C Primer Plus第十三章编程练习答案

学完C语言之后，我就去阅读《C Primer Plus》这本经典的C语言书籍，对每一章的编程练习题都做了相关的解答，仅仅代表着我个人的解答思路，如有错误，请各位大佬帮忙点出！

由于使用的是命令行参数常用于linux系统或者vscode，但此代码是运行于vs2022的，测试截图就不弄了。

1.修改程序清单13.1中的程序，要求提示用户输入文件名，并读取用户输入的信息，不使用命令行参数。

#include <stdio.h>
#include <stdlib.h>
#define LEN 50
int main(int argc, char* argv[])
{
    int ch;
    FILE* fp;
    int count = 0;
    char filename[LEN];

    printf("Please enter a filename: ");
    scanf("%s", filename);

    fp = fopen(filename, "r");
    if (fp == NULL)
    {
        printf("Can't open %s\n", filename);
        exit(0);
    }
    while ((ch = getc(fp)) != EOF)
    {
        putc(ch, stdout);
        count++;
    }
    fclose(fp);
    printf("\nFile %s has %d characters\n", filename, count);

    return 0;
}

2.编写一个文件拷贝程序，该程序通过命令行获取原始文件名和拷贝文件名。尽量使用标准I/O和二进制模式。

#include <stdio.h>
#include <stdlib.h>
#define BUF 512
int main(int argc, char* argv[])
{
    int bytes;
    FILE* source;
    FILE* target;
    char temp[BUF];

    if ((source = fopen("test.txt", "rb")) == NULL)
    {
        printf("Can't open %s\n", "test.txt");
        exit(0);
    }
    if ((target = fopen("copy.txt", "wb")) == NULL)
    {
        printf("Can't open %s\n", "copy.txt");
        exit(0);
    }
    while ((bytes = fread(temp, sizeof(char), BUF, source)) > 0)
    {
        fwrite(temp, sizeof(char), bytes, target);
    }

    fclose(source);
    fclose(target);
    
    return 0;
}

3.编写一个文件拷贝程序，提示用户输入文本文件名，并以该文件名作为原始文件名和输出文件名。该程序要使用 ctype.h 中的 toupper()函数，在写入到输出文件时把所有文本转换成大写。使用标准I/O和文本模式。

#include <stdio.h>
#include <ctype.h>
#include <stdlib.h>
#define LEN 50
int main(int argc, char* argv[])
{
    int ch;
    FILE* fp;
    char filename[LEN];

    printf("Please enter a filename: ");
    scanf("%s", filename);
    fp = fopen(filename, "r+");
    if (fp == NULL)
    {
        printf("Can't open %s\n", filename);
        exit(0);
    }
    while ((ch = getc(fp)) != EOF)
    {
        fseek(fp, -1L, SEEK_CUR);
        putc(toupper(ch), fp);
        fseek(fp, 0L, SEEK_CUR); 
    rewind(fp);
    printf("File %s:\n", filename);
    while ((ch = getc(fp)) != EOF)
    {
        putchar(ch);
    }
    printf("\n");

    fclose(fp);

    return 0;
}

4.编写一个程序，按顺序在屏幕上显示命令行中列出的所有文件。使用 argc控制循环。

#include <stdio.h>
#include <stdlib.h>
int main(int argc, char* argv[])
{
    int i, temp;
    FILE* source;

    if (argc == 1)
    {
        printf("Usage: %s filename[s]\n", argv[0]);
        exit(0);
    }
    for (i = 1; i < argc; i++)
    {
        source = fopen(argv[i], "r");
        if (source == NULL)
        {
            printf("Can't open file %s\n", argv[i]);
            continue;
        }
        printf("File %s:\n", argv[i]);
        while ((temp = getc(source)) != EOF)
        {
            putchar(temp);
        }
        printf("\n");
        fclose(source);
    }

    return 0;
}

5.修改程序清单13.5中的程序，用命令行界面代替交互式界面。

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define BUFSIZE 4096
#define SLEN 81
void append(FILE* source, FILE* dest)
{
    size_t bytes;
    static char temp[BUFSIZE];
    while ((bytes = fread(temp, sizeof(char), BUFSIZE, source)) > 0)
    {
        fwrite(temp, sizeof(char), bytes, dest);
    }
    return;
}
int main(int argc, char* argv[])
{
    int i, ch;
    int files = 0;
    FILE* fa;
    FILE* fs;

