Mảng, Chuỗi ký tự & Xử lý văn bản trong C

// Cach 1: Khai bao roi gan gia tri tung phan tu
int scores[5];
scores[0] = 85;
scores[1] = 92;
scores[2] = 78;
scores[3] = 90;
scores[4] = 88;

// Cach 2: Khoi tao truc tiep khi khai bao
int scores[] = {85, 92, 78, 90, 88}; // Compiler tu dong tinh size = 5

// Cach 3: Khoi tao mot phan (phan tu con lai = 0)
int data[10] = {1, 2, 3}; // data[3]..data[9] deu bang 0

// Cach 4: Khoi tao tat ca bang 0
int zeros[100] = {0};

int arr[3] = {10, 20, 30};

// NGUY HIEM: Truy cap ngoai pham vi — Undefined Behavior!
printf("%d\n", arr[5]);   // Doc gia tri rac tu vung nho khong thuoc mang
arr[-1] = 99;             // Ghi de len vung nho khong thuoc mang — co the crash

#include <stdio.h>

void findMaxMin(int arr[], int size, int *max, int *min) {
    *max = arr[0];
    *min = arr[0];
    for (int i = 1; i < size; i++) {
        if (arr[i] > *max) *max = arr[i];
        if (arr[i] < *min) *min = arr[i];
    }
}

int main() {
    int numbers[] = {42, 17, 93, 8, 56, 71, 3, 88};
    int size = sizeof(numbers) / sizeof(numbers[0]); // = 8, chi dung o day

    int max, min;
    findMaxMin(numbers, size, &max, &min);

    printf("Max = %d, Min = %d\n", max, min); // Max = 93, Min = 3
    return 0;
}

// Khai bao ma tran 3x4
int matrix[3][4] = {
    {1,  2,  3,  4},    // Hang 0
    {5,  6,  7,  8},    // Hang 1
    {9, 10, 11, 12}     // Hang 2
};

// Truy cap phan tu: matrix[hang][cot]
printf("%d\n", matrix[1][2]); // In ra: 7

#include <stdio.h>
#define ROWS 3
#define COLS 4

int main() {
    int matrix[ROWS][COLS] = {
        {1,  2,  3,  4},
        {5,  6,  7,  8},
        {9, 10, 11, 12}
    };

    int transposed[COLS][ROWS]; // Dao kich thuoc

    for (int i = 0; i < ROWS; i++) {
        for (int j = 0; j < COLS; j++) {
            transposed[j][i] = matrix[i][j];
        }
    }

    printf("Ma tran chuyen vi:\n");
    for (int i = 0; i < COLS; i++) {
        for (int j = 0; j < ROWS; j++) {
            printf("%3d ", transposed[i][j]);
        }
        printf("\n");
    }
    return 0;
}

#include <stdio.h>
#include <stdlib.h>

int main() {
    int *rows[3];              // Mang 3 con tro
    int sizes[] = {2, 5, 3};   // Moi hang co kich thuoc khac nhau

    for (int i = 0; i < 3; i++) {
        rows[i] = (int*) malloc(sizes[i] * sizeof(int));
        for (int j = 0; j < sizes[i]; j++) {
            rows[i][j] = (i + 1) * 10 + j; // Gan gia tri vi du
        }
    }

    // In ra mang rang cua
    for (int i = 0; i < 3; i++) {
        printf("Hang %d (%d phan tu): ", i, sizes[i]);
        for (int j = 0; j < sizes[i]; j++) {
            printf("%d ", rows[i][j]);
        }
        printf("\n");
        free(rows[i]); // Giai phong bo nho
    }
    return 0;
}

// Cach 1: Con tro tro toi string literal (CHI DOC — nam trong text segment)
char *greeting = "Hello";
// greeting[0] = 'h';  // NGUY HIEM! Undefined behavior — co the crash

// Cach 2: Mang ky tu (CO THE GHI — nam tren stack)
char name[] = "Hello";
name[0] = 'h';  // OK! name bay gio la "hello"

// Cach 3: Khai bao tuong minh voi null terminator
char city[6] = {'H', 'a', 'N', 'o', 'i', '\0'};

// Cach 4: Khai bao mang voi kich thuoc lon hon
char buffer[100] = "Hello"; // 5 ky tu + '\0', 94 byte con lai = 0

#include <stdio.h>
#include <string.h>

int main() {
    char csv_line[] = "Nguyen Van A,25,Ha Noi,Developer";
    char *token;
    int field = 0;
    const char *labels[] = {"Ten", "Tuoi", "Thanh pho", "Nghe nghiep"};

