# C language 解題

「你所不知道的 C 語言」系列講座
https://hackmd.io/@sysprog/c-prog/%2F%40sysprog%2Fc-programming

> Test Case

#include <stdio.h>

int main()
{
    char str[]="Hello";
    char *p=str;
    int *x;
    double *y;
    short *z;
    long *a;
    int n=10;
    char b;
    short c;
    int d;
    long e;
    float f;
    double g;
    //printf("%ld\n", sizeof(str));
    printf("%ld\n", sizeof(p));
    printf("%ld\n", sizeof(n));
    printf("%ld\n", sizeof(x));
    printf("%ld\n", sizeof(y));
    printf("%ld\n", sizeof(z));
    printf("%ld\n", sizeof(a));
    printf("==============\n");
    printf("%ld\n", sizeof(b));
    printf("%ld\n", sizeof(c));
    printf("%ld\n", sizeof(d));
    printf("%ld\n", sizeof(e));
    printf("%ld\n", sizeof(f));
    printf("%ld\n", sizeof(g));
    return 0;
}

###

> Swap two variables without using a temporary variable
> XOR

#include <stdio.h>

int main()
{
    int a = 123;
    int b = 456;
    
    a = a ^ b;
    b = a ^ b;
    a = a ^ b;
    
    printf("%d,%d\n", a , b);

    return 0;
}

> Sum the Integers from 1 to N
> Pointer and formula

#include <stdio.h>

int main()
{
    int n = 123;
    sum(&n);
    printf("%d", n);

    return 0;
}

void sum(int *n){
    *n = (*n * (*n+1))/2;
}

> How to find the missing integer in an array
> XOR

#include <stdio.h>

int find(int *array, int size){
    int missingint=0;
    for (int n=0; n < size; n++){
        missingint ^= array[n];
        
        }
    return missingint;
}

int main()
{
    int num[] = {1,1,2,2,3,3,4};
    int size = (sizeof(num) / sizeof(num[0]));
    int missingint = find(num, size);
    
    printf("%d", missingint);

    return 0;
}

> Test Case.

#include <stdio.h>

int main()
{
    int a[]={6,7,8,9,10};
    int *p=a; // p == &a[0] 
    *(p++)+=123; // 先拿出P[0] 之後p[0]++ 指標所指位置++ 變成p[1]
    *(++p)+=123; // 先p[1]++ 變成p[2]位置 之後拿出 p[2]
    printf("%d", a[2]);
    
    
    return 0;
}

Get bit

#include <stdio.h>

char get_bit7(char in)
{
    if (in & (1<<7)) return 1;
    else return 0;
    //return (in & (1<<7) ? 1 : 0);
}

char set_bit7(char in)
{
    return (in | (1<<7));
}

char clear_bit(char in, int bit)
{
    return (in & (~(1<<bit)));
}

int main()
{
    int a = 255;
    int b = get_bit7(a);
    printf("%d\n", b);
    return 0;
}

> 1. Please write a subroutine to print out one character’s ASCII code in 十進位 and 十六進位. For example, a subroutine printA(‘a’) will print out 97, 0x61.

#include <stdio.h>
#include <stdlib.h>

int main(void)
{
	char string[1024];
    int i;
	printf("ASCII Input: ");
	gets(string);
    printf(string);
	printf("\n");
	printf("Decimal Ouput:");
	for(i = 0; i < strlen(string); i++)
	{
		printf("%d ", string[i]); //%d TX Decimal
	}
    printf("\n");
	printf("Hexadecimal Output: ");
	for(i = 0; i < strlen(string); i++)
	{
		printf("0x%x ", string[i]); //%x TX Hexadecimal
	}
}

%c：以字元方式輸出
%d：10 進位整數輸出
%o：以 8 進位整數方式輸出
%u：無號整數輸出
%x、%X：將整數以 16 進位方式輸出
%f：浮點數輸出
%e、%E：使用科學記號顯示浮點數
%g、%G：浮點數輸出，取 %f 或 %e（%f 或 %E），看哪個表示精簡
%%：顯示 %
%s：字串輸出
%lu：long unsigned 型態的整數
%p：指標型態

> 2. Please write a subroutine for sorting 3 input values to (max, middle, min). For example, a subroutine my_sort(12, 10, 20) then print out (20, 12, 10).

