{"id":3517,"date":"2019-03-02T00:01:29","date_gmt":"2019-03-02T08:01:29","guid":{"rendered":"https:\/\/c-for-dummies.com\/blog\/?p=3517"},"modified":"2019-02-16T15:57:07","modified_gmt":"2019-02-16T23:57:07","slug":"a-curious-thing-about-array-notation","status":"publish","type":"post","link":"https:\/\/c-for-dummies.com\/blog\/?p=3517","title":{"rendered":"A Curious Thing About Array Notation"},"content":{"rendered":"

It’s nuts. All through your C training when you learned about arrays, you were taught the format: array[x]<\/code> where array<\/code> is the name of the array and x<\/code> is the element number. But this expression can also be written as x[array]<\/code>.<\/p>\n

Mind. Blown.
\n
\nThis oddity is related to how arrays are dealt with internally by the compiler. As I’ve written about in my books and described in my online training, there is a strong relationship between arrays and pointers. It’s often said that array notation is a shortcut for pointers, though that’s not 100 percent accurate. Instead, it’s better to say that arrays translate into pointers easily. And I’ll add that using pointers is faster. Anyway.<\/p>\n

\r\n#include <stdio.h>\r\n\r\nint main()\r\n{\r\n    int a[5] = { 10, 20, 30, 40, 50 };\r\n\r\n    printf(\"%d\\n\",a[3]);\r\n    printf(\"%d\\n\",3[a]);\r\n\r\n    return(0);\r\n}<\/pre>\n
Here’s the output:<\/p>\n
40
\n40<\/code><\/p>\n
Both expressions a[3]<\/code> and 3[a]<\/code> reference the fourth element of array a[]<\/code>. Upon learning such thing I freaked out. “What if I’ve been doing it wrong!”<\/p>\n
Don’t worry: I must maintain that proper and consistent representation of an array in the C language is important. In the original K&R manual, arrays are written name<\/em>[size<\/em>]<\/code>. That’s it and that’s how everyone should do it. In fact, you cannot declare an array like this:<\/p>\n
int 5[a];<\/code><\/p>\n
This statement generates a host of errors should you attempt to compile the code. But the expression 3[a]<\/code> does in fact represent the fourth element of the array. How?<\/p>\n
Internally, the compiler sees the name of the array as a base address. It’s not really a pointer, but the a<\/code> in a[3]<\/code> refers to a memory location. It’s similar to the pointer expression *(a+3)<\/code>.<\/p>\n
When you reference an array element as 3[a]<\/code>, internally the compiler casts the expression as *(3+a)<\/code>. Therefore:<\/p>\n
a[3] == 3[a]<\/code> because *(a+3) == *(3+a)<\/code><\/p>\n
I hope my explanation removes any puzzlement you may have over the 3[a]<\/code> type of expression. In practical use, I strongly recommend that you write a[3]<\/code> to keep your code consistent and readable. But you ever stumble across some joker who reverses his array notation for whatever reason, now you know he’s just being goofy.<\/p>\n","protected":false},"excerpt":{"rendered":"
Prepare to completely lose focus on what you thought was array notation. 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-3517","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\/3517","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=3517"}],"version-history":[{"count":4,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/3517\/revisions"}],"predecessor-version":[{"id":3525,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/3517\/revisions\/3525"}],"wp:attachment":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=3517"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=3517"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=3517"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}