使用C语言编写一个数据结构的一元多项式计算器，多项式序列按指数降序排列，实现两个多项式的相加、相减、相乘，并将结果输出。并输出未知数为1时的值

好的，下面是使用C语言编写的一元多项式计算器的代码。该计算器使用链表实现多项式的存储和操作，多项式节点包含系数和指数两个成员变量。具体实现如下：

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

// 定义多项式节点结构体
typedef struct poly_node {
    float coef; // 系数
    int expn;   // 指数
    struct poly_node *next; // 指向下一个多项式节点的指针
} PolyNode;

// 函数声明
PolyNode* create_poly();
void add_term(PolyNode *poly, float coef, int expn);
void print_poly(PolyNode *poly);
PolyNode* add_poly(PolyNode *poly1, PolyNode *poly2);
PolyNode* sub_poly(PolyNode *poly1, PolyNode *poly2);
PolyNode* mul_poly(PolyNode *poly1, PolyNode *poly2);
float eval_poly(PolyNode *poly, float x);

// 主函数
int main()
{
    // 创建多项式1
    PolyNode *poly1 = create_poly();
    add_term(poly1, 3.0, 4);
    add_term(poly1, 2.0, 2);
    add_term(poly1, 1.0, 0);
    printf("多项式1：");
    print_poly(poly1);

    // 创建多项式2
    PolyNode *poly2 = create_poly();
    add_term(poly2, 4.0, 3);
    add_term(poly2, -2.0, 2);
    add_term(poly2, 1.0, 0);
    printf("多项式2：");
    print_poly(poly2);

    // 多项式加法
    PolyNode *sum = add_poly(poly1, poly2);
    printf("多项式加法：");
    print_poly(sum);

    // 多项式减法
    PolyNode *diff = sub_poly(poly1, poly2);
    printf("多项式减法：");
    print_poly(diff);

    // 多项式乘法
    PolyNode *prod = mul_poly(poly1, poly2);
    printf("多项式乘法：");
    print_poly(prod);

    // 未知数为1时的值
    printf("未知数为1时的值：%.2f\n", eval_poly(prod, 1.0));

    return 0;
}

// 创建多项式
PolyNode* create_poly()
{
    PolyNode *poly = (PolyNode*)malloc(sizeof(PolyNode));
    poly->coef = 0.0;
    poly->expn = -1;
    poly->next = NULL;
    return poly;
}

// 添加一项到多项式中
void add_term(PolyNode *poly, float coef, int expn)
{
    PolyNode *p = poly;
    while (p->next && p->next->expn > expn) {
        p = p->next;
    }
    if (p->next && p->next->expn == expn) {
        p->next->coef += coef;
        if (p->next->coef == 0.0) {
            PolyNode *tmp = p->next;
            p->next = tmp->next;
            free(tmp);
        }
    } else {
        PolyNode *new_node = (PolyNode*)malloc(sizeof(PolyNode));
        new_node->coef = coef;
        new_node->expn = expn;
        new_node->next = p->next;
        p->next = new_node;
    }
}

// 打印多项式
void print_poly(PolyNode *poly)
{
    PolyNode *p = poly->next;
    if (!p) {
        printf("0.0\n");
        return;
    }
    while (p) {
        if (p->coef > 0.0 && p != poly->next) {
            printf("+");
        }
        if (p->coef != 1.0 || p->expn == 0) {
            printf("%.2f", p->coef);
        }
        if (p->expn > 1) {
            printf("x^%d", p->expn);
        } else if (p->expn == 1) {
            printf("x");
        }
        p = p->next;
    }
    printf("\n");
}

// 多项式加法
PolyNode* add_poly(PolyNode *poly1, PolyNode *poly2)
{
    PolyNode *sum = create_poly();
    PolyNode *p1 = poly1->next;
    PolyNode *p2 = poly2->next;
    while (p1 && p2) {
        if (p1->expn > p2->expn) {
            add_term(sum, p1->coef, p1->expn);
            p1 = p1->next;
        } else if (p1->expn < p2->expn) {
            add_term(sum, p2->coef, p2->expn);
            p2 = p2->next;
        } else {
            add_term(sum, p1->coef + p2->coef, p1->expn);
            p1 = p1->next;
            p2 = p2->next;
        }
    }
    while (p1) {
        add_term(sum, p1->coef, p1->expn);
        p1 = p1->next;
    }
    while (p2) {
        add_term(sum, p2->coef, p2->expn);
        p2 = p2->next;
    }
    return sum;
}

// 多项式减法
PolyNode* sub_poly(PolyNode *poly1, PolyNode *poly2)
{
    PolyNode *diff = create_poly();
    PolyNode *p1 = poly1->next;
    PolyNode *p2 = poly2->next;
    while (p1 && p2) {
        if (p1->expn > p2->expn) {
            add_term(diff, p1->coef, p1->expn);
            p1 = p1->next;
        } else if (p1->expn < p2->expn) {
            add_term(diff, -p2->coef, p2->expn);
            p2 = p2->next;
        } else {
            add_term(diff, p1->coef - p2->coef, p1->expn);
            p1 = p1->next;
            p2 = p2->next;
        }
    }
    while (p1) {
        add_term(diff, p1->coef, p1->expn);
        p1 = p1->next;
    }
    while (p2) {
        add_term(diff, -p2->coef, p2->expn);
        p2 = p2->next;
    }
    return diff;
}

// 多项式乘法
PolyNode* mul_poly(PolyNode *poly1, PolyNode *poly2)
{
    PolyNode *prod = create_poly();
    PolyNode *p1 = poly1->next;
    while (p1) {
        PolyNode *p2 = poly2->next;
        while (p2) {
            add_term(prod, p1->coef * p2->coef, p1->expn + p2->expn);
            p2 = p2->next;
        }
        p1 = p1->next;
    }
    return prod;
}

// 计算多项式在x处的值
float eval_poly(PolyNode *poly, float x)
{
    float sum = 0.0;
    PolyNode *p = poly->next;
    while (p) {
        sum += p->coef * pow(x, p->expn);
        p = p->next;
    }
    return sum;
}

注：这个计算器中的多项式序列按指数降序排列，即从高到低排列。