{"id":6540,"date":"2024-08-31T00:01:18","date_gmt":"2024-08-31T07:01:18","guid":{"rendered":"https:\/\/c-for-dummies.com\/blog\/?p=6540"},"modified":"2024-08-24T10:28:37","modified_gmt":"2024-08-24T17:28:37","slug":"more-messing-with-array-subscripts","status":"publish","type":"post","link":"https:\/\/c-for-dummies.com\/blog\/?p=6540","title":{"rendered":"More Messing with Array Subscripts"},"content":{"rendered":"

I’ve written before<\/a> about array subscripts and how they’re secretly expressions. But another weird aspect of array subscripts also looms on the horizon: The format a[1]<\/code> can be written as 1[a]<\/code>.
\n
\nBefore I explain what’s going on, here’s sample code:<\/p>\n

2024_08_31-Lesson-a.c<\/a><\/h3>\n
\r\n#include <stdio.h>\r\n\r\nint main()\r\n{\r\n    int a[] = { 1, 2, 3, 4, 5 };\r\n\r\n    printf(\"%d\\n\",a[2]);\r\n    printf(\"%d\\n\",2[a]);\r\n\r\n    return 0;\r\n}<\/pre>\n
The two printf()<\/em> statements use both forms to access the third array element, traditionally written as a[2]<\/code>. Here’s the output:<\/p>\n
3
\n3<\/code><\/p>\n
Both arrangements generate the same result, accessing the third element in array a[]<\/code>. But how?<\/p>\n
You’re probably aware of the weird relationship between pointers and arrays. They aren’t the same thing; array notation isn’t shorthand for pointer notation. The big difference is how the compiler stores the data in memory. Regardless, internally the compiler uses pointer offsets to access array elements.<\/p>\n
For example, a[2]<\/code> translates into *(a+2)<\/code>. Because this translation takes each part of the array notation and shoves it into the pointer format, when you write 2[a]<\/code> you get *(2+a)<\/code>.<\/p>\n
Yes, the expression *(2+a)<\/code> looks awkward, but the math is the same as *(a+2)<\/code>. You can use either format to obtain the same result.<\/p>\n
The weirder thing, which harkens to my earlier blog post, is whether this format works with expressions. Here’s the code I used to test:<\/p>\n
2024_08_31-Lesson-b.c<\/a><\/h3>\n
\r\n#include <stdio.h>\r\n#include <stdlib.h>\r\n#include <time.h>\r\n\r\nint main()\r\n{\r\n    int a[] = { 10, 20, 30, 40, 50 };\r\n\r\n    srand( (unsigned)time(NULL) );\r\n    printf(\"%d\\n\", (rand()%5)[a] );\r\n\r\n    return 0;\r\n}<\/pre>\n
This code updates code presented in the Lesson from May. The expression rand()%5<\/code> generates a random value from 0 through 4. It’s prefixed before the brackets with the array name inside the brackets. The expression is enclosed in parentheses, otherwise the compiler would try to fetch the non-existent fifth element from array a[]<\/code>. (The order of operations binds the 5 tight to the brackets.)<\/p>\n
The output shows a random element from the array, just as if the expression rand()%5<\/code> were in the brackets. So it works!<\/p>\n
While I’m delighted in the funky nature of swapping an array name with its element number (or expression), I would never use such a format in my code. Yes, it’s wonderfully cryptic. Obfuscators would enjoy using this trick, but I aim for readable code.<\/p>\n
A bigger question, one that I haven’t attempted, is whether this trick works in Python or other languages. It’s not that using the backwards format does anything special, just that such things are entertaining to nerds such as myself.<\/p>\n","protected":false},"excerpt":{"rendered":"
Here’s something truly weird that works — and makes sense when you know what’s going on behind the scenes. Continue reading →<\/span><\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"comment_status":"open","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[2],"tags":[],"class_list":["post-6540","post","type-post","status-publish","format-standard","hentry","category-main"],"_links":{"self":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/6540","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=6540"}],"version-history":[{"count":4,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/6540\/revisions"}],"predecessor-version":[{"id":6558,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/6540\/revisions\/6558"}],"wp:attachment":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=6540"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=6540"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=6540"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}