    if (argc < 3)
    {
        printf("Usage: %s appendfile sourcefile.\n", argv[0]);
        exit(0);
    }
    if ((fa = fopen(argv[1], "a+")) == NULL)
    {
        printf("Can't open %s\n", argv[1]);
        exit(0);
    }
    if (setvbuf(fa, NULL, _IOFBF, BUFSIZE) != 0)
    {
        fputs("Can't create output buffer\n", stderr);
        exit(0);
    }
    for (i = 2; i < argc; i++)
    {
        if (strcmp(argv[i], argv[1]) == 0)
        {
            fputs("Can't append file to itself\n", stderr);
        }
        else if ((fs = fopen(argv[i], "r")) == NULL)
        {
            printf("Can't open %s\n", argv[i]);
        }
        else
        {
            if (setvbuf(fs, NULL, _IOFBF, BUFSIZE) != 0)
            {
                fputs("Can't create input buffer\n", stderr);
                continue;
            }
            append(fs, fa);
            if (ferror(fs) != 0)
            {
                printf("Error in reading file %s.\n", argv[i]);
            }
            if (ferror(fa) != 0)
            {
                printf("Error in writing file %s.\n", argv[1]);
            }
            fclose(fs);
            files++;
            printf("File %s appended.\n", argv[i]);
        }
    }
    printf("Done appending. %d files appended.\n", files);
    rewind(fa); 
    printf("%s contents:\n", argv[1]);
    while ((ch = getc(fa)) != EOF)
    {
        putchar(ch);
    }
    printf("\n");
    printf("Done displaying.");
    fclose(fa);

    return 0;
}

6.使用命令行参数的程序依赖于用户的内存如何正确地使用它们。重写程序清单 13.2 中的程序，不使用命令行参数，而是提示用户输入所需信息。

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define LEN 40
int main(int argc, char* argv[])
{
    FILE* in, * out;
    int ch;
    char name[LEN];
    int count = 0;
    char temp[LEN];

    printf("Please enter a filename:\n");
    scanf("%s", temp);
    in = fopen(temp, "r");
    if (in == NULL)
    {
        printf("I could't open the file \"%s\"\n", temp);
        exit(0);
    }
    strncpy(name, temp, LEN - 5);
    name[LEN - 5] = '\0';
    strcat(name, ".red");
    out = fopen(name, "w");
    if (out == NULL)
    {
        printf("Can't create output file.\n");
        exit(1);
    }
    while ((ch = getc(in)) != EOF)
    {
        if (count++ % 3 == 0)
        {
            putc(ch, out);
        }
    }
    fclose(in);
    fclose(out);

    return 0;
}

7.编写一个程序打开两个文件。可以使用命令行参数或提示用户输入文件名。

a.该程序以这样的顺序打印：打印第1个文件的第1行，第2个文件的第1 行，第1个文件的第2行，第2个文件的第2行，以此类推，打印到行数较多文件的最后一行。

b.修改该程序，把行号相同的行打印成一行。

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#define LEN 256
int main(int argc, char* argv[])
{
    FILE* f1;
    FILE* f2;
    char str1[LEN], str2[LEN];

    printf("请输入两个文件的名字:");
    scanf("%s %s", str1, str2);
    f1 = fopen(str1, "r");
    if (f1 == NULL)
    {
        printf("Can't open file %s\n", str1);
        exit(0);
    }
    f2 = fopen(str2, "r");
    if (f2 == NULL)
    {
        printf("Can't open file %s\n", str2);
        exit(0);
    }

    char* s1 = fgets(str1, LEN, f1);
    char* s2 = fgets(str2, LEN, f2);
    while (s1 || s2)
    {
        int len1 = strlen(str1), len2 = strlen(str2);
        if (str1[len1 - 1] == '\n')
        {
            str1[len1 - 1] = '\0';
        }
        if (str2[len2 - 1] == '\n')
        {
            str2[len2 - 1] = '\0';
        }
        if (s1)
        {
            printf("%s\n", str1);
        }
        if (s2)
        {
            printf("%s\n", str2);
        }
        s1 = fgets(str1, LEN, f1), s2 = fgets(str2, LEN, f2);
    }
    fclose(f1);
    fclose(f2);

