CS 筆記 (pure C)
===
**[CS 筆記 (C++)](/odFJ4Kz0Re6oA1HLWpAbjw)**
**[STL (C++)](/dEohSyu8Q4OOwTonjp2lkQ)**
**[Data Structure 資料結構](/LzdZ7TChRu2a0ZtHzIRoyA)**
**[String 字串](/LVjWmeW0RtCVXyyG6hnxag)**
**[Sorting 演算法](/X5mITXctTRWcIndQeFoFEw)**
**[Graph](/bnlpuDcjSTmHyQutAbWgyg)**
**[Struct](/2rOZs4ewRdyWszr9cXbytw)**
**[變數宣告](/zlht-GAQQ9WbO9r_Ca3ZcA)**
**[檔案處理](/O1rrjJa1RPSQ2TyIj5MQig)**
**[其他](/Fjz_cSjaR4SRtqHVdPLzaQ)**
## 指標
用`int*` 宣告`int`的指標
```c=
int main(void)
{
int x=1;
int* z=&x;
printf("address of x : %p\n",z);
printf("address of x : %p\n",&x);
printf("x = %d\n",x);
printf("x = %d\n",*z);
}
```
Result
```
address of x : 0x7fffffffdc4c
address of x : 0x7fffffffdc4c
x = 1
x = 1
```
### swap
#### 方法一
```c
// ^ xor
a = a^b;
b = a^b;
a = a^b;
```
`a` `b` 交換
#### 方法二
```c=
#include <stdio.h>
int swap(int* a,int* b);
int main(void)
{
int x,y;
printf("x = ");
scanf("%d",&x);
printf("y = ");
scanf("%d",&y);
printf("x = %d ; y = %d\n",x,y);
swap(&x,&y);
printf("x = %d ; y = %d\n",x,y);
}
int swap(int* a,int* b)
{
int tmp = *a;
*a = *b;
*b = tmp;
}
```
指標string
value` s[i]` is equivalent to `*(s+i)`
addrs`&s[i]` is equivalent to ` (s+i)`
```c=
#include <stdio.h>
#include <string.h> //strlen
int main(void)
{
//char *s = (char*)"HI!";
char s[] = "HI!";
printf("%s\n",s); //HI!
//ver1
printf("%c",*s);
printf("%c",*(s+1));
printf("%c",*(s+2));
printf("\n");
//ver2
printf("%c",s[0]);
printf("%c",s[1]);
printf("%c",s[2]);
printf("\n");
//ver3
for(int i=0 , n = strlen(s); i<n ; i++)
{
printf("%c",s[i]);
}
printf("\n");
}
```
```c=
#include <stdio.h>
int main(void)
{
char* str[]={"piece","of","cake"};
int i,j;
for(i=0;i<3;i++)
{
printf("%s",str[i]);
printf(" ");
}
printf("\n");
}
```
scanf
```c=
#include <stdio.h>
#include <stdlib.h>
int main(void)
{
//int
int x;
printf("int x = ");
scanf("%d", &x);
printf("x = %d\n", x);
//char
char c;
printf("char c = ");
scanf(" %c", &c);
printf("c = %c\n", c);
//string
//scanf 不需&
char s[8];
printf("string s = ");
scanf(" %7s", s);
printf("s = %s", s);
}
```
## malloc
透過副程式 malloc
```c=
#include <stdio.h>
#include <stdlib.h>
void malloc_float2(float** p,unsigned int sz);
int main(void)
{
float* ptr = NULL;
int i, n = 100;
malloc_float2(&ptr,n);
for(i=0;i<n;i++)
{
ptr[i] = (float) rand()/RAND_MAX;
printf("%d : %f\n",i,ptr[i]);
}
free(ptr);
}
void malloc_float2(float** p,unsigned int sz)
{
//p=&ptr ptr=*p
*p = (float*)malloc(sz*sizeof(float));
}
```
:::warning
use `valgrind` to check memory leak
:::
## 動態記憶體配置(Dynamical Memory Allocation)
### 一維陣列
:::info
```c
double* ptd;
ptd = (double*) malloc(array_size * sizeof(double) );
```
:::
```c=
#include <stdio.h>
#include <stdlib.h>
int main(void)
{
double* ptd;
int array_size,i;
printf("# of doubles you want?\n");
scanf("%d",&array_size);
ptd = (double*) malloc(array_size * sizeof(double) );
if(ptd == NULL)
{
printf("memory allocation failed\n");
exit(EXIT_FAILURE);
}
for(i=0;i<array_size;i++)
{
ptd[i] = (double) rand()/RAND_MAX; //0~1之間
if(ptd[i]>0.5)
{
printf("%d : %f\n",i,ptd[i]);
}
}
free(ptd);
return 0;
}
```
#### 透過副程式 malloc
### 二維陣列
```c=
#include <stdio.h>
#include <stdlib.h>
int main(void)
{
int row,col;
int i,j;
printf("row col = \n");
scanf("%d %d",&row,&col);
int** array_2D = (int**) malloc(col*sizeof(int*));
for (i = 0; i < row; i++)
{
array_2D[i] = (int*) malloc(row*sizeof(int));
}
//assign
for(i = 0; i < row*col ;i++)
{
array_2D[i/col][i%col] = i;
}
for (i = 0; i < row; i++)
{
for (j = 0; j < col; j++)
{
printf("%d ",array_2D[i][j]); //輸出元素的地址
}
printf("\n");
}
//free
