `if...else...` Statement

```C
if (expression)
    statement1;
else
    statement2;
```

### The `if...else if...else` Statement

```C
if (expression1)
    statement1;
else if (expression2)
    statement2;
else
    statement3;
```

### Nested `if`

### The Conditional Operator

```C
Expression_1 ? Expression_2 : Expression_3
```

The value of this expression depends on whether **Expression_1** is true :

* If Expression_1 is $\textcolor{green}{\text{True}}$: value of the expression is that of Expression_2;
* If Expression_1 is $\textcolor{red}{\text{False}}$: value of the expression is that of Expression_3.

> We already have `if...else...`, why we need conditional operator?
> 
> * Compatible with older programming languages
> * Less lines

### The `switch` Statement

The $\textcolor{red}{\text{switch}}$ statement is for multi-way selection. The syntax is:

```C
switch (Expression)
{
    case constant_1:
        statement_1;
        break;
    case constant_2:
        statement_2;
        break;
    case constant_3:
        statement_3;
        break;
    default:
        statement_d;
}
```

* `switch`/`case`/`break`/`default` are reserved words.
* The result of expression in `()` must be an $\textcolor{blue}{\text{integer type}}$. (An integer or character)
* `constant_1`, `constant_2` are called $\textcolor{green}{\text{labels}}$. Each must be an **integer constant** $(\mathrm{e.g.}\ 1)$, a **character constant** $(\mathrm{e.g.}'a')$ or an **integer character constant** $(\mathrm{e.g.}\ 4+5,1+'a')$.$\textcolor{red}{\text{It cannot be a variable.}}$
* Each of the labels must deliver a $\textcolor{blue}{\text{unique}}$ integer value. Duplicates are **not allowed**.
* We may have multiple labels for a statement.
* If we $\textcolor{red}{\text{do not use}}$ `break` after some statements, execution will continue with the statements below until it meets a `break` statement or the end of `switch` statement. This is called a *fall through* situation.

> The type `char` can have $\textcolor{red}{\text{one character only}}$, interpreted as an integer using the ASCII code.

## Looping

### Two types of loops

* **Counter-controlled loops**: [由计数器控制] The loop body is repeated for a number of times, and the number of repetitions is $\textcolor{blue}{\text{known}}$ before the loop starts execution.
* **Sentinel-controlled loops**: [由flag控制] The number of repetitions is $\textcolor{blue}{\text{not known}}$ before the loop starts execution. Usually, a $\textcolor[RGB]{30,144,255}{\text{sentinel value}}$ (such as -1, different from regular data) is used to determine whether to execute the loop body.
  
  <img src="https://cdn.dabigu.xyz/QQ图片20220223093748.png" width="600px">

### The `while` Loop

```C
while (test)
    statement;
```

For example:

```c
#include <stdio.h>
int main(void)
{
	int sum, item;
	printf(“Enter the list of integers:\n”);
	scanf(“%d”, &item);
	while (item != -1) {
		/* Sentinel-controlled */
		sum += item;
		scanf(“%d”, &item);
	}
	printf(“The sum if %d\n”, sum);
	return 0;
}
```

The user inputs `-1` if he wants the loop to stop.

### The `for` Loop

```c
for (initialize;test;update)
    statement;
```

* Normally, `test` is a $\textcolor{green}{\text{relational expression}}$ to control iterations. ($\mathrm{i.e.}$ when to end the loop)
* `update` is frequently used to update some loop **control variable** before repeating the loop.
* $\textcolor{blue}{\text{Any}}$ of the expressions can be *omitted*. However, if the `test` expression is missing, it will become an **infinite loop**.

For example,

```C
for (i = 1 ; i <= 500 ; i++)
{
    printf("Hello World!")
}
```

does the same as the following codes in `python`:

```python
for i in range(501):
    print("Hello World!")
```

### The `do-while` Loop

```c
do {
    statement;
}while (test);
```

The condition `test` is performed $\textcolor{red}{\text{after}}$ executing the statement every time, which means that the loop will be executed **at least once**. (The `while` or `for` loop may not be executed even once.)

### The `break` Statement

To alter flow of control inside loop (as well as the `switch` statement), execution of `break` causes $\textcolor{blue}{\text{immediate termination}}$ of the $\textcolor{red}{\text{inner most}}$ enclosing loop or `switch` statement.