    return 0;
}

8.编写一个程序，以一个字符和任意文件名作为命令行参数。如果字符后面没有参数，该程序读取标准输入；否则，程序依次打开每个文件并报告每个文件中该字符出现的次数。文件名和字符本身也要一同报告。程序应包含错误检查，以确定参数数量是否正确和是否能打开文件。如果无法打开文件，程序应报告这一情况，然后继续处理下一个文件。

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
int search(int ch, FILE* fp)
{
    int find;
    int n = 0;

    while ((find = getc(fp)) != EOF)
    {
        if (ch == find)
        {
            ++n;
        }
    }
    return n;
}
int main(int argc, char* argv[])
{
    FILE* fp;
    int i, ch, ct;
    int count = 0;

    if (argc < 2)
    {
        printf("Usage: %s character filename[s]\n", argv[0]);
        exit(-1);
    }
    if (strlen(argv[1]) != 1)
    {
        printf("The second argument must be a character!\n");
        exit(-1);
    }
    if (argc == 2)
    {
        fp = stdin;
        ch = argv[1][0];
        printf("Please enter a string (EOF to quit): ");
        count = search(ch, fp);
        printf("%c appeared %d times.\n", ch, count);
    }
    else if (argc > 2)
    {
        ch = argv[1][0];
        for (i = 2; i < argc; i++)
        {
            if ((fp = fopen(argv[i], "r")) == NULL)
            {
                printf("Can't open file %s\n", argv[i]);
                continue;
            }
            printf("File %s:\n", argv[i]);
            while ((ct = getc(fp)) != EOF)
            {
                putchar(ct);
            }
            rewind(fp);
            count = search(ch, fp);
            printf("%c appeared %d times in file %s\n", ch, count, argv[i]);
            fclose(fp);
        }
    }

    return 0;
}

9.修改程序清单 13.3 中的程序，从 1 开始，根据加入列表的顺序为每个单词编号。当程序下次运行时，确保新的单词编号接着上次的编号开始。

#include <stdio.h>
#include <stdlib.h>
#define MAX 41
int main(void)
{
    FILE* fp;
    int ct = 0;
    char words[MAX];

    fp = fopen("wordy.txt", "a+");
    if ( fp == NULL)
    {
        printf("Can't open \"wordy\" file.\n");
        exit(-1);
    }

    while (fgets(words, MAX, fp) != NULL)
    {
        ct++;
    }
    rewind(fp);
    printf("Enter words to add to the file; press the #\n");
    printf("key at the beginning of a line to terminate.\n");
    while ((fscanf(stdin, "%s", words) == 1) && (words[0] != '#'))
    {
        fprintf(fp, "%-2d : %s\n", ++ct, words);
    }

    printf("File contents:");
    rewind(fp);
    while (fgets(words, MAX, fp) != NULL)
    {
        fputs(words, stdout);
    }
    printf("Done!\n");
    fclose(fp);

    return 0;
}

10.编写一个程序打开一个文本文件，通过交互方式获得文件名。通过一个循环，提示用户输入一个文件位置。然后该程序打印从该位置开始到下一个换行符之前的内容。用户输入负数或非数值字符可以结束输入循环。

#include <stdio.h>
#include <stdlib.h>
#define LEN 50

int main(int argc, char* argv[])
{
    int ch;
    FILE* fp;
    int number;
    char filename[LEN];

    printf("Please enter a filename: ");
    scanf("%49s", filename);

    fp = fopen(filename, "r");
    if ( fp == NULL)
    {
        printf("Can't open file %s\n", filename);
        exit(-1);
    }
    printf("Please enter a file position (<0 or q to quit): ");
    while (scanf("%d", &number) == 1 && number >= 0)
    {
        fseek(fp, number, SEEK_SET);
        while ((ch = getc(fp)) != '\n' && ch != EOF)
        {
            putchar(ch);
        }
        if (ch == EOF)
        {
            printf("The position reaches EOF.\n");
            break;
        }
        printf("\nYou can enter again (<0 or q to quit): ");
    }
    printf("Done.\n");
    fclose(fp);

