# String and Sorting
###### tags: `EMBEDDED_C`
- [x] String and Pointer
- [x] String Copy
- [x] String Length
- [ ] <font style="color:red"> **String Reverse** </font>
- [ ] isPalidome
- [ ] StrStr
- [ ] Strdup vs strcpy
- [ ] 以一個單字為單位 反轉一個輸入字串(不是每個字元反轉過來)
- [ ] 給定一個prefix(第一行輸入)，輸出不包含此prefix的string
- [ ] Quick Sort
- [ ] Binary Search
- [ ] Bubble Sort
## 待整理題目
#### 2. 以一個單字為單位 反轉一個輸入字串(不是每個字元反轉過來)
(如:I am a boy 變成 boy a am I)
#### 給定一個prefix(第一行輸入)，輸出不包含此prefix的string
input:
```
aaaa
aaaabbbbccc
badssssd
mdjeld
bbaaaakkdkd
```
output:
```
badssssd
mdjeld
bbaaaakkdkd
```
## String abd Pointer
```clike=
int myStrLength(const char *p_mystr) {
int count = 0;
while( *p_mystr != '\0'){
count ++;
p_mystr++;
}
return count;
}
int main(void){
char *test = "Suck My Dick";
char strArry[36] = "Suck My Dick!";
printf("The length of string: %s is %d\n",\
&strArry[0], myStrLength(strArry));
return 0;
}
```
## String Copy
寫出一個字串拷貝程式：
void StrCpy(char* dst , char* src) ;
* 如果想要優化？ ??? **NEON** ???
```cpp=
// array version
void strcpy(char *dst, char *src){
int i = 0;
while((src[i] = dst[i]) != '\0')
i++;
}
// pointer version
void strcpy(const char *s, char *t)
while((*s = *t) != '\0') {
s++;
t++;
}
}
```
## String Length
Implement the funcytion `int strlen( const char *str )`
```cpp=
int strlen( const char *str ){
int len;
while( (*str++) != '\0' ){
len++;
}
//while((*str++))
return len;
}
```
* My version
```cpp=
int str_len(const char* str){
int len;
while(*str != '\0'){
len++;
str++;
}
return len;
}
```
## Swap implementation comparsion
- Environment: Macbook Air M1
- Command: clang -g -O3 -c ./swap_compare.c -o a.out && /opt/homebrew/opt/binutils/bin/gobjdump -dSl ./a.out | less
- Integer
- Use temp
- C code
```c=
void swap(int * a, int * b)
{
int tmp = *a;
*a = *b;
*b = tmp;
}
```
- Dissaembly
```asm=
swap_int():
./swap_compare.c:4
void swap_int(int * a, int * b)
{
int tmp = *a;
0: b9400008 ldr w8, [x0]
./swap_compare.c:5
*a = *b;
4: b9400029 ldr w9, [x1]
8: b9000009 str w9, [x0]
./swap_compare.c:6
*b = tmp;
c: b9000028 str w8, [x1]
./swap_compare.c:7
}
10: d65f03c0 ret
```
- Use XOR
- C code
```c=
void swap_int_xor(int *a, int *b)
{
*a ^= *b;
*b ^= *a;
*a ^= *b;
}
```
- Disassembly
```asm=
0000000000000014 <_swap_int_xor>:
swap_int_xor():
./swap_compare.c:11
void swap_int_xor(int *a, int *b)
{
*a ^= *b;
14: b9400028 ldr w8, [x1]
18: b9400009 ldr w9, [x0]
1c: 4a080128 eor w8, w9, w8
20: b9000008 str w8, [x0]
./swap_compare.c:12
*b ^= *a;
24: b9400029 ldr w9, [x1]
28: 4a080128 eor w8, w9, w8
2c: b9000028 str w8, [x1]
./swap_compare.c:13
*a ^= *b;
30: b9400009 ldr w9, [x0]
34: 4a080128 eor w8, w9, w8
38: b9000008 str w8, [x0]
./swap_compare.c:14
}
3c: d65f03c0 ret
```
- char
- Use temp
- C code
```c=
void swap(char **a, char **b)
{
char ** tmp = *a;
**a = **b;
**b = **tmp;
}
```
- Disassembly
```asm=
0000000000000014 <_swap_int_xor>:
swap_int_xor():
./swap_compare.c:11
void swap_int_xor(int *a, int *b)
{
*a ^= *b;
14: b9400028 ldr w8, [x1]
18: b9400009 ldr w9, [x0]
1c: 4a080128 eor w8, w9, w8
20: b9000008 str w8, [x0]
./swap_compare.c:12
*b ^= *a;
24: b9400029 ldr w9, [x1]
28: 4a080128 eor w8, w9, w8
2c: b9000028 str w8, [x1]
./swap_compare.c:13
*a ^= *b;
30: b9400009 ldr w9, [x0]
34: 4a080128 eor w8, w9, w8
38: b9000008 str w8, [x0]
./swap_compare.c:14
}
3c: d65f03c0 ret
```
- Use XOR
- C code
```c=
void swap_char_xor(char **a, char **b)
{
**a ^= **b;
**b ^= **a;
**a ^= **b;
}