> 即遇到`break`语句时，立刻跳出其所在的一层循环，执行该层循环后的代码。与`python`的逻辑结构相同。

### The `continue` Statement

The control immediately **passed to** ($\textcolor{green}{\text{the update}}$, if any) and $\textcolor{blue}{\text{test}}$ condition of the $\textcolor{red}{\text{nearest}}$ enclosing loop. All subsequent statements after the `continue` statement are not executed for **this** particular iteration.

> 即遇到`continue`语句时，立刻回到其所在循环的开头。循环体中在`continue`后的代码不会执行。

For example:

```C
#include <stdio.h>
int main(void)
{
	int i;
	float data, sum = 0.0;
	/* read 8 numbers */
	for (i=0; i < 8; i++) {
		scanf(“%f”, &data);
		if (data < 0.0)
			continue;
		sum += data;
	}
	printf(“The sum is %f\n”, sum);
	return 0;
}
```

> 当用户输入的`data` $<0$时，`if`条件判断成立，`continue`被执行，因而跟着其后的语句`sum += data`不会被执行，也就是说该次输入不会被统计到`sum`中。

### Nested Loops

Last modification：February 25, 2022

Support if you like my work! qwq

版权声明：若文中无特殊说明，则本文为原创文章，版权归 Bi_Gu 所有。

本文链接：https://dabigu.xyz/index.php/archives/94/

所有原创文章采用知识共享署名-非商业性使用 4.0 国际许可协议进行许可。
您可以自由的转载和修改，但请务必注明文章来源并且不可用于商业目的。

随机文章 Maths Analysis II - Homework #14 20220407 Financial Economics Lecture #2 ABCs of Finance Discrete Mathematics Homework #8

CS100 Lecture #4 20220223

Bi_Gu • 2022 年 02 月 23 日

# CS100 Lecture#4 20220223

## Control Flow

### The `if` Statement

```C
if (expression)
    statement;
/* simple or compound statement enclosed with brackets */
```

The boundary of `if` statement is decided by brackets `{}`. If there are no brackets, the statement executed when the expression is `True` would be and only be **one line below**.

### The `if...else...` Statement

```C
if (expression)
    statement1;
else
    statement2;
```

### The `if...else if...else` Statement

```C
if (expression1)
    statement1;
else if (expression2)
    statement2;
else
    statement3;
```

### Nested `if`

### The Conditional Operator

```C
Expression_1 ? Expression_2 : Expression_3
```

The value of this expression depends on whether **Expression_1** is true :

> We already have `if...else...`, why we need conditional operator?
> 
> * Compatible with older programming languages
> * Less lines

### The `switch` Statement

The $\textcolor{red}{\text{switch}}$ statement is for multi-way selection. The syntax is:

> The type `char` can have $\textcolor{red}{\text{one character only}}$, interpreted as an integer using the ASCII code.

## Looping

### Two types of loops

### The `while` Loop

```C
while (test)
    statement;
```

For example:

The user inputs `-1` if he wants the loop to stop.

### The `for` Loop

```c
for (initialize;test;update)
    statement;
```

For example,

```C
for (i = 1 ; i <= 500 ; i++)
{
    printf("Hello World!")
}
```

does the same as the following codes in `python`:

```python
for i in range(501):
    print("Hello World!")
```

### The `do-while` Loop

```c
do {
    statement;
}while (test);
```

### The `break` Statement

> 即遇到`break`语句时，立刻跳出其所在的一层循环，执行该层循环后的代码。与`python`的逻辑结构相同。

### The `continue` Statement

> 即遇到`continue`语句时，立刻回到其所在循环的开头。循环体中在`continue`后的代码不会执行。

For example:

> 当用户输入的`data` $<0$时，`if`条件判断成立，`continue`被执行，因而跟着其后的语句`sum += data`不会被执行，也就是说该次输入不会被统计到`sum`中。

### Nested Loops

CS100 Lecture #4 20220223

Leave a Comment Cancel reply

《Phigros》游戏记录

使用RD Client实现Windows远程桌面

基于opencv和pyautogui的腾讯会议自动入会

Discrete Mathematics Homework #13

Discrete Mathematics Homework #7

Diary Set #3 20220228-20220306

Maths Analysis II - Homework #3 20220222

Maths Analysis II - Mid-term Homework

Maths Analysis II - Homework #27 20220602

Linear Algebra Notes - Section 2.1

CS100 Lecture #4 20220223