C_TA_Website/source/_posts/answer6.md

---
title: 第六次作业（答案）
date: 2020-11-20 17:40:16
tags:
---

# 冒泡排序

```C
#include <stdio.h>
#include <stdlib.h>
#include <time.h>

#define LEN 10

void swap(int a[], int i, int j)
{
    int tmp = a[i];
    a[i] = a[j];
    a[j] = tmp;
}

void bubble_sort(int a[], int n)
{
    int done = 0;
    for (int i = 0; i < n - 1; ++i) {
        if (done)
            return;
        done = 1;
        for (int j = 0; j < n - 1 - i; ++j) {
            if (a[j] > a[j + 1]) {
                swap(a, j, j + 1);
                done = 0;
            }
        }
    }
}

void rand_array(int a[], int n)
{
    for (int i = 0; i < n; ++i) {
        int tmp = rand() % 20 + 1;
        a[i] = tmp;
    }
}

void print_array(int a[], int n)
{
    for (int i = 0; i < n; ++i) {
        printf("%d ", a[i]);
    }
    printf("\n");
}

int main()
{
    srand(time(0));
    int a[LEN];
    printf("使用随机数填充数组：\n");
    rand_array(a, LEN);
    print_array(a, LEN);
    bubble_sort(a, LEN);
    printf("排序过后的数组：\n");
    print_array(a, LEN);
    return 0;
}
```

# 矩阵乘法

```C
#include <stdio.h>

void input_matrix(double mat[][100], int *row, int *column)
{
    printf("行：");
    scanf("%d", row);
    printf("列：");
    scanf("%d", column);
    printf("数据：\n");
    for (int i = 0; i < *row; ++i) {
        for (int j = 0; j < *column; ++j) {
            scanf("%lf", &mat[i][j]);
        }
    }
}

void print_matrix(double mat[][100], int row, int column)
{
    for (int i = 0; i < row; ++i) {
        for (int j = 0; j < column; ++j) {
            printf("%f ", mat[i][j]);
        }
        printf("\n");
    }
}

void matrix_mul(double mat_a[][100], double mat_b[][100], double mat_c[][100],
        int a_row, int a_column, int b_column)
{
    for (int i = 0; i < a_row; ++i) {
        for (int j = 0; j < b_column; ++j) {
            mat_c[i][j] = 0;
            for (int k = 0; k < a_column; ++k) {
                mat_c[i][j] += mat_a[i][k] * mat_b[k][j];
            }
        }
    }
}

int main()
{
    double mat_a[100][100];
    double mat_b[100][100];
    double mat_c[100][100];
    int a_row;
    int a_column;
    int b_row;
    int b_column;
    printf("请输入矩阵A：\n");
    input_matrix(mat_a, &a_row, &a_column);
    printf("请输入矩阵B：\n");
    input_matrix(mat_b, &b_row, &b_column);
    if (a_column != b_row) {
        printf("A和B无法相乘！\n");
        return 0;
    }
    matrix_mul(mat_a, mat_b, mat_c, a_row, a_column, b_column);
    print_matrix(mat_c, a_row, b_column);
    return 0;
}
```

# P166 T13

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

void mystrcat(char a[], char b[])
{
    int i = strlen(a) - 1;
    for (int j = 0; b[j] != '\n'; ++i, ++j) {
       a[i] = b[j];
    }
    a[i] = '\0';
}

int main()
{
    printf("请输入第一个字符串，回车结束：\n");
    char a[200];
    fgets(a, 200, stdin);
    printf("请输入第二个字符串，回车结束：\n");
    char b[100];
    fgets(b, 100, stdin);
    mystrcat(a, b);
    printf("连接之后的字符串是\n%s\n", a);
    return 0;
}
```

# P216 T13

```C
#include <stdio.h>

double P(int n, double x)
{
    if (n == 0) {
        return 1;
    } else if (n == 1) {
        return x;
    } else {
        return ((2 * n - 1) * x - P(n - 1, x) - (n - 1) * P(n - 2, x)) / n;
    }
}

int main()
{
    printf("请输入x: ");
    double x;
    scanf("%lf", &x);
    printf("请输入n: ");
    int n;
    scanf("%d", &n);
    printf("P_n(x) = %lf\n", P(n, x));
    return 0;
}
```

# P216 T14

```C
#include <stdio.h>

double score[5][3];
double average[5];

void Average_Student(void)
{
    for (int i = 0; i < 5; ++i) {
        double avg = 0;
        for (int j = 0; j < 3; ++j) {
            avg += score[i][j];
        }
        avg /= 3;
        average[i] = avg;
        printf("第%d个学生的平均成绩是%f\n", i + 1, avg);
    }
}

void Average_Lesson(void)
{
    for (int i = 0; i < 3; ++i) {
        double avg = 0;
        for (int j = 0; j < 5; ++j) {
            avg += score[j][i];
        }
        avg /= 5;
        printf("第%d门课程的平均成绩是%f\n", i + 1, avg);
    }
}

void Find_Max(void)
{
    double max = score[0][0];
    int max_i = 0, max_j = 0;
    for (int i = 0; i < 5; ++i) {
        for (int j = 0; j < 3; ++j) {
            if (max < score[i][j]) {
                max = score[i][j];
                max_i = i;
                max_j = j;
            }
        }
    }
    printf("成绩最高的是第%d个学生的第%d门课程的成绩。\n", max_i + 1, max_j + 1);
}

void Calc_Var(void)
{
    double sum_xi = 0;
    double sum_xi2 = 0;
    for (int i = 0; i < 5; ++i) {
        sum_xi += average[i];
        sum_xi2 += average[i] * average[i];
    }
    double sigma = sum_xi2 / 5 - sum_xi * sum_xi / 25;
    printf("方差是%f\n", sigma);
}

int main()
{
    for (int i = 0; i < 5; ++i) {
        for (int j = 0; j < 3; ++j) {
            printf("请输入第%d个学生的第%d门成绩：", i + 1, j + 1);
            scanf("%lf", &score[i][j]);
        }
    }
    Average_Student();
    Average_Lesson();
    Find_Max();
    Calc_Var();
    return 0;
}
```