C语言中使字符串右对齐的方法

C语言没有提供可使字符串右对齐的标准库函数，但是，编写这样的一个函数是很方便的。请看下例：

#include <stdio.h>
#include <string.h>
#include <malloc.h>
char * rjust (char * );
char * rtrim(char * );
void main (void)
{
    char * rjust_str = "This string is not righ-justified. " ;
    /* Show the status of the string before calling the rjust() function. */
    printf("Before calling rjust(), rjust_str is ' %s'\n. " , rjust_str);
    /* Call the rjustO function to right-justify this string. */
    rjust(rjust_str) ;
    /* Show the status of the string after calling the rjust() function. */
    printf ("After calling rjust() , rjust_str is ' %s'\n. " , rjust_str) ;
}
/* The rjust() function right-justifies a string. */
char * r just (char * str)
{
    int n = strlen(str); /* Save the original length of the string. */
    char* dup_str;
    dup_str = strdup(str); /* Make an exact duplicate of the string. */
    rtrim(dup_str); /* Trim off the trailing spaces. */
    /* Call sprintf () to do a virtual "printf" back into the original
        string. By passing sprintf () the length of the original string,
        we force the output to be the same size as the original, and by
        default the sprintf() right-justifies the output. The sprintf()
        function fills the beginning of the string with spaces to make
        it the same size as the original string. */
    sprintf(str, "%*. * s", n, n, dup_str);
    free(dup-str) ; /* Free the memory taken by the duplicated string. */
    return str;\ /* Return a pointer to the string. */
}
/* The rtrim() function removes trailing spaces from a string. */
char * rtrim(char * str)
{
    int n = strlen(str)-l; /* Start at the character BEFORE the null character (\0). */
    while (n>0) /* Make sure we don't go out of bounds... */
    {
        if ( * (str+n) ! = ' ') /* If we find a nonspace character: */
        {
            * (str + n + 1) = '\0';( /* Put the null character at one character past our current position. */
             break; /* Break out of the loop. */
        }
        else /* Otherwise, keep moving backward in the string. */
            n—;
    }
    return str ; /* Return a pointer to the string. */
}

在上例中，使字符串右对齐的工作是由用户编写的rjust()函数完成的，该函数调用了6．2的例子中的rtrim()函数和几个标准函数。rjust()函数的工作过程如下所示：

1. 将原字符串的长度存到变量n中。这一步是不可缺少的，因为输出字符串和原字符串的长度必须相同。
2. 调用标准C库函数strdup()，将原字符串复制到dup_str中。原字符串需要有一份拷贝，因为经过右对齐处理的字符串要写到原字符串中。
3. 调用rtrim()函数，删去dup_str尾部的空格。
4. 调用标准C库函数sprinf()，将dup_str写到原字符串中。由于原字符串的长度(存在n中)被传递给sprintf()函数，所以迫使输出字符串的长度和原字符串相同。因为sprintf()函数缺省使输出字符串右对齐，因此输出字符串的头部将被加入空格，以使它和原字符串长度相同，其效果实际上就是使原字符串右对齐。
5. 调用标准库函数free()，释放由strdup()函数分配给dup_str的动态内存。