Thread: Help understanding conditional operator

Code:

/* Inputs a list of strings from the keyboard, sorts them */
/* in ascending or descending order, then displays them */

#include <stdio.h>
#include <string.h>
#include <process.h>
#include <malloc.h>

#define MAXLINES 25

int	get_lines(char *lines[]);
void	sort(char *p[], int n, int sort_type);
void	print_strings(char *p[], int n);
int	alpha(char *p1, char *p2);
int	reverse(char *p1, char *p2);

char	*lines[MAXLINES];

void main(void)
{
	int number_of_lines, sort_type;

	/* Read in the lines from keyboard */
	number_of_lines = get_lines(lines);

	if (number_of_lines < 0)
	{
		puts("Malloc error");
		exit(-1);
	}

	puts("Enter 0 for reverse order sort, 1 for alphabetical:");
	scanf("%d", &sort_type);

	sort(lines, number_of_lines, sort_type);
	print_strings(lines, number_of_lines);
}

int get_lines(char *lines[])
{
	int n = 0;
	char buffer[80]; /* temp storage for each line */
	puts("Enter on line at a time; enter blank when done");

	while (n < MAXLINES && gets(buffer) != 0 && buffer[0] != '\0')
	{
		if ((lines[n] = (char *)malloc(strlen(buffer) + 1)) == NULL)
			return -1;
			strcpy(lines[n++], buffer); /* copy buffer to lines at [n] and increment n */
	}
	return n; /* return number of lines entered */
}

void sort(char *p[], int n, int sort_type)
{
	int a, b;
	char *x;

	/* Pointer to function */
	int (*compare)(char *s1, char *s2);

	/* Initialize the pointer to point at the proper comparison */
	/* function depending on the argument sort_type */
	compare = (sort_type) ? reverse : alpha;

	for (a = 1; a < n; a++)
	{
		for (b = 0; b < n-1; b++)
		{
			if (compare(p[b], p[b+1]) > 0)
			{
				x = p[b];
				p[b] = p[b+1];
				p[b+1] = x;
			}
		}
	}
}

void print_strings(char *p[], int n)
{
	int count;

	for (count = 0; count < n; count++)
		printf("\n%s ", p[count]);
}

int alpha(char *p1, char *p2)
/* Alphabetical comparison */
{
	return(strcmp(p2, p1));
}

int reverse(char *p1, char *p2)
/* Reverse alphabetical comparison */
{
	return(strcmp(p1, p2));
}

I'm having a little difficulty understanding the usage of the conditional operator (in blue). The user is supposed to enter 0 for a reverse sort and 1 for an alphabetical sort. If I understand the conditional op, if sort_type is true (i.e. non-zero) then the statement evaluates to reverse, otherwise it evaluates to alpha. But, the value to be entered for reverse is zero - so wouldn't that result in a false, and then set compare to alpha? I'm confused...

If I understand you correctly, the problem I have is not with the logic, but with the fact that the sort function names are actually backwards? Well, that's what you get from a "teach Yourself C in 10 Minutes" book.

Also, I have no idea why process.h is in there, nor do I see the need for void main(void). In your opinion, what would be the better definition for main()?

First rule of book buying:
If you see

Code:

void main(void)

put it down and move on to another book. If you already own it, then burn the book.

Ahhh, relief...