{"id":4919,"date":"2021-08-14T00:01:59","date_gmt":"2021-08-14T07:01:59","guid":{"rendered":"https:\/\/c-for-dummies.com\/blog\/?p=4919"},"modified":"2021-08-07T08:39:58","modified_gmt":"2021-08-07T15:39:58","slug":"negative-array-elements","status":"publish","type":"post","link":"https:\/\/c-for-dummies.com\/blog\/?p=4919","title":{"rendered":"Negative Array Elements"},"content":{"rendered":"

I enjoy it when an experienced coder reviews my stuff. I’m happy with the feedback, and occasionally they toss me a bone. This time it was a rather obscure bone, but I see the point: Avoid using negative array elements.
\n
\nSomewhere in a code example, I use a variable to set an array’s dimension. This construction is rare, but it’s possible. Specifically, I was admonished for using an unsigned<\/em> variable to initialize the array. So I got thinking . . .<\/p>\n

Consider the source code below, where signed int<\/em> variable a<\/code> is initialized to the value -10. This variable then sets the number of elements in int<\/em> array values[]<\/code>. The program outputs the amount of memory allocated for the array as well as the number of elements.<\/p>\n

2021_08_14-Lesson.c<\/a><\/h3>\n
\r\n#include <stdio.h>\r\n\r\nint main()\r\n{\r\n    signed a = -10;\r\n    int values[a];\r\n\r\n    printf(\"Array 'values' occupies %zu bytes of storage\\n\",sizeof(values));\r\n    printf(\"It contains %ld elements\\n\",sizeof(values)\/sizeof(int));\r\n\r\n    return(0);\r\n}<\/pre>\n
Here’s a sample run:<\/p>\n
Array 'values' occupies 17179869144 bytes of storage
\nIt contains 4294967286 elements<\/code><\/p>\n
Let me translate: When initialized to -10 elements, the array consumes 17 gigabytes of storage. It contains over 4 billion elements.<\/p>\n
My computer has 32GB of RAM, so these are legitimate values. Further, a 12 line C program run at the command prompt grabbed over half the RAM in the system. Impressive.<\/p>\n
Obviously, what’s going on is that the compiler is reading the value -10 as unsigned<\/em>, not signed<\/em>. For a 32-bit integer value, the unsigned<\/em> equivalent of -10 is 17179869144 or 0x3FFFFFFD8. I think. I’m not even sure. All I know, is that it’s a Bad Thing to do.<\/p>\n
This approach is the only way you can assign negative array elements. If you try to be direct:<\/p>\n
int values[-10];<\/code><\/p>\n
The compiler pukes up an error message; can’t be done!<\/p>\n
Still, you can<\/em> use a variable set array size. If you do so, ensure that you use an unsigned<\/em> data type. Or, do what I do in these cases: Allocate a pointer instead.<\/p>\n","protected":false},"excerpt":{"rendered":"
Perhaps not in this universe, but are negative array elements even a thing? 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-4919","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\/4919","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=4919"}],"version-history":[{"count":3,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/4919\/revisions"}],"predecessor-version":[{"id":4930,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/4919\/revisions\/4930"}],"wp:attachment":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=4919"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=4919"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=4919"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}