I’ve covered the miraculous qsort()<\/em> function elsewhere in this blog<\/a>. It’s nifty, allowing you to sort all kinds of data with your only job being to code a compare()<\/em> function. Most coders these days just copy-paste this function and call it good. But for sorting an array of structures, more brain muscle is required. The The compar()<\/em> function has a very specific format. Here\u2019s what most programmers purloin from StackOverflow:<\/p>\n Effectively, the return<\/em> statement determines the difference between two elements in the array, generating a value negative, zero, or positive. The qsort()<\/em> function uses this result to sort the list. Changing the expression’s variable order reverses the sort.<\/p>\n And that’s about as much thought as anyone puts into the task — unless faced with something like the sample code shown below.<\/p>\n An array of James Bond films is presented as The qsort()<\/em> function is called at Line 30, which sorts the list by year. A for<\/em> loop outputs the results, which looks like this:<\/p>\n Your task is to code the compare()<\/em> function. You must deal with the Yes, this task is a bastard. When faced with this challenge myself a few months back, I gave up and did something other than a quick sort. But the problem persisted because I knew a solution was possible! You can discover it as well, providing you properly deal with the const void<\/em> pointers and somehow mangle them properly to obtain the Have fun! Click here<\/a> to view my solution, but try this exercise on your own before you peek to see what I did.<\/p>\n","protected":false},"excerpt":{"rendered":" Yes, you can use the qsort()<\/em> (quick sort) to sort just about anything, even an array of structures. Getting the compare()<\/em> function correct, however, is a tough pickle. Continue reading
\n
\nAs a review, the qsort()<\/em> function has four arguments:<\/p>\nvoid qsort(void *base, size_t nel, size_t width, int (*compar)(const void *, const void *));<\/code><\/p>\n*base<\/code> is the start of the array or data to sort; nel<\/code> is the number of elements; width<\/code> is the size of each element (most likely the datatype), and *compar()<\/code> is a function that compares the data, spawning the sort.<\/p>\nint compare(const void *a, const void *b)
\n{
\n return( *(int *)a - *(int *)b);
\n}<\/code><\/p>\n2022_12_01-Lesson.c<\/a><\/h3>\n
\r\n#include <stdio.h>\r\n#include <stdlib.h>\r\n\r\nstruct film {\r\n char title[32];\r\n int year;\r\n};\r\n\r\nint compare(const void *a, const void *b)\r\n{\r\n}\r\n\r\nint main()\r\n{\r\n const int size = 9;\r\n struct film bond[size] = {\r\n { \"Casino Royale\", 2006 },\r\n { \"Dr. No\", 1962 },\r\n { \"GoldenEye\", 1995 },\r\n { \"Goldfinger\", 1964 },\r\n { \"Moonraker\", 1979 },\r\n { \"Octopussy\", 1983 },\r\n { \"Skyfall\", 2012 },\r\n { \"Spectre\", 2015 },\r\n { \"Thunderball\", 1965 }\r\n };\r\n int x;\r\n\r\n \/* sort the list by year *\/<\/span>\r\n qsort(bond,size,sizeof(struct film),compare);\r\n\r\n \/* output results *\/<\/span>\r\n for( x=0; x<size; x++ )\r\n printf(\"%d, %s\\n\",\r\n bond[x].year,\r\n bond[x].title\r\n );\r\n\r\n return(0);\r\n}<\/pre>\nbond[]<\/code>. Each structure contains a movie name and the year it was released.<\/p>\n1962, Dr. No
\n1964, Goldfinger
\n1965, Thunderball
\n1979, Moonraker
\n1983, Octopussy
\n1995, GoldenEye
\n2006, Casino Royale
\n2012, Skyfall
\n2015, Spectre<\/code><\/p>\nyear<\/code> member in the film<\/em> structure, comparing each value and processing the results to create a proper sort.<\/p>\nyear<\/code> member from each structure passed to the compare()<\/em> function.<\/p>\n