    return 0;
}

11.编写一个程序，接受两个命令行参数。第1个参数是一个字符串，第 2个参数是一个文件名。然后该程序查找该文件，打印文件中包含该字符串的所有行。因为该任务是面向行而不是面向字符的，所以要使用fgets()而不是getc()。使用标准C库函数strstr()（11.5.7节简要介绍过）在每一行中查找指定字符串。假设文件中的所有行都不超过255个字符。

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#define SLEN 256
int main(int argc, char* argv[])
{
    FILE* fp;
    char line[SLEN];

    if (argc != 3)
    {
        printf("Usage: %s string filename\n", argv[0]);
        exit(-1);
    }
    if ((fp = fopen(argv[2], "r")) == NULL)
    {
        printf("Can't open file %s", argv[2]);
        exit(-1);
    }
    while (fgets(line, SLEN, fp) != NULL)
    {
        if (strstr(line, argv[1]) != NULL)
        {
            fputs(line, stdout);
        }
    }
    fclose(fp);

    return 0;
}

12.创建一个文本文件，内含20行，每行30个整数。这些整数都在0～9 之间，用空格分开。该文件是用数字表示一张图片，0～9表示逐渐增加的灰度。编写一个程序，把文件中的内容读入一个20×30的int数组中。一种把这些数字转换为图片的粗略方法是：该程序使用数组中的值初始化一个20×31 的字符数组，用值0 对应空格字符，1 对应点字符，以此类推。数字越大表示字符所占的空间越大。例如，用#表示9。每行的最后一个字符（第31个）是空字符，这样该数组包含了20个字符串。最后，程序显示最终的图片（即，打印所有的字符串），并将结果储存在文本文件中。例如，下面是开始的数据：

#include <stdio.h>
#include <stdlib.h>
#define ROWS 20
#define COLS 30
#define LEVELS 10
#define LEN 50
const char trans[LEVELS + 1] = " .':~*=&%#"; 
void change(int(*a)[COLS], char(*b)[COLS], int rows)
{
    int row, col;

    for (row = 0; row < ROWS; row++)
    {
        for (col = 0; col < COLS; col++)
        {
            b[row][col] = trans[a[row][col]];
        }
    }
}
int main(void)
{
    int row, col;
    FILE* infile;
    char filename[LEN];
    int figure[ROWS][COLS];
    char picture[ROWS][COLS];

    printf("Please enter a filename: ");
    scanf("%s", filename);

    infile = fopen(filename, "r+");
    if ( infile == NULL)
    {
        printf("Can't open file %s\n", filename);
        exit(-1);
    }
    for (row = 0; row < ROWS; row++)
    {
        for (col = 0; col < COLS; col++)
        {
            fscanf(infile, "%d", &figure[row][col]);
        }
    }

    change(figure, picture, ROWS);
    printf("File %s have been transformed pictures:\n", filename);
    for (row = 0; row < ROWS; row++)
    {
        for (col = 0; col < COLS; col++)
        {
            putchar(picture[row][col]);
        }
        printf("\n");
    }

    fseek(infile, 0L, SEEK_END);
    putc('\n', infile);      
    for (row = 0; row < ROWS; row++)
    {
        for (col = 0; col < COLS; col++)
        {
            fprintf(infile, "%c", picture[row][col]);
        }
        putc('\n', infile);
    }
    fclose(infile);

    return 0;
}

13.用变长数组（VLA）代替标准数组，完成编程练习12。

#include <stdio.h>
#include <stdlib.h>
#define ROWS 20
#define COLS 30
#define LEVELS 10
#define LEN 50
const char trans[LEVELS + 1] = " .':~*=&%#";
void change(int rows, int cols, int a[rows][cols], char b[rows][cols])
{
    int row, col;

