第五章 二

練習5-7

重寫readlines，將輸入的文本行存儲到由main函數提供的一個數組中，而不是alloc分配的存儲空間中。

int readlines(char *lineptr[], int maxline, char *linetostore)
{
    int len, nlines;
    char *p = linetostore;
    char *linetop = p + ALLLEN;

    nlines = 0;
    while (len = getline(line, MAXLEN) > 0) { /* 讀入行不爲空 */
        if (nlines >= maxline || p + len > linetop)
            return -1;
        else {
            line[len - 1] = '\0';
            strcpy(p, line);
            lineptr[nlines++] = p;
            p += len;
        }
    }
    return nlines;
}

練習5-9

用指針方式替代數組下標改寫函數day_of_year和month_day。

編寫這個程序的時候遇到很多問題，關鍵是指針沒有理解好。下面那代碼來比較，先列出可運行代碼。

（代碼1）

#include <stdio.h>

int day_of_year(int year, int month, int day);
void month_day(int year, int day, int *pmonth, int *pday);


int main()
{
    int year = 2012;
    int month = 1;
    int day = 2;
    int pmonth;
    int pday;
    int mday = day_of_year(year, month, day);
    month_day(2013, 65, &pmonth, &pday);
    printf("%d  %d\n", pmonth, pday);
    return 0;
}

static daytab[2][13] = {
    {0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31},
    {0, 31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31}
};

int day_of_year(int year, int month, int day)
{
    int i, leap;

    leap = year % 4 == 0 && year % 100 != 0 || year % 400 == 0;
    int *p = daytab[leap];
    for(i = 0; i < month; i++)
        day += *p++;
    return day;
}

void month_day(int year, int day, int *pmonth, int *pday)
{
    int i, leap;

    int *p;
    leap = year % 4 == 0 && year % 100 != 0 || year % 400 == 0;
    p = daytab[leap];
    for (i = 0; day > *++p; i++)
        day -= *p;
         ++i;
    *pmonth = i;
    *pday = day;
}

下面是有問題的代碼：（代碼2）

#include <stdio.h>

int day_of_year(int year, int month, int day);
void month_day(int year, int day, int *pmonth, int *pday);


int main()
{
    int year = 2012;
    int month = 1;
    int day = 2;
    int *pmonth;
    int *pday;
    int mday = day_of_year(year, month, day);
    month_day(2012, 2, pmonth, pday);
    printf("%d  %d\n", *pmonth, *pday);
    return 0;
}

static daytab[2][13] = {
    {0, 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31},
    {0, 31, 29, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31}
};

int day_of_year(int year, int month, int day)
{
    int i, leap;

    leap = year % 4 == 0 && year % 100 != 0 || year % 400 == 0;
    int *p = daytab[leap];
    for(i = 0; i < month; i++)
        day += *p++;
    return day;
}

void month_day(int year, int day, int *pmonth, int *pday)
{
    int i, leap;
    int temp = 0;
    int *p;
    leap = year % 4 == 0 && year % 100 != 0 || year % 400 == 0;
    p = daytab[leap];
    for (i = 0; day > *++p; i++)
        day -= *p;
        printf("i = %d",i);
        temp = ++i;
    *pmonth = i;
    printf("pmonth = %d temp = %d\n",*pmonth,temp);
    *pday = day;
}

出錯原因：只定義了指針pmonth和pday，沒有初始化。也就是沒有給指針分配空間，當（代碼2）執行47行時肯定會出現問題。（代碼1）在定義時給int類型的pmonth和pday賦值分配空間，調用函數時傳的是引用。執行函數時，改變兩個引用所指向的空間的值。