Q. How can I remove the leading spaces from a string?
The C language does not provide a standard function that removes leading spaces from a string. It is easy, however, to build your own function to do just this. you can easily construct a custom function that uses the rtrim() function in conjunction with the standard C library function strrev() to remove the leading spaces from a string. Look at how this task is performed:
#include <stdio.h>#include <string.h>voidmain(void);
char* ltrim(char*);
char* rtrim(char*);
voidmain(void)
{char* lead_str =" This string has leading spaces in it.";
/* Show the status of the string before calling the ltrim()
function. */printf("Before calling ltrim(), lead_str is '%s'\n", lead_str);
printf("and has a length of %d.\n", strlen(lead_str));
/* Call the ltrim() function to remove the leading blanks. */
ltrim(lead_str);/* Show the status of the string
after calling the ltrim() function. */printf("After calling ltrim(), lead_str is '%s'\n", lead_str);
printf("and has a length of %d.\n", strlen(lead_str));
}/* The ltrim() function removes leading spaces from a string. */char* ltrim(char* str)
{strrev(str);/* Call strrev() to reverse the string. */
rtrim(str);/* Call rtrim() to remove the "trailing" spaces. */
strrev(str);/* Restore the string's original order. */
returnstr;/* Return a pointer to the string. */
}/* The rtrim() function removes trailing spaces from a string. */char* rtrim(char* str)
{intn = strlen(str) -1;/* 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--; }returnstr;/* Return a pointer to the string. */
}Notice that the ltrim() function performs the following tasks: First, it calls the standard C library functionstrrev(), which reverses the string that is passed to it. This action puts the original string in reverse order, thereby creating "trailing spaces" rather than leading spaces. Now, the rtrim() function is used to remove the "trailing spaces" from the string. After this task is done, the strrev() function is called again to "reverse" the string, thereby putting it back in its original order.
Q. How can I right-justify a string?
Even though the C language does not provide a standard function that right-justifies a string, you can easily build your own function to perform this action. Using the rtrim() function, you can create your own function to take a string and right-justify it. Here is how this task is accomplished:
#include <stdio.h>#include <string.h>#include <malloc.h>voidmain(void);
char* rjust(char*);
char* rtrim(char*);
voidmain(void)
{char* rjust_str ="This string is not right-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 rjust() 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* rjust(char* str)
{intn = 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. */returnstr;/* Return a pointer to the string. */
}/* The rtrim() function removes trailing spaces from a string. */char* rtrim(char* str)
{intn = strlen(str) -1;/* 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--; }returnstr;/* Return a pointer to the string. */
}The rjust() function first saves the length of the original string in a variable named n. This step is needed because the output string must be the same length as the input string. Next, the rjust() function calls the standard C library function named strdup() to create a duplicate of the original string. A duplicate of the string is required because the original version of the string is going to be overwritten with a right-justified version. After the duplicate string is created, a call to the rtrim() function is invoked (using the duplicate string, not the original), which eliminates all trailing spaces from the duplicate string.
Next, the standard C library function sprintf() is called to rewrite the new string to its original place in memory. The sprintf() function is passed the original length of the string (stored in n), thereby forcing the output string to be the same length as the original. Because sprintf() by default right-justifies string output, the output string is filled with leading spaces to make it the same size as the original string. This has the effect of right-justifying the input string. Finally, because the strdup() function dynamically allocates memory, the free() function is called to free up the memory taken by the duplicate string.
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