CS100 Lecture #5 20220228

Author： Bi_Gu
发布时间：February 28, 2022
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# CS100 Lecture#5 20220228

## Functions

### Function Definition in C

A $\textcolor{blue}{\text{function}}$ is a self-contained unit of code to carry out a specific task; it contains

* A header
* A opening curly brace `{`
* A body
* A closing curly brace `}`
  
  ```c
  float findMaximum (float x, float y)
  {
      //variable declaration
      float maxnum;
      //find the max number
      if (x >= y)
          maxnum = x;
      else
          maxnum = y;
      return maxnum;
  }
  ```

### Function header

Its format is like

```c
Return_type Function_name(Argument_list)
```

* $\textcolor{blue}{\text{Arguments}}$ define the $\textcolor{green}{\text{data}}$ passed into the function. A function can have one or many arguments.
  
  Each parameter in the list should have a **data type**; A function can have no parameter.
* The date type for $\textcolor{red}{\text{each}}$ argument must be declared.
  
  The function **assumes** that these inputs will (all) be supplied to the function when it is being called.
* $\textcolor{blue}{\text{Return type}}$ is the data type returned from the function.
  
  It can be `int`, `float`, `char`, `void`, or *nothing*. [缺省]
  
  * `void` - The function will not return any value.
  * *nothing* - If defined with no type, the **default** type is `int`.

> $\textcolor{red}{\text{Parameter}}$ is the variable passed into the function while $\textcolor{green}{\text{defining}}$ the function.
> 
> 形参是函数被定义时传递到函数中的变量，只能在函数体中使用，不能在函数外使用；
> 
> $\textcolor{red}{\text{Argument}}$ is the values passed to those variables while $\textcolor{green}{\text{calling}}$ the function.
> 
> 实参是函数被引用时传递给形参的变量值，只能在函数体外使用。

### The `Return` Statement

It may appear in $\textcolor{green}{\text{any}}$ place and in **more than one** place inside the function body.

### Function Prototypes

* This is to **declare** a function. A function declaration is called a $\textcolor[RGB]{30,144,255}{\text{function prototype}}$.[函数原型]
* It provides information about
  
  * The (data) **type** of the function;
  * The **name** of the function;
  * The **number** and (data) **type** of the arguments.
* The declaration may be the same as the function `header` terminated by a semicolon `;`. For example,
  
  ```c
  void hello_n_times(int n);
  ```
* Or the function is declared without giving the parameter names:
  
  ```C
  double distance(double, double);
  ```
* The declaration has to be done $\textcolor{red}{\text{before}}$ the function is called:
  
  * Before the `main()` header
  * Inside the `main()` body
  * Inside **any** function body which uses it(used multiple times = declared multiple times)

Usually, a function is declared in a $\textcolor{red}{\text{header file}}$.

* A header file `(*.h)` can contain declarations of multiple functions;
* A header can also define multiple $\textcolor{blue}{\text{global}}$ variable declarations.

## Scope of Variables & Parameter Passing

### Scope of Variables in a Function

Variables declared in a function is $\textcolor{red}{\text{only}}$ visible **within** that function.

### Parameter Passing: Call by Value

Communications between a **function** and the **calling body** is done through $\textcolor{blue}{\text{arguments}}$ and the $\textcolor{blue}{\text{return value}}$ of a function. For example,

```C
# include <stdio.h>
int add1(int); //function prototype

int main()
{
    int num = 5;
    num = add1(num); // num is an argument
    printf("The value of num is: %d", num);
    return 0;
}
int add1(int value) //function definition; value is a parameter
{
    return ++value; //returns the value to where the function is called
}
```

> 这里当函数被调用时，将实参`num`的值传递给形参`value`，在函数中完成操作后通过`return`函数返回给实参`num`。

## Function Decomposition

Function decomposition basically means $\textcolor[RGB]{30,144,255}{\text{Stepwise Refinement}}$. [逐步求精]

In `C`, function decomposition starts with the high-level description of the program and decomposes the program($\mathrm{i.e.}$ the `main()` function) into successive **smaller** functions until we arrive at *suitably sized* functions.