```
- Disassembly
```asm=
0000000000000060 <_swap_char_xor>:
swap_char_xor():
./swap_compare.c:25
void swap_char_xor(char **a, char **b)
{
**a ^= **b;
60: f9400028 ldr x8, [x1]
64: 39400108 ldrb w8, [x8]
68: f9400009 ldr x9, [x0]
6c: 3940012a ldrb w10, [x9]
70: 4a080148 eor w8, w10, w8
74: 39000128 strb w8, [x9]
./swap_compare.c:26
**b ^= **a;
78: f9400008 ldr x8, [x0]
7c: 39400108 ldrb w8, [x8]
80: f9400029 ldr x9, [x1]
84: 3940012a ldrb w10, [x9]
88: 4a080148 eor w8, w10, w8
8c: 39000128 strb w8, [x9]
./swap_compare.c:27
**a ^= **b;
90: f9400028 ldr x8, [x1]
94: 39400108 ldrb w8, [x8]
98: f9400009 ldr x9, [x0]
9c: 3940012a ldrb w10, [x9]
a0: 4a080148 eor w8, w10, w8
a4: 39000128 strb w8, [x9]
./swap_compare.c:28
}
a8: d65f03c0 ret
```
## isPalidome
## Reverse String
```cpp=
void reverseString(char *p_str){
char *str_end = pstr + str_len(p_str) -1;
char tmp = 0;
while(str_end >> p_str){
tmp = str;
}
}
```
* 巧虎
```cpp=
#include <stdio.h>
static void swap(char **a, char **b)
{
**a ^= **b;
**b ^= **a;
**a ^= **b;
}
static void reverse(char *s)
{
char *p = s;
// this time, p point to \0
while(p && *++p != '\0'){}
for(p--; s < p; p--, s++)
swap(&p, &s);
}
int main(int argc, char **argv)
{
if(argc < 2){
printf("%s [string]\n", argv[0]);
return -1;
}
printf("Before : %s\n", argv[1]);
reverse(argv[1]);
printf("After reverse: %s\n", argv[1]);
return 0;
}
```
### c語言遞迴反向輸出標準字符串[AMD]
## `strcpy` vs `strdup`
## `strdup`
## `strstr`
```cpp=
if (NULL != strstr())
```
```cpp=
static void string_cpy(const char *src_p, char *dst_p){
while(*src_p != '\0'){
*dst_p = *src_p;
src_p++;
dst_p++;
}
}
static int string_length(const char *strlen_p){
int count = 0;
while(*strlen_p != '\0'){
count++;
strlen_p++;
}
}
static void string_reverse(char *reverse_p){
int count = string_length(reverse_p) -1;
char *start_p = reverse_p;
char *end_p = reverse_p;
char ch_tmp;
end_p = reverse_p + count;
while(end_p > start_p){
ch_tmp = *start_p;
*start_p = *end_p;
*end_p = ch_tmp;
end_p++;
reverse_p++;
}
}
```
## Other
1. Waht'e the result?
```cpp=
char *ptr = "Supermicro USA"
*ptr ++;
printf("%s\n", ptr);
ptr++;
printf("%s\n", ptr);
```
## Review
```cpp=
int str_len(const char *src) {
int count = 0;
while(*src != '\0') {
src++;
count++;
}
return count;
}
void str_cpy(const char *src, char *dst) {
while(*src != '\0') {
*dst = *src;
src++;
dst++;
}
}
bool isPalidome(const char *src) {
char *forward = src;
char *backward = src;
int len = (str_len(src) - 1) /2;
forward += len;
backward += len;
while(*forward != '\0') {
if(*forward != *backward) {
return false;
}
forward++;
backward--;
}
return true;
}
int main(void) {
char a[32] = "Suck My Dick!";
char c[32] = "ABCDCBA";
char d[32] = "ABCDDCBA";
printf("%s's length is %d\n", c, str_len(c));
if( isPalidome(c) == true)
printf("%s is Palidome\n", c);
else
printf("%s is not Palidome\n", c);
}
```
###
```c=
void transform(int *ptr) {
int arry[8] = {0};
for(int i = 7; i >= 0; i--) {
arry[i] = (*ptr) % 10;
*ptr = (*ptr) / 10;
}
*ptr = 0;
for(int i = 0; i < 8; i++) {
printf("arry[%d]:%d\n", i, arry[i]);
}
for(int i = 4; i < 8; i++) {
*ptr = (*ptr) * 10;
*ptr = *ptr + arry[i];
}
for(int i = 0; i < 4; i++) {
*ptr = (*ptr) * 10;
*ptr = *ptr + arry[i];
}
}
```
## Quick Sort
## Bubble Sort
## Binary Search
## Reference
* [[請益] 發哥上機考準備 DCARD](https://www.dcard.tw/f/tech_job/p/236515024)
*
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