# #C Programming (II) - 3
## Function
English: Lei KuoLiang
nicolaslouis@mail.fcu.edu.tw
Chinese(TW): Wang M.H
---
### This week's course catalog
0. Review
Function
* Function announcement
* parameters
* Return value
2. C standard library (C standard library)
* math.h
---
## Last week's course review
----
### 2D Array
Declare an int array a of size 3x4
And preset values
```c
int a[3][4] = {{1, 2, 3, 4},
{5, 6, 7, 8},
{9, 10, 11, 12}};
```
----
The size of the first dimension can also be omitted when declaring
The computer will automatically detect
```c
int a[][4] = {{1, 2, 3},
{5, 6, 7, 8},
{9, 10}};
//At this time, a is a 3x4 array.
```
But can't omit the size of the second dimension
```c
int a[][] = {{1, 2}, {3, 4}, {5, 6}}; //ERROR!!
int b[3][] = {{1, 2}, {3, 4}, {5, 6}}; //ERROR!!
```
----
Enter the values into the array in sequence, and then output them sequentially
```c
int a[5][6];
//Enter in order
for(int i = 0; i < 5; i++)
for(int j = 0; j < 6; j++)
scanf("%d", &a[i][j]);
//Sequential output
for(int i = 0; i < 5; i++) {
for(int j = 0; j < 6; j++)
printf("%d ", a[i][j]);
printf("\n");
}
```
----
### Two-dimensional character array
```c
//Declare
char str[3][10] = {"Jim", "Hello", "Brian"};
//Enter in order
for(int i = 0; i < 3; i++)
scanf("%s", str[i]);
//Sequential output
for(int i = 0; i < 3; i++)
printf("%s", str[i]);
```
----
### Bubble sorting
```c
int num[N];
//After entering the value
for(i = N - 1; i > 0; i--) {
for(j = 0; j < i; j++) {
if(num[j] > num[j + 1]){
//num[j]、num[j + 1]Numerical exchange
int tmp = num[j];
num[j] = num[j + 1];
num[j + 1] = tmp;
}
}
}
```
---
## Function
You can use it many times after writing it.
----
Using the function can solve the following problems:
1. The main program is too long
2. Many repetitive program blocks
3. Length is not conducive to debug
4. I want to change the name and think about it.
...
----
Give an example of a homework:
----
Write a subroutine **print_player**
----
After modification:
---
### Function Declaration
```c
Return value type function name (incoming parameter) {
//Calculation......
return Return value;
}
```
E.g：
```c
int add(int a, int b) {
int c = a + b;
return c;
}
```
----
In fact, the main program we wrote is a function.
```c
int main(void) {
//......
return 0;
}
```
Compared to the main program, other functions are called subprograms.
----
The subroutine needs to be declared in front of the main program.
```c
int add(int a, int b) {
int c = a + b;
return c;
}
int main(void) {
printf("%d\n", add(3, 4));
return 0;
}
```
----
You can also declare the function first, then define it later.
```c
int add(int, int);
//Can also add variable names int add (int a, int b);
int main(void) {
printf("%d\n", add(3, 4));
return 0;
}
int add(int a, int b) {
int c = a + b;
return c;
}
```
---
### Parameter
----
Parameters can be no, there can be only one, there can be many
----
Case without parameters
```c
void printHelloWorld() {
printf("Hello World\n");
}
```
Or
```c
void printHelloWorld(void) {
printf("Hello World\n");
}
```
----
There is only one parameter
```c
int isLeapYear(int year) {
//(is a multiple of 4 and not a multiple of 100) or (a multiple of 400) is a leap year
if ((!(year % 4) && (year % 100)) || !(year % 400))
return 1;
//Otherwise it is a normal year
return 0;
}
```
----
Multiple parameters
```c
double power(double val, int n) {
double result = 1;
if(n > 0)
for(int i = 0; i < n; i++)
result *= val;
else
for(int i = 0; i > n; i--)
result /= val;
return result;
}
```
----
The parameters can also be arrays, don't forget to tell the size of the function array
(or handle the boundaries of the data)
```c
void bubbleSort(int num[], int n) {
for(i = n - 1; i > 0; i--)
for(j = 0; j < i; j++)
if(num[j] > num[j + 1]){
//num[j]、num[j + 1]Numerical exchange
int tmp = num[j];
num[j] = num[j + 1];
num[j + 1] = tmp;
}
}
```
----
When the parameter is a multi-dimensional array, the size of the array after the second dimension is written (same as the declaration rule of the multi-dimensional array)
```c
void functionA(int data[][5]);
```
```c
void functionB(int data[][6][4]);
```
----
#### Call by value
If the argument is a numeric value (int, float, double, char...), then ++ does not affect the value of the original variable when doing a function operation.
```c
int add(int a, int b) {
a += b;//Will not affect the val1 in main
return a;
}
int main() {
int val1 = 5, val2 = 6, val3;
val3 = add(val1, val2);
printf("val1 = %d\n", val1);
printf("val3 = %d\n", val3);
}
```
Operation result
```
val1 = 5
val3 = 11
```
----
#### Call by reference
If the argument is an array, ++ will affect the value of the original variable when doing a function operation.