* What is a **suitably sized** function?
  * Smaller functions are *easier* to understand;
  * Smaller functions promote software **reusability**;
  * Function size should not be longer than $\textcolor{green}{\text{a page}}$. Better yet, a function should not be longer than $\textcolor{green}{\text{half a page}}$.
* Why do we need Function Decomposition?
  * Program better structured;
  * Program better to understand and modify;
  * Shorter program;
  * Easier to debug.

### Placing Functions into Different Files

For example, one file `mainF.c` contains the `main()`. The `main` function calls `function1()` and `function2()`.

These two functions (definition) are in another file `support.c`. There are two constants defined by `#define` and used by the program, `CONST1` and `CONST2`.

The $\textcolor{green}{\text{constant definitions}}$ and the $\textcolor{green}{\text{function declarations}}$ are in a header file `def.h`.

* file1 - `mainF.c`
  
  ```C
  #include <stdio.h>
  #include "def.h" //double quotes mean the file is in the current directory
  int main(void)
  {
      ...
      count = function1(h, k);
      function2(&h, &k);
      ...
  }
  ```
* file2 - `support.c`
  
  ```C
  #include <stdio.h>
  #include "def.h"
  int function1(int f, int g)
  {
      ...
  }
  void function2(int *p, float *q)
  {
      ...
  }
  ```
* file3 - `def.h`
  
  ```c
  #define CONST1 80
  #define CONST2 100
  int function1(int, int);
  void function2(int *, float *);
  ```

> If we do not use a `header` file, then these lines in `header.h` have to both in the header of `mainF.c` and `support.c`.

### Compiling Program with Several Files

Last modification：February 28, 2022

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CS100 Lecture #5 20220228

Bi_Gu • 2022 年 02 月 28 日

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# CS100 Lecture#5 20220228

## Functions

### Function Definition in C

A $\textcolor{blue}{\text{function}}$ is a self-contained unit of code to carry out a specific task; it contains

### Function header

Its format is like

```c
Return_type Function_name(Argument_list)
```

### The `Return` Statement

It may appear in $\textcolor{green}{\text{any}}$ place and in **more than one** place inside the function body.

### Function Prototypes

Usually, a function is declared in a $\textcolor{red}{\text{header file}}$.

* A header file `(*.h)` can contain declarations of multiple functions;
* A header can also define multiple $\textcolor{blue}{\text{global}}$ variable declarations.

## Scope of Variables & Parameter Passing

### Scope of Variables in a Function

Variables declared in a function is $\textcolor{red}{\text{only}}$ visible **within** that function.

### Parameter Passing: Call by Value

Communications between a **function** and the **calling body** is done through $\textcolor{blue}{\text{arguments}}$ and the $\textcolor{blue}{\text{return value}}$ of a function. For example,

```C
# include <stdio.h>
int add1(int); //function prototype

> 这里当函数被调用时，将实参`num`的值传递给形参`value`，在函数中完成操作后通过`return`函数返回给实参`num`。

## Function Decomposition

Function decomposition basically means $\textcolor[RGB]{30,144,255}{\text{Stepwise Refinement}}$. [逐步求精]

### Placing Functions into Different Files

For example, one file `mainF.c` contains the `main()`. The `main` function calls `function1()` and `function2()`.

These two functions (definition) are in another file `support.c`. There are two constants defined by `#define` and used by the program, `CONST1` and `CONST2`.

The $\textcolor{green}{\text{constant definitions}}$ and the $\textcolor{green}{\text{function declarations}}$ are in a header file `def.h`.

> If we do not use a `header` file, then these lines in `header.h` have to both in the header of `mainF.c` and `support.c`.

### Compiling Program with Several Files

CS100 Lecture #5 20220228

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