    for (row = 0; row < rows; row++)
    {
        for (col = 0; col < cols; col++)
        {
            b[row][col] = trans[a[row][col]];
        }
    }
    return;
}
int main(void)
{
    int row, col;
    FILE* infile;
    char filename[LEN];
    int figure[ROWS][COLS];
    char picture[ROWS][COLS];

    printf("Please enter a filename: ");
    scanf("%s", filename);

    infile = fopen(filename, "r+");
    if ( infile == NULL)
    {
        printf("Can't open file %s\n", filename);
        exit(-1);
    }
    for (row = 0; row < ROWS; row++)
    {
        for (col = 0; col < COLS; col++)
        {
            fscanf(infile, "%d", &figure[row][col]);
        }
    }

    change(ROWS, COLS, figure, picture);
    printf("File %s have been transformed pictures:\n", filename);
    for (row = 0; row < ROWS; row++)
    {
        for (col = 0; col < COLS; col++)
        {
            putchar(picture[row][col]);
        }
        printf("\n");
    }

    fseek(infile, 0L, SEEK_END);
    putc('\n', infile);     
    for (row = 0; row < ROWS; row++)
    {
        for (col = 0; col < COLS; col++)
        {
            fprintf(infile, "%c", picture[row][col]);
        }
        putc('\n', infile);
    }
    fclose(infile);

    return 0;
}

14.数字图像，尤其是从宇宙飞船发回的数字图像，可能会包含一些失真。为编程练习12添加消除失真的函数。该函数把每个值与它上下左右相邻的值作比较，如果该值与其周围相邻值的差都大于1，则用所有相邻值的平均值（四舍五入为整数）代替该值。注意，与边界上的点相邻的点少于4 个，所以做特殊处理。

#include <stdio.h>
#include <stdlib.h>
#define ROWS 20
#define COLS 30
#define LEVELS 10
#define LEN 50
const char trans[LEVELS + 1] = " .':~*=&%#";
int clean_distortion(int(*a)[COLS], int rows, int x, int y)
{
    int i = 0, average = 0, cnt = 0, sur = 0;
    int d[4][2] = { {0, 1}, {1, 0}, {0, -1}, {-1, 0} };
    for (i = 0; i < 4; ++i)
    {
        int dx = x + d[i][0], dy = y + d[i][1];
        if (dx < 0 || dx >= ROWS || dy < 0 || dy >= COLS) continue;
        ++sur;
        if (a[x][y] - a[dx][dy] > 1)
        {
            ++cnt, average += a[dx][dy];
        }
    }
    if (cnt != sur) return a[x][y];
    return 1.0 * average / cnt + 0.5;
}
void change(int(*a)[COLS], char(*b)[COLS], int rows)
{
    int row, col;

    for (row = 0; row < ROWS; row++)
    {
        for (col = 0; col < COLS; col++)
        {
            b[row][col] = trans[a[row][col]];
        }
    }
}
int main(void)
{
    int row, col;
    FILE* infile;
    char filename[LEN];
    int figure[ROWS][COLS];
    char picture[ROWS][COLS];

    printf("Please enter a filename: ");
    scanf("%s", filename);

    infile = fopen(filename, "r+");
    if ( infile == NULL)
    {
        printf("Can't open file %s\n", filename);
        exit(-1);
    }
    for (row = 0; row < ROWS; row++)
    {
        for (col = 0; col < COLS; col++)
        {
            fscanf(infile, "%d", &figure[row][col]);
        }
    }
    for (row = 0; row < ROWS; row++)
    {
        for (col = 0; col < COLS; col++)
        {
            figure[row][col] = clean_distortion(figure, ROWS, row, col);
        }
    }
    change(figure, picture, ROWS);
    printf("File %s have been transformed pictures:\n", filename);
    for (row = 0; row < ROWS; row++)
    {
        for (col = 0; col < COLS; col++)
        {
            putchar(picture[row][col]);
        }
        printf("\n");
    }

    fseek(infile, 0L, SEEK_END);
    putc('\n', infile);
    for (row = 0; row < ROWS; row++)
    {
        for (col = 0; col < COLS; col++)
        {
            fprintf(infile, "%c", picture[row][col]);
        }
        putc('\n', infile);
    }
    fclose(infile);

    return 0;
}