用c语言写：完成增加商品，按照学号删除商品，修改商品信息，查询商品信息的

代码如下：

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

#define MAX_LEN 50
#define MAX_PRODUCTS 20

typedef struct {
    char id[MAX_LEN];
    char name[MAX_LEN];
    double price;
} Product;

int find_product_index(Product products[], int num_products, char id[]) {
    for (int i = 0; i < num_products; i++) {
        if (strcmp(products[i].id, id) == 0) {
            return i;
        }
    }
    return -1;
}

void add_product(Product products[], int *num_products) {
    if (*num_products == MAX_PRODUCTS) {
        printf("Error: Maximum number of products reached!\n");
        return;
    }
    Product new_product;
    printf("Enter product ID: ");
    scanf("%s", new_product.id);
    printf("Enter product name: ");
    scanf("%s", new_product.name);
    printf("Enter product price: ");
    scanf("%lf", &new_product.price);
    products[*num_products] = new_product;
    (*num_products)++;
    printf("Product added successfully!\n");
}

void delete_product(Product products[], int *num_products) {
    char id[MAX_LEN];
    printf("Enter product ID to delete: ");
    scanf("%s", id);
    int index = find_product_index(products, *num_products, id);
    if (index == -1) {
        printf("Error: Product not found!\n");
        return;
    }
    for (int i = index; i < (*num_products) - 1; i++) {
        products[i] = products[i + 1];
    }
    (*num_products)--;
    printf("Product deleted successfully!\n");
}

void modify_product(Product products[], int num_products) {
    char id[MAX_LEN];
    printf("Enter product ID to modify: ");
    scanf("%s", id);
    int index = find_product_index(products, num_products, id);
    if (index == -1) {
        printf("Error: Product not found!\n");
        return;
    }
    Product modified_product;
    printf("Enter new product name (press enter to skip): ");
    scanf("%s", modified_product.name);
    printf("Enter new product price (press 0 to skip): ");
    scanf("%lf", &modified_product.price);
    if (strlen(modified_product.name) > 0) {
        strcpy(products[index].name, modified_product.name);
    }
    if (modified_product.price != 0) {
        products[index].price = modified_product.price;
    }
    printf("Product modified successfully!\n");
}

void display_product(Product product) {
    printf("%-10s %-20s $%.2lf\n", product.id, product.name, product.price);
}

void display_all_products(Product products[], int num_products) {
    printf("%-10s %-20s %s\n", "ID", "Name", "Price");
    for (int i = 0; i < num_products; i++) {
        display_product(products[i]);
    }
}

int main() {
    Product products[MAX_PRODUCTS];
    int num_products = 0;
    int choice;
    do {
        printf("\n1. Add product\n");
        printf("2. Delete product\n");
        printf("3. Modify product\n");
        printf("4. Display all products\n");
        printf("0. Exit\n");
        printf("Enter your choice: ");
        scanf("%d", &choice);
        switch (choice) {
            case 1:
                add_product(products, &num_products);
                break;
            case 2:
                delete_product(products, &num_products);
                break;
            case 3:
                modify_product(products, num_products);
                break;
            case 4:
                display_all_products(products, num_products);
                break;
            case 0:
                printf("Exiting program...\n");
                break;
            default:
                printf("Invalid choice!\n");
        }
    } while (choice != 0);
    return 0;
}

这个程序定义了一个 `Product` 结构体，包含商品的 ID、名称和价格。然后实现了增加商品、按学号删除商品、修改商品信息和查询商品信息的功能。其中，查询商品信息的功能是通过 `display_all_products` 函数实现的，该函数将所有商品的信息输出到控制台上。