#include<stdio.h>


void bubble_sort(int num[], int size)
{
    for(int i=size-1 ; i > 0 ; i--)
    {
        int sp = 1;     
        for(int j = 0 ; j < i ; j++)
            if (num[j] > num[j+1]) //if True and then transfer values.
            {
                int temp = num[j]; 
                num[j] = num[j+1];
                num[j+1] = temp;
                
                sp = 0;
            }
            if(sp==1) {break;} //if swap == 1 and then break.
    }
}

void print_array(int num[], int size)
{
    for (int i = size - 1 ; i >= 0 ; i--)
    {
        printf("%d ", num[i]);
    }
}


int main(void) 
{
    int num[] = {12, 10, 20, 8, 7, 100};
     
    printf("Before:\n");    
    print_array(num, (int)(sizeof(num) / sizeof(num[0]))); //print values of array.
    printf("\n");
    
    bubble_sort(num, (int)(sizeof(num) / sizeof(num[0]))); // call bubble sorting.
    
    printf("After:\n");
    print_array(num, (int)(sizeof(num) / sizeof(num[0])));
    
    return 0 ;
}

> 3. Please write a subroutine to concatenate two strings (i.e. write a function that implements strcat() ) to one string . And also write how to call this function. For example my_strcat(“abc”, “def”) will print out abcdef .

void my_strcat(char str1[], char str2[], int size)
{
    char concated[size];
    memset(concated,"\0", size); //memset set memory to specific value.
    
    strcat(concated, str1);
    strcat(concated, str2);
    
    printf("%s\n", concated);
}

int main(void) {
    char str1[] = "abc";
    char str2[] = "def";     
    
    int size = strlen(str1) + strlen(str2) + 1; //get string size and then plus 1 without overflow.
    
    my_strcat(str1, str2, size); //call my_strcat.

    return 0;
}

> 4. You could refer the following function to answer the question of 4-1 and 4-2:
> int rand(void);
> the Return value will be randomly generated between 0 to 65535.
> 4-1 Please write a program to randomly generate 6 values within 1 ~ 10000.
> 4-2 Please write a program to randomly generate 6 different values within 1 ~ 100.
> Note: Any of the 6 values must be different to other 5 values.

#include <stdio.h>
#include <stdlib.h>
#include <time.h>

int myrand(int values)
{
    srand(time(NULL));
    int randvalues = (rand()%(values-0+1))+0;
    printf("The 0 ~ 65535 Random Number is %d .\n", randvalues);
    
    return 0;
}

int rand_num(int values, int num)
{
    srand(time(NULL));
    int randvalues;

    for (int i = 0 ; i < num  ; i++)
    {
           randvalues = (rand()%(values-1+1))+1;
           printf("The 1 ~ 100000 Random Number is %d .\n", randvalues);
    }
    return 0;
}

int rand_diffnum_card(int values, int num)
{
    srand(time(NULL)); // The add getpid() can raise the random.
    
    int randvalues;
    int i ;
    int arraynum[values + 1];
    
    for (i = 1 ; i < values ; i++) { arraynum[i]=i; }
    
    for (i = 0 ; i < values; i++)
    {
            randvalues = (rand()%((values-1)+1))+1;
            int temp =  arraynum[i];
            arraynum[i] = randvalues;           
            arraynum[randvalues] = temp;
            //printf("The 1 ~ 100 Different Random Number is %d .\n", arraynum[i]);
    }
       
    for (i = 0 ; i < num  ; i++) { 
        printf("The 1 ~ 100 Different Random Number is %d .\n", arraynum[i]); 
    }
        
    
    return 0;
}

int main(void) 
{
    #define diffnumrang 100
    #define gernum 6
    #define setrand  65535 //0~65535
    #define setrang  10000 // 1~100000
    
    myrand(setrand);
    printf("-------------- \n");
    rand_num(setrang, gernum);
    printf("-------------- \n");
    rand_diffnum_card(diffnumrang, gernum);
    
    return 0;
}

> 5. Write a function that reverses a string. The input string is given as an array of characters char[].
> Example 1: Input: ["h","e","l","l","o"] Output: ["o","l","l","e","h"]
> Example 2: Input: ["H","a","n","n","a","h"] Output: ["h","a","n","n","a","H"]
>
> void reverseString(char* s){
> // Please write your answer here Note : char* s => input string
>
> return reverses_string;
> }

#include <stdio.h>
#include <string.h>

int main()
{
    char str[50];
    gets(str);
    reverseString(str);   
    printf("Reverse a String : %s", str); 
    return 0;
}
int reverseString(char *str){
    int i,j;
    char temp;
    j = strlen(str)-1;
    printf("Entered a string and size: %s , %d \n", str , strlen(str));
	for(i = 0 ; i<j ; i++)
    {
        temp = str[i];
        str[i] = str[j];
        str[j] = temp;
        j--;
    }

    return str;
}