C study notes[1]

the introduction of c language

1. the first C standard called ANSI C was born in 1983 for the strongly demand of C community,was adopted in 1989,usualy intituled as C89.the C99 standard was constructed starting in 1994 which adds a lot of important and pressing functionalities such as supporting international character sets,allowing to single line notation,new data types,Variable-length array and so on.the standard c00 ommitee’s developing of new C standard began in 2007,released in 2011.however,the C standard always abide by a pricinple that C language is supposed to keep simple and small.
2. the source code of C will be compilied to an executable which can run on several OS such as windows,linux,unix,macos and so on .
3. outputing to screen(terminal) is so easy, immediately use the printf function.

#include <stdio.h>
int main(void) {
	int num = 88;
	printf("the number is:");
	printf("%d",num);
}

the first printf fucntion take out the string “the number is:” for outputing to terminal,without line break.the second use two parameters which has one more parameter than the first printf,the newly added parameter is character format and in this example %d represents integer value.there are few formats as similar as %d as follows.

%d 	decimal integer
%f 	floating number
%c 	single char
%s 	string 
%x 	Hexadecimal integer
%p 	pointer address
%% 	Output percentage sign

• a c program starts its running with main() function,means the main function is the entry of all program .
• all variables in a C program must be declared earlier than using them.

4. all variables must belong to a specific data type.

• basic data types are shown as follows.

int: Integer type, typically 4 bytes (platform-dependent). Range: -2,147,483,648 to 2,147,483,647 (32-bit).

char: Character type, 1 byte. Range: -128 to 127 or 0 to 255 (unsigned).

float: Single-precision floating-point, 4 bytes. Range: ~1.2E-38 to 3.4E+38.

double: Double-precision floating-point, 8 bytes. Range: ~2.3E-308 to 1.7E+308.

void: Represents "no type" (used for functions with no return value or pointers to generic data).

• Type Modifiers

  signed: Allows positive and negative values (default for int and char).

  unsigned: Only non-negative values (doubles the positive range).

  short: Smaller integer (usually 2 bytes). Range: -32,768 to 32,767.

  long: Larger integer (4 or 8 bytes). Range: -2³¹ to 2³¹-1 (32-bit) or -2⁶³ to 2⁶³-1 (64-bit).

  long long: Extended integer (at least 8 bytes). Range: -2⁶³ to 2⁶³-1.

• Derived Data Types

  Arrays: Contiguous collection of elements of the same type (e.g., int arr[10]).

  Pointers: Stores memory addresses (e.g., int *ptr).

  Structures (struct): Groups variables of different types under one name.

  Unions (union): Similar to struct, but members share memory.

• User-Defined Types

  typedef: Creates aliases for existing types (e.g., typedef int myInt;).

  Enums (enum): Defines named integer constants (e.g., enum {RED, GREEN, BLUE};).

#include <stdio.h>

int main() {
    int a = 10;
    float b = 3.14;
    char c = 'A';
    double d = 123.456e-10;
    unsigned long e = 1000000UL;

    printf("int: %d, float: %f, char: %c, double: %lf, ulong: %lu\n", a, b, c, d, e);
    return 0;
}

5. The four operations can be accomplished by mathematical operator

#include <stdio.h>

int main() {
    float num1, num2;
    char op;

    printf("请输入两个数字和运算符（如：3 + 5）：");
    scanf("%f %c %f", &num1, &op, &num2);

    switch (op) {
        case '+':
            printf("结果: %.2f\n", num1 + num2);
            break;
        case '-':
            printf("结果: %.2f\n", num1 - num2);
            break;
        case '*':
            printf("结果: %.2f\n", num1 * num2);
            break;
        case '/':
            if (num2 != 0) {
                printf("结果: %.2f\n", num1 / num2);
            } else {
                printf("错误：除数不能为 0！\n");
            }
            break;
        default:
            printf("错误：无效的运算符！\n");
    }

    return 0;
}

of couse,C language already provide a little more complicated computation through math library.

#include <stdio.h>
#include <math.h>

int main() {
    double x = 2.0;

    printf("e^%.2f = %.2f\n", x, exp(x));      // e^x
    printf("2^%.2f = %.2f\n", x, exp2(x));     // 2^x
    printf("ln(%.2f) = %.2f\n", x, log(x));    // 自然对数
    printf("log2(%.2f) = %.2f\n", x, log2(x));  // 以2为底的对数

    return 0;
}

if you handle more advanced computer knowledge ,so much so that Reverse Polish Expression can be written with C to achieve more tasks.

references

1. deepseek