C
Overview
Hello World
#include <stdio.h>
int main() {
printf("Hello, World!\n");
return 0;
}
// Output: Hello, World!
Variable - Constant
// variable
int num = 20;
// constant
const double PI = 3.14;
// multi-var
int x = 3, y = 5;
// assign value
y = x += 2;
Data Type
// integer 【修饰：short, long, unsigned(正数), signed(正负数均可)】
int num = 20;
// 浮点数：float、double、long double
double PI = 3.14;
float price = 123.672;
// char
char c = 'z';
// string
char name[] = "Tom Smith";
char text[100];
强制转换
double price = 123.67;
printf("%d \n", (int)price); // 123
内存占用大小
// sizeof()
printf("size (short): %d \n", sizeof(short)); // 2
printf("size (int): %d \n", sizeof(int)); // 4
printf("size (long): %d \n", sizeof(long)); // 8
printf("size (float): %d \n", sizeof(float)); // 4
printf("size(double): %d \n", sizeof(double)); // 8
printf("size (char): %d \n", sizeof(char)); // 1
printf("size(string): %d \n", sizeof(char[10])); // 10
Output - Input
input
getchar() // char
gets() // string
fgets()
scanf() // important
example
char s[100];
fget(s, 100, stdin);
int num;
float price;
char name[100];
scanf("%d %f %c %[^\n]s", num, price, c, name);
output
printf()
putchar() // append a char
puts() // append a string
fput()
example
printf("int: %d, float: %f, double: %.2f, character: %c, string: %s", num, price, salary, letter, name);
char s[] = "hello";
putchar('-');
puts("world");
// output: hello-world
fput(s, stdout);
Comments
// This is a single line comments.
/*
* This is a multi-line comments.
* 1. ***
* 2. ***
*/
/**
* This is a documents comments.
* @author: shville
* @date: 2023-06-11
*/
Operator
/**
* arithmetic operators : + - * / %
* assignment operators : += -= *= /= %=
* increment & Decrement: i++ i-- ++i --i
* comparison operators : == != < > <= >=
* && || !
* a ? b : c
*/
Core
String
#include <string.h>
// init
char s[6];
char s[6] = "hello"; // size: 6
char s[6] = {'h', 'e', 'l', 'l', 'o', '\0'};
char s[ ] = {'h', 'e', 'l', 'l', 'o', '\0'};
char *s;
char *s[6];
char *s = "hello";
strlen(s)
strcat(s1, s2)
strcpy(dst, src)
sizeof s
// assign (error: s = "Tmo")
sprintf(s, "Batman Joker")
strcpy(s, "Batman Joker")
string input-output
char s[100];
printf("Enter a text line: ");
fgets(s, 100, stdin); // scanf("%[^\n]s", s);
fputs(s, stdout); // printf("%s\n", s);
string-int
#include <stdlib.h>
char s[100] = "120";
int num = atoi(s) // 120
char
#include <ctype.h>
toupper() // c-'a'+'A'
tolower() // c-'A'+'a'
Arrays
// array init
int nums[5];
int nums[5] = {1, 3, 5, 7, 9};
int nums[ ] = {1, 3, 5, 7, 9};
// change element
nums[1] = 9;
// print the value of element
printf("nums[1]: %d\n", nums[1]); // 9
// iter
for (int i = 0; i < 5; i++) {
printf("[%d]: %d\n", i, nums[i]);
}
// two-dimensional arrays
int nums[2][3];
int nums[2][3] = {{1,2,3}, {4,5,6}};
for(int i = 0; i < 2; i++) {
for (int j = 0; j < 3; j++) {
printf("%d ", nums[i][j]);
}
printf("\n");
}
Pointer
pointer-variable
void *ptr;
int x = 12;
float y = 3.14;
char c = 'z';
ptr = &x;
printf("%d\n", *(int*)ptr); // 12
ptr = &y;
printf("%.2f\n", *(float*)ptr); // 3.14
ptr = &c;
printf("%c\n", *(char*)ptr); // z
pointer-function
#include <stdio.h>
void* square(const void* num);
int main() {
int x = 6;
printf("output: %d\n", square(&x)); // 36
return 0;
}
void* square(const void* num) {
return (*(int*)num) * (*(int*)num);
}
pointer-function-template
preprocessor
/**
* preprocessor
* #include
* #define, #undef
