c 语言学习笔记01

1. c 语言1.1.1.helloworld文件名：test_pointer.c文件内容：#include <stdio.h>#include <malloc.h>int main(){printf("hello world\n\n");return 0;}编译加运行：[root@localhost c_study]# gcc test_pointer.c ;./a.outhello world1.1.2.获取键盘输入文件名：getKeyboardInptStr.c文件内容：#include <stdio.h>#include <malloc.h> int main(){char *a= malloc(sizeof(char *));char *b= malloc(sizeof(char *));scanf("%s",a);scanf("%s",b);printf("%s \t%s\n",a,b);free(a);free(b);return 0;}编译：gcc getKeyboardInptStr.c -o hello.exe运行[root@localhost tcp_ip]# ./hello.exe abcdddabc ddd[root@localhost tcp_ip]#1.1.2.1.从键盘输入构造数组文件名keyboard_input.c文件内容：#include <stdio.h>#include <malloc.h>int main(){char * strs[4];char *a;int i;for(i=0;i<4;i++){a= malloc(sizeof(char *));scanf("%s",a);strs[i]=a;//printf("%s \t\n",a);//free(a);}printf("=====================================\n");for(i=0;i<4;i++){printf("%s \t\n",strs[i]);}return 0;}1.1.3.读取文件读取文件有两个操作：打开流（fopen）和关闭流（fclose）#include <stdio.h>#include <stdlib.h>#include <malloc.h>int main(int ac,char**av){int exit_status=EXIT_SUCCESS;FILE *input;while( *++av!=NULL){input=fopen(*av,"r");if(input==NULL){perror(*av);exit_status=EXIT_FAILURE;continue;}if(fclose(input)!=0){perror("fclose");exit(EXIT_FAILURE);}}return exit_status;}1.1.3.1.打开文件流1.1.3.1.1.ungetc 用法文件名：ungetc_test.c文件内容：#include<stdio.h>#include<ctype.h>int main(){int value;int ch;value=0;while((ch=getchar())!=EOF && isdigit(ch)){value=value*10;value+=ch-'0';}//ungetc(ch,stdin);printf("%d\n",value);ch=getchar();printf("last char is %c\n",ch);return value;}若注释掉红色部分：[root@localhost read_file]# gcc ungetc_test.c;echo 234abc|./a.out 234last char is b否则：[root@localhost read_file]# gcc ungetc_test.c;echo 234abc|./a.out 234last char is a1.1.3.2.关闭流1.1.3.3.读取文本文件文件名称：read_file03.c文件内容：#include <stdio.h>#include <stdlib.h>#include <malloc.h>int main(){int value;int ch;char *input_file="ab.txt";FILE *input;input=fopen(input_file,"r");while((ch=fgetc(input))!=EOF){printf("%c",ch);}return 0;}1.2.指针1.2.1.取地址和取值取地址：&取值：*1.2.1.1.取地址int age=23;int *age_adrr=&age;1.2.1.2.取值参阅取地址。

printf("My age is \t%d\n",*age_adrr);1.2.1.3.范例01：文件名：test_pointer.c#include <stdio.h>#include <malloc.h>int main(){int age=23;int *age_adrr=&age;printf("My age is \t\t%d\n",age);printf("My age address is \t%d\n",age_adrr);printf("My age is \t\t%d\n",*age_adrr);//printf("hello world\n\n");return 0;}运行：[root@localhost c_study]# gcc test_pointer.c ;./a.outMy age is 23My age address is -1074593988My age is 23[root@localhost c_study]#1.2.2.左值和右值1.2.3.字符串操作1.2.3.1.split#include <string.h>#include <stdio.h>#include <malloc.h>char** StringSplit(char* string, char* split) {char** result;/*First assign a char * of memory, and then dynamicly allocate the rest of the memory*/ result = (char **) malloc(sizeof(char *));memset(result, 0, sizeof(char *));/*Define a pointer for ergodic and a pointer for searching for position*/char* p = string;char* pos = string;/*No matter whether there is the division string, it will be split into a string definitely*/ int count = 0;while (*p != '\0') {char* temp;char* tt;/*Find the string*/pos = strstr(p, split);/*result is zero indicate that there is not this character in the rest of string*/if (pos == 0) {result = (char **) realloc(result, sizeof(char *) * (count + 2));//result[0] = count;result[count] = p;result[count + 1] = NULL;return result;}temp = (char *) malloc(sizeof(char) * (pos - p + 1));memset(temp, 0, sizeof(char) * (pos - p + 1));/*set head pointer ,in order to use for assignment */tt = temp;while (p <= pos) {*temp++ = *p++;}*--temp = '\0';result = (char * *) realloc(result, sizeof(char *) * (count + 1));//result[0] = count;result[count] = tt;count++;/*Set pointer for the next ergodic duration (important). When split length is more than 1, don't set will be more assignment unnecessary string*/p += strlen(split) - 1;}return result;}int main() {char **nameArr = StringSplit("set work_mem to 2048;", " ");// char *name = StringSplit("set work_mem to 2048;", " ")[0];printf("%s----\n", nameArr[0]);}运行结果：[root@localhost split2]# gcc split_test02.c ;./a.outset----1.2.3.2.