Sometimes programming can be a snap, especially when you have a known data set and pretty much all the values are static. You can whip out code right away, it runs well and looks beautiful — but that’s because everything is known. The fewer variables, the easier to code.
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\nAs an example, consider a list of items:<\/p>\n
\r\n char *fruit[] = {\r\n \"apple\", \"orange\", \"blueberry\", \"date\", \"melon\",\r\n \"grape\", \"peach\", \"mango\", \"watermelon\", \"pear\",\r\n \"olive\", \"plum\", \"nectarine\", \"kiwi\", \"loquat\",\r\n \"honeydew\", \"lime\", \"grapefruit\", \"cherry\",\r\n \"avocado\", \"tomato\", \"banana\", \"cantaloupe\",\r\n \"huckleberry\", \"kumquat\", \"tangerine\", \"fig\",\r\n \"raspberry\", \"papaya\", \"lychee\", \"guava\"\r\n };<\/pre>\nThe fruit[]<\/code> array consists of strings (char<\/em> pointers). If you were to sort this array and present the list, the following code does the job:<\/p>\n\r\n#include <stdio.h>\r\n#include <stdlib.h>\r\n#include <string.h>\r\n\r\n#define ITEMS 30\r\n\r\nint compare(const void *a, const void *b);\r\n\r\nint main()\r\n{\r\n char *fruit[] = {\r\n \"apple\", \"orange\", \"blueberry\", \"date\", \"melon\",\r\n \"grape\", \"peach\", \"mango\", \"watermelon\", \"pear\",\r\n \"olive\", \"plum\", \"nectarine\", \"kiwi\", \"loquat\",\r\n \"honeydew\", \"lime\", \"grapefruit\", \"cherry\",\r\n \"avocado\", \"tomato\", \"banana\", \"cantaloupe\",\r\n \"huckleberry\", \"kumquat\", \"tangerine\", \"fig\",\r\n \"raspberry\", \"papaya\", \"lychee\"\r\n };\r\n int x;\r\n\r\n\/* quicksort the list *\/<\/span>\r\n qsort(fruit,ITEMS,sizeof(char *),compare);\r\n\r\n\/* display sorted strings *\/<\/span>\r\n for(x=0;x<ITEMS;x++)\r\n {\r\n printf(\"%s\\n\",fruit[x]);\r\n }\r\n\r\n return(0);\r\n}\r\n\r\n\/* compare routine for quicksort *\/<\/span>\r\nint compare(const void *a, const void *b)\r\n{\r\n const char **pa, **pb;\r\n\r\n pa = (const char **)a;\r\n pb = (const char **)b;\r\n return( strcmp(*pa,*pb) );\r\n}<\/pre>\nThe meat of this code is the quicksort function, qsort()<\/em> at Line 23. You can read the details here<\/a>, and see that the bulk of the code was copied from that earlier Lesson.<\/p>\nThe output shows the sorted list of 30 items. The code relies upon the ITEMS<\/code> constant, which reflects the number of elements in the array. So the programmer (me) assumes that the number of elements won’t change, which is okay — so far.<\/p>\nNow assume that The Boss likes the code, but the single-column list is too long. So he directs you to present the list in rows and columns. That modification is simple enough: Only the output part of the code changes:<\/p>\n
\r\n\/* display sorted strings *\/<\/span>\r\n x = 0;\r\n while(x < ITEMS)\r\n {\r\n printf(\"%-12s\",fruit[x]);\r\n x++;\r\n \/* new row each 5 items *\/<\/span>\r\n if( x%5 == 0)\r\n putchar('\\n');\r\n }<\/pre>\nI elected to use a while<\/em> loop because it’s easier to process the elements one at a time, using a single variable x<\/code> as a counter, than to mess with an extra variable required for nested for<\/em> loops.<\/p>\nIn the printf()<\/em> statement, the %-12s<\/code> placeholder left-aligns each string within 12 spaces, which helps line-up the output. (Relevant Lesson here<\/a>.) And 12 is chosen because the longest string in the array, huckleberry<\/code>, is 11 characters long; it’s a known value.<\/p>\nThe if( x%5 == 0 )<\/code> statement pops out a newline after every five elements. The output presents the sorted list in six rows of five columns:<\/p>\napple avocado banana blueberry cantaloupe \r\ncherry date fig grape grapefruit \r\nhoneydew huckleberry kiwi kumquat lime \r\nloquat lychee mango melon nectarine \r\nolive orange papaya peach pear \r\nplum raspberry tangerine tomato watermelon<\/code><\/pre>\nIt looks good to me, but I’m only the programmer. The Boss, on the other hand, is puzzled that the list isn’t sorted by columns. In other words, he wants this output instead:<\/p>\n
apple date kiwi melon pear \r\navocado fig kumquat nectarine plum \r\nbanana grape lime olive raspberry \r\nblueberry grapefruit loquat orange tangerine \r\ncantaloupe honeydew lychee papaya tomato \r\ncherry huckleberry mango peach watermelon<\/code><\/pre>\nIn the first column is apple<\/code> through cherry<\/code>, then date<\/code> through mango<\/code> in the second column, and so on. The code for this output is presented in next week’s Lesson<\/a>. Even so, this Lesson began as an Exercise, one that proved more difficult than I had planned. So if you want to concoct your own code for the above output, please do so.<\/p>\n","protected":false},"excerpt":{"rendered":"It’s easy to spew out a list of items. When you mix in rows and columns, things can get twisted fast. Continue reading