* #ifdef, #ifndef, #if, #elif, #else, #endif
* #pragma
* #error, #warning
*/
#define
// define a var
#define PI 3.14
// define a function
#define AREA(r) PI*r*r
// define a template
#define __Tmp__ template <class T>
/* == grammar == */
// a) line break `\`
#define VERY_LONG_CONSTANT \
23.678901
// b) ignore space
# define SQUARE(x) \
x*x
// c) bracket `()`
#define MAX(A, B) ((A) > (B) ? (A) : (B))
example
#include <stdio.h>
#define PI 3.14
#define AREA(r) PI*r*r
#define MAX(A, B) ((A) > (B) ? (A) : (B))
#define VERY_LONG_CONSTANT \
23.678901
# define SQUARE(x) \
x*x
int main() {
float r = 2;
printf("PI: %3.2f\n", PI); // 3.14
printf("Area: %5.2f\n", AREA(r)); // 12.56
printf("%d\n", MAX(3, 5)); // 5
return 0;
}
#define pre
printf("%s %s\n", __TIME__, __DATE__);
printf("This is a line %d\n", __LINE__);
printf("File Name: %s\n", __FILE_NAME__);
// int std_c;
// std_c = __STDC__;
// printf("STDC is %d\n", std_c);
printf("STDC is %d\n", __STDC__);
#ifndef
#ifdef / #ifndef
#else
#endif
#define / #undef
// 1. #ifndef
#ifndef TERM
#define TERM 24
#endif
// 2. #ifdef
// #ifdef RATE
// #undef RATE
// #define RATE 0.068
// #else
// #define RATE 0.068
// #endif
// 2. #ifdef-#undef-#endif
#ifdef RATE
#undef RATE
#endif
#define RATE 0.068
#ifdef TRIPLE
#define SQR(X) (x)*(x)
#else
#define TRIPLE(x) (x)*3
#endif
#if-statement
// 1. #if-statement: only print once result.
// 2. #if defined(LEVEL) = #ifdef LEVEL
// #if !defined(LEVEL) = #ifndef LEVEL
#if
#elif
#else
#endif
example
#include <stdio.h>
#define LEVEL 7
// 1. #if-statement: only print once result.
// 2. #if defined(LEVEL) = #ifdef LEVEL
// #if !defined(LEVEL) = #ifndef LEVEL
int main() {
int num = 0;
// only exec once
#if LEVEL > 6
// statement
num = 1;
#elif LEVEL > 5
// statement
num = 2;
#elif LEVEL > 4
// statement
num = 3;
#else
// statement
num = 4;
#endif
printf("num: %d\n", num);
// output: 1
// defined()
#if defined(_CLASS) || defined(LEVEL)
printf("%s\n", "defined lever");
#else
printf("%s\n", "not defined");
#endif
// output: defined lever
#ifndef LEVEL
printf("%s\n", "not defined lever");
#else
printf("%s\n", "defined lever");
#endif
// output: defined lever
return 0;
}
macro-operator
// #x -> "x"
#define TO_STR(x) #x
// x##y -> xy is a variable
#define VAR(name, num) name##num // VAR(x, 3) -> x3
#define CONCAT(x, y) x##y // CONCAT(x, y) -> xy
example
#include <stdio.h>
#include <string.h>
// #x: stringification operator(convert a parameter to a string constant).
// #x -> "x"
#define TO_STR(x) #x
#define STRLEN(x) strlen(x)
// x##y -> xy is a variable
#define VAR(name, num) name##num // VAR(x, 3) -> x3
#define CONCAT(x, y) x##y // CONCAT(x, y) -> xy
int main() {
printf("%s\n", TO_STR(123\12)); // 123
printf("%s\n", TO_STR(123\\12)); // 123\12
printf("%s\n", TO_STR(123\\\12)); // `123\`
printf("%s\n", TO_STR(123\\\\12)); // 123\\12
printf("%d\n", STRLEN(TO_STR(12345))); // strlen("12345"): 5
int x1 = 125, x2 = 250, x3 = 500;
printf("%d\n", VAR(x, 1)); // x1: 125
printf("%d\n", VAR(x, 2)); // x2: 250
printf("%d\n", VAR(x, 3)); // x3: 500
int x = 4, y = 5;
int CONCAT(x, y) = x + y; // CONCAT(x, y) -> xy = x + y
printf("%d\n", xy); // 9
return 0;
}
#error
#if defined(LEVEL)
#error "_DARWIN_UNLIMITED_STREAMS specified, but -miphoneos-version-min version does not support it."
#elif
#error "_DARWIN_UNLIMITED_STREAMS specified, but -mmacosx-version-min version does not support it."
#endif