求char** 的长度1.3.链表文件名称：struct_test.c文件内容：#include <stdio.h>#include <malloc.h>typedef struct {int age;char *name;} Student;int main(){Student *student=(Student *)malloc(sizeof(Student));student->age=3;char *name_tmp="huangwei";student->name=name_tmp;printf("My age is %d\n",student->age);printf("My name is %s\n",student->name);return 0;}运行：[root@localhost c_study]# gcc struct_test.c ;./a.out;rm -f a.out My age is 3My name is huangwei1.3.1.1.链表定义的两种种方式方式一：struct Node{int age;char *name;struct Node *next;} Student;方式二：typedef struct Node{int age;char* name;struct Node* next;} Student;文件名：struct_test03内容：#include <stdio.h>#include <stdlib.h>#include <malloc.h>struct Node{int age;char *name;struct Node *next;} Student;int main(){struct Node *student1=(struct Node *)malloc(sizeof(Student));struct Node *student2=(struct Node *)malloc(sizeof(Student));student2->name="jian";student2->age=22;student1->age=3;char *name_tmp="huangwei";student1->name=name_tmp;student1->next=student2;printf("%s age is %d\n",student1->name,student1->age);printf("%s is %d\n",student1->next->name,student1->next->age);return 0;}运行：[root@localhost c_study]# gcc struct_test03.c ;./a.outhuangwei age is 3jian is 221.3.1.2.求链表的长度int length_p(Student* p){if (p==NULL){return 0;}int leng_l=0;while(p->next!=NULL){leng_l++;p=p->next;}if(p!=NULL){leng_l++;}return leng_l;}1.4.预处理语句预处理命令#include<stdio.h>#define PI 3.14int main(){printf("PI is %f\n",PI);return 0;}运行结果：[root@localhost define3]# gcc define_test01.c ;./a.out PI is 3.140000#include<stdio.h>#define PI 3.14#define SUM(x,y) x+y#define CHENG(X,Y) (X)*(Y)int main(){printf("PI is %f\n",PI);printf("The sum is %d\n",SUM(3,4));printf("The cheng is %d\n",CHENG(4,2));return 0;}运行结果：[root@localhost define3]# gcc define_test01.c ;./a.out PI is 3.140000The sum is 7The cheng is 8#include<stdio.h>#define NAME "whuang"int main(){#undef NAMEprintf("My name is %s\n",NAME);return 0;}运行结果：会报错：[root@localhost define3]# gcc define_test02.c ;./a.outdefine_test02.c: In function ‘main’:define_test02.c:6: 错误：‘NAME’未声明(在此函数内第一次使用) 修改为：#include<stdio.h>#define NAME "whuang"int main(){printf("My name is %s\n",NAME);#undef NAMEreturn 0;}就OK 了1.4.1.ifdef#include<stdio.h>#define NAME "whuang"int main(){printf("My name is %s\n",NAME);#ifdef NAMEprintf("def NAME\n");#endifreturn 0;}运行结果：[root@localhost define3]# gcc define_test03.c ;./a.out My name is whuangdef NAME1.4.2.else#include<stdio.h>#define NAME "whuang"int main(){printf("My name is %s\n",NAME);#undef NAME#ifdef NAMEprintf("def NAME\n");#elseprintf("NOT def NAME\n");#endifreturn 0;}运行结果：[root@localhost define3]# gcc define_test03.c ;./a.out My name is whuangNOT def NAME1.4.3.ifndef#include<stdio.h>#define NAME "whuang"int main(){printf("My name is %s\n",NAME);#undef NAME#ifndef NAMEprintf("NOT def NAME\n");#elseprintf("def NAME\n");#endifreturn 0;}运行结果：[root@localhost define3]# gcc define_test04_ifndef.c ;./a.out My name is whuangNOT def NAME1.5.经典摘录1，读取文件时，返回的为什么是int，而不是char？2，xx1.6.常见错误1.6.1.在C99 模式之外使用‘for’循环初始化声明报错信息如下：[root@localhost read_file]# gcc keyboard_input.ckeyboard_input.c: In function ‘main’:keyboard_input.c:7: 错误：在C99 模式之外使用‘for’循环初始化声明1.7.内存对齐文件名：memery_test01.c文件内容：#include <stdio.h>struct Test1 {char c1;char c2;short s;int i;};struct Test2 {char c1;short s;char c2;int i;};int main(){struct Test1 a;struct Test2 b;printf("%d\n", sizeof(a));printf("%d\n", sizeof(b));return 0;}运行结果：[root@localhost memery3]# gcc memery_test01.c ;./a.out812说明：两个结构体，其中是相同的数据类型，只不过顺序不同，但是占用的空间却不同。