```c
void zeroing(int a[], int n) {
for(int i = 0; i < n; i++)
a[i] = 0;
}
int main() {
int arr[5] = {1, 2, 3, 4, 5};
zeroing(arr, 5);
for(int i = 0; i < 5; i++)
printf("%d ", arr[i]);
}
```
Operation result
```
0 0 0 0 0
```
---
### Return value
----
If the function has a return value (type is not void), add "`return passback value;`" at the end of the function or where you want to end.
```c
int isPrime(int num){
if (num == 1)
return 0; //End function, return 0
else
for (int i = 2; i * i <= num; i++) {
if (num % i == 0)
return 0; //End function, return 0
}
return 1; //End function, return 1
}
```
----
If the function has no return value (type void), add "`return;`" at the end of the function or where you want to end, or automatically return the braces at the end of the function.
```c
//Continue the code of the previous page
void printPrime(int max) {
if (max < 2)
return; //End function
for (int i = 2; i <= max; i++) {
if(isPrime(i))
printf("%d\n", i);
}
} //End function
```
----
Receive return value
```c
int square(int n){
return n * n;
}
int main(){
int a = square(2);
int b = square(a);
printf("%d\n", square(b));
if(square(b) == 16)
printf("4 * 4 = 16\n");
}
```
----
In fact, EOF is the return value of scanf.
```c
while(scanf("%d", %a) != EOF){
//....
}
```
The following is the virtual code of scanf
```c
int scanf(const char * Read format,...) {
Read the data of the stdin data stream;
if(Read the end of the material)
return EOF;
Store data in variables;
return Number of variables stored;
}
```
---
### Exercise one
Please complete a subroutine called **maximum**
Call maximum(a, b, c), return the largest number after judgment
(all use int variables)
```
Sample input：
3 1 2
7 9 9
5 -2 4
Sample output：
3
9
5
```
---
## C standard library
C standard library
----
++C Standard Library ++ is a collection of all standard-compliant header files in C programming, as well as commonly used library implementations (such as input and output and string control).
----
Common header
| Header | Description |
| -------- | -------- |
| <stdio.h> | Define input and output functions. |
| <stdlib.h> | Define numerical conversion functions, pseudo-random number generation functions, dynamic memory allocation functions... |
| <math.h> | Define the C language math function. |
| <time.h> | Defines the time and date of the acquisition, the time and date data manipulation functions, and formatting. |
----
Common header
| Header | Description |
| -------- | -------- |
| <ctype.h> | A character classification function is defined to test whether a character belongs to an alphabetic character, a control character, and so on. |
| <string.h> | Defines the C language string processing routine. |
| <limits.h> | Some limit values for integer types (such as INT_MIN, INT_MAX) are defined. |
| <float.h> | Defines some limit values for floating point types (such as FLT_MIN, FLT_MAX). |
----
Of course, there are many libraries~~
Interested to go to [here] (http://www.cplusplus.com/reference/clibrary/), or go to Google or wiki.
---
## math.h
a library of mathematical functions
Before use #`include <math.h>`
----
### Trigonometric function
|||
| -------- | -------- |
| double sin (double); | sine |
| double cos (double); | cosine |
| double tan (double); | Tangent |
| double asin (double); | arcsin (arcsin)|
| double acos (double); | inverse cosine (arccos) |
| double atan (double); | arctangent (arctan) |
----
### Index logarithm
|||
| -------- | -------- |
Double exp (double);|exponential function (exp(n) = *e*^n^)
Double sqrt (double);|open square root
Double log (double);|natural logarithm (log(n) = ln n = log~e~ n)
Double log10 (double);|base-10 logarithm (log10(n) = log~10~ n)
Double pow(double x, double y);|calculates the power of y with base x (pow(x, y)=x^y^)
----
### Take integer, absolute value
|||
| -------- | -------- |
Double ceil (double);| take the whole (unconditional entry)
Double floor (double);|take down the whole (unconditionally rounded)
Double round (double);|rounded up (after version C99)
Int abs(int);| Find the absolute value of the integer
Double fabs (double);|for absolute value
---
### Exercise 2
* We know that the formula for the proportional series is $\displaystyle\sum_{x=0}^{n-1}ar^x=\dfrac{a(1-r^n)}{1-r}$
* Please write a function **float geometricSeries (float a, float r, int n)**,
Return the value calculated by $a\times n$ when `r = 1.0`.
Otherwise return the value calculated by $\dfrac{a(1-r^n)}{1-r}$.
----
* Input has a lot of measurement, each line of measurement is one line, the measurement has 2 floating point numbers a, r, and 1 integer n.
* Call `geometricSeries(a, r, n)` in the main program and print out the return value.
```
Sample input：
2.0 1.0 10
2.0 3.0 5
1.5 0.5 6
Sample output：
20.000000
242.000000
2.953125
```
----
* HINT:
If the floating point number is equal to whether the two numbers are equal, do not use `==`,
Although there is nothing wrong with the grammar, because ++ may have a slight error relationship with ++, it is recommended to use "when the difference is less than a certain value, it is considered equal", for example:
```c
if (fabs(r - 1.0) < 0.000001) {
//...
}
```
---
### Homework
----
* The first week of homework
* This program does not add new features, simply modify the program structure
* Add at least the following function in the code ++ at least ++:
* EditNewChar()
* PrintCharData()
* SelectAndBattle()
* Battling()
* Battling() must be called by SelectAndBattle()
----
---
###### tags: `1082 Ai-Mod-Eng-LKL`
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