for (int i = 0; i < col; i++) //記得釋放空間
{
free(array_2D[i]);
}
free(array_2D);
return 0;
}
```
### dangling pointer
副程式 回傳指標時
:::danger
```c=
#include <stdio.h>
int* f(void)
{
int a = 3;
return &a;
}
```
```c=
#include <stdio.h>
int* f(void)
{
//int a = 3;
int a[]={1,2,4};
return a;
}
```
:::
:::success
```c=
#include <stdio.h>
int* f(void)
{
int* a;
a = (int*)malloc(10*sizeof(int));
return a;
}
```
:::
## getchar
```c=
#include <stdio.h>
int main(void)
{
char c;
printf("char:\n");
while ((c = getchar())!='\n')
{
//putchar(c);
printf("%c",c);
}
//putchar('\n');
printf("\n");
}
```
### scan any string (<100) and printf
```c=
#include <stdio.h>
int main(void)
{
int i=0;
char c[100];
printf("char:\n");
while ((c[i] = getchar())!='\n')
{
i++;
}
c[i+1] = '\0';
printf("%s\n",c);
}
```
only scan first char
```c=
#include <stdio.h>
int main(void)
{
int i = 0;
while(getchar() != 'y' )
{
i++;
printf("n = %d ?",i);
while(getchar() != '\n');
}
printf("\n");
}
```
## fgets
## freopen fopen
freopen
```c
//讀檔案 到 input
freopen("file_name.txt","r",stdin);
//寫檔案 到 output
freopen("file_name.txt","w",stdout);
```
fopen
```c
FILE *fin; //typedef FILE
fin = fopen("file_name.txt","r");
fscanf(fin,"%d",&in_data);
```
```clike=
#include <stdio.h>
int data[100000];
int main(void)
{
int i,j,tmp,num_data;
FILE *fin,*fout;
fin = fopen("unsorted.txt","r");
fscanf(fin,"%d",&num_data);
for(i=0;i<num_data;i++)
{
fscanf(fin,"%d",&data[i]);
}
//sorting data[i]
for(i=0;i<num_data-1;i++)
{
for(j=0;j<num_data-j-1;j++)
{
if(data[j]>data[j+1])
{
tmp = data[j];
data[j] = data[j+1];
data[j+1] = tmp;
}
}
}
fout = fopen("sorted.txt","w");
fprintf(fout,"%d\n\n",num_data);
for(i=0;i<num_data;i++)
{
fprintf(fout,"%d\n",data[i]);
}
}
```
Primer Number
===
Sieve of Eratosthenes [(wiki link)](https://en.wikipedia.org/wiki/Sieve_of_Eratosthenes)
```c=
algorithm Sieve of Eratosthenes is
input: an integer n > 1.
output: all prime numbers from 2 through n.
let A be an array of Boolean values
initially all set to true.
for i = 2, 3, 4, ..., sqrt(n) do
if A[i] is true
for j = i^2, i^2+i, i^2+2i, i^2+3i, ..., n do
set A[j] := false
return all i such that A[i] is true.
```
```c=
#include <stdio.h>
#define N 1000
int A[N+1]; //N is too large
int main(void)
{
int i,j;
for(i=0;i<=N;i++) A[i]=1;
for(i=2;i*i<=N;i++)
{
if(A[i])
{
for(j=i*i;j<=N;j=j+i)
{
A[j]=0;
}
}
}
for(i=2;i<=N;i++)
{
if(A[i]) printf("%d, ",i);
}
printf("\n");
}
```
## gcd
```c=
#include <stdio.h>
#include <string.h>
int gcd(int a,int b);
int main(void)
{
int a,b;
while(1)
{
printf("a = ");
scanf("%d",&a);
printf("b = ");
scanf("%d",&b);
printf("gcd(a,b) = %d\n",gcd(a,b));
}
}
int gcd(int a,int b)
{
if(b>a)
return gcd(b,a);
else if(a%b==0)
return b;
else
return gcd(b,a%b);
}
```
## 四則運算
PreFix 前序式 -> InFix 中序式
```c=
#include <stdio.h>
#include <ctype.h>
int calcul(void);
int main(void)
{
while (1)
{
printf("Enter Prefix : (space between char) \n");
printf(" = %d\n",calcul());
}
}
int calcul(void)
{
int c;
int ans;
int op1,op2;
c = getchar();
if(isspace(c))
{
ans = calcul();
}
else if(c == '+')
{
printf("(");
op1 = calcul();
printf("+");
op2 = calcul();
printf(")");
ans = op1 + op2;
}
else if(c == '-')
{
printf("(");
op1 = calcul();
printf("-");
op2 = calcul();
printf(")");
ans = op1 - op2;
}
else if(c == '*')
{
printf("(");
op1 = calcul();
printf("*");
op2 = calcul();
printf(")");
ans = op1 * op2;
}
else if(isdigit(c))
{
ungetc(c,stdin);
scanf("%d",&ans);
printf("%d",ans);
}
return ans;
}
```
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