    // Lan goi dau tien: truyen chuoi goc
    token = strtok(csv_line, ",");

    while (token != NULL) {
        printf("%s: %s\n", labels[field], token);
        field++;
        // Cac lan goi tiep theo: truyen NULL de tiep tuc tach
        token = strtok(NULL, ",");
    }

    return 0;
}
// Ket qua:
// Ten: Nguyen Van A
// Tuoi: 25
// Thanh pho: Ha Noi
// Nghe nghiep: Developer

#include <stdio.h>
#include <string.h>

int main() {
    char name[50];

    printf("Nhap ho ten cua ban: ");
    // fgets doc toi da 49 ky tu + '\0', bao gom ca dau cach
    if (fgets(name, sizeof(name), stdin) != NULL) {
        // Xoa ky tu xuong dong '\n' o cuoi (fgets giu lai no)
        size_t len = strlen(name);
        if (len > 0 && name[len - 1] == '\n') {
            name[len - 1] = '\0';
        }
        printf("Xin chao, %s!\n", name);
    }

    // Mau an toan: fgets + sscanf de doc so
    char line[100];
    int age;
    printf("Nhap tuoi: ");
    if (fgets(line, sizeof(line), stdin) != NULL) {
        if (sscanf(line, "%d", &age) == 1) {
            printf("Tuoi cua ban la: %d\n", age);
        } else {
            printf("Du lieu nhap khong hop le!\n");
        }
    }

    return 0;
}

#include <stdio.h>
#include <string.h>

int main() {
    char password[8] = "secret";
    char input[8];

    printf("Nhap mat khau: ");
    // NGUY HIEM: gets() khong kiem tra kich thuoc
    // Neu nguoi dung nhap hon 7 ky tu, se tran sang bien password!
    // gets(input);  // KHONG BAO GIO DUNG HAM NAY!

    // AN TOAN: Dung fgets thay the
    fgets(input, sizeof(input), stdin);
    size_t len = strlen(input);
    if (len > 0 && input[len-1] == '\n') input[len-1] = '\0';

    if (strcmp(input, password) == 0) {
        printf("Truy cap thanh cong!\n");
    } else {
        printf("Sai mat khau!\n");
    }

    return 0;
}

#include <stdio.h>
#include <string.h>

void reverseString(char *str) {
    int left = 0;
    int right = strlen(str) - 1;

    while (left < right) {
        // Hoan doi 2 ky tu o 2 dau
        char temp = str[left];
        str[left] = str[right];
        str[right] = temp;
        left++;
        right--;
    }
}

int main() {
    char word[] = "Hello World";
    printf("Truoc: %s\n", word);
    reverseString(word);
    printf("Sau:   %s\n", word); // "dlroW olleH"
    return 0;
}

#include <stdio.h>
#include <string.h>
#include <ctype.h>

int isPalindrome(const char *str) {
    int left = 0;
    int right = strlen(str) - 1;

    while (left < right) {
        // Bo qua ky tu khong phai chu/so
        while (left < right && !isalnum(str[left]))  left++;
        while (left < right && !isalnum(str[right])) right--;

        // So sanh khong phan biet hoa thuong
        if (tolower(str[left]) != tolower(str[right])) {
            return 0; // Khong phai palindrome
        }
        left++;
        right--;
    }
    return 1; // La palindrome
}

int main() {
    printf("'racecar': %s\n", isPalindrome("racecar") ? "Palindrome" : "Khong");
    printf("'hello':   %s\n", isPalindrome("hello")   ? "Palindrome" : "Khong");
    printf("'A man a plan a canal Panama': %s\n",
           isPalindrome("A man a plan a canal Panama") ? "Palindrome" : "Khong");
    return 0;
}

#include <stdio.h>
#include <ctype.h>

int myAtoi(const char *str) {
    int result = 0;
    int sign = 1;
    int i = 0;

    // Bo qua khoang trang o dau
    while (isspace(str[i])) i++;

    // Xu ly dau + hoac -
    if (str[i] == '+' || str[i] == '-') {
        sign = (str[i] == '-') ? -1 : 1;
        i++;
    }

    // Chuyen tung ky tu so thanh gia tri
    while (isdigit(str[i])) {
        result = result * 10 + (str[i] - '0');
        i++;
    }

    return sign * result;
}

int main() {
    printf("'  -42'  => %d\n", myAtoi("  -42"));   // -42
    printf("'12345'  => %d\n", myAtoi("12345"));    // 12345
    printf("'+100'   => %d\n", myAtoi("+100"));     // 100
    printf("'abc'    => %d\n", myAtoi("abc"));      // 0
    return 0;
}