{"id":441,"date":"2014-02-08T00:01:31","date_gmt":"2014-02-08T08:01:31","guid":{"rendered":"http:\/\/c-for-dummies.com\/blog\/?p=441"},"modified":"2014-05-06T08:08:23","modified_gmt":"2014-05-06T15:08:23","slug":"an-array-of-pointers","status":"publish","type":"post","link":"https:\/\/c-for-dummies.com\/blog\/?p=441","title":{"rendered":"An Array of Pointers"},"content":{"rendered":"

Here’s a scary thing for most people: **blorf<\/code>. No, not the name blorf<\/em>, although it’s one of my favorite made-up words. What drives programmers insane — and all programmers, not just C programmers — are those double disaster asterisks.
\n
\nA pointer is a variable that holds a memory location, specifically the address of another variable.<\/p>\n

The pointer-to-a-pointer thing is confusing, I confess, but it doesn’t need to be. As an example, consider the following code, which uses a pointer array months<\/code> to create a buncha strings:<\/p>\n

\r\n#include \r\n\r\nint main()\r\n{\r\n    char *months[] = {\r\n        \"January\", \"February\", \"March\", \"April\",\r\n        \"May\", \"June\", \"July\", \"August\",\r\n        \"September\", \"October\", \"November\", \"December\" };\r\n    int x;\r\n\r\n    for(x=0;x<12;x++)\r\n        printf(\"%s\\n\",months[x]);\r\n\r\n    return(0);\r\n}<\/pre>\n
An array of pointers is created at Line 5. It’s a common shortcut used by many programmers who need an array of strings. It’s not really an array of strings, however: It’s a list of 12 memory locations, each of which is the starting address of a string stored in memory.<\/p>\n
Figure 1 is a screenshot from the Code::Blocks debugger. It illustrates how the memory locations, or pointer values, are stored in the array.<\/p>\n
\"Figure
Figure 1. How the months<\/code> array looks in memory.<\/p><\/div>\n
As you can see in Figure 1, each element of the array is an address, a memory location. The first element, months[0]<\/code>, holds the address 0x403024<\/code>, which references the string “January.”<\/p>\n
The code displays the strings by using the printf()<\/em> function at Line 12, courtesy of the for<\/em> loop at Line 11. Simple, standard fare.<\/p>\n
Well, it looks simple because array notation is used. Yet, months<\/code> is really a list of addresses, pointers. Just like the *argv[]<\/code> argument for the main()<\/em> function, it can also be written as **months<\/code>.<\/p>\n
Please do not flee in terror!<\/p><\/blockquote>\n
The **months<\/code> construction works because months<\/code> is a pointer and it references other pointers. An array is pretty-much a gussied up pointer. Therefore, the months<\/code> variable itself references the starting address of a string. That’s how it can be used to display the strings; months<\/code> references each string’s starting address.<\/p>\n
Let that sink in as you view this next code sample. Line 12 (the printf()<\/em> function) substitutes a pointer for the array notation used in the original example:<\/p>\n
\r\n#include \r\n\r\nint main()\r\n{\r\n    char *months[] = {\r\n        \"January\", \"February\", \"March\", \"April\",\r\n        \"May\", \"June\", \"July\", \"August\",\r\n        \"September\", \"October\", \"November\", \"December\" };\r\n    int x;\r\n\r\n    for(x=0;x<12;x++)\r\n        printf(\"%s\\n\",*(months+x));\r\n\r\n    return(0);\r\n}<\/pre>\n
The variable *(months+x)<\/code> walks through the list of addresses stored in the months<\/code> array. It starts with 0x403024<\/code> (from Figure 1) and then goes to 0x40302c<\/code>, and so on.<\/p>\n
Now you can’t use **months<\/code> to declare an array of strings. That’s because an array must be declared by using array notation. So although **months<\/code> and *months[]<\/code> are equivalent elsewhere in the code, **months<\/code> isn’t an array and cannot be used to declare one. To review:<\/p>\n
**months<\/code> is a pointer variable that holds the value of another pointer variable.<\/p>\n
*months[]<\/code> is a pointer variable that references an array of pointer variables.<\/p>\n
To help illustrate how this nonsense applies you can bring in another pointer-to-a-pointer variable, a second dratted **<\/code> thing, such as the variable **blorf<\/code>.<\/p>\n
The following code demonstrates how **months<\/code> and *months[]<\/code> are equivalent:<\/p>\n
\r\n#include <stdio.h>\r\n\r\nint main()\r\n{\r\n    char *months[] = {\r\n        \"January\", \"February\", \"March\", \"April\",\r\n        \"May\", \"June\", \"July\", \"August\",\r\n        \"September\", \"October\", \"November\", \"December\" };\r\n    int x;\r\n    char **blorf;      \/* here is the bastard *\/\r\n\r\n    blorf = months;\r\n    for(x=0;x<12;x++)\r\n        printf(\"%s\\n\",*(blorf+x));\r\n\r\n    return(0);\r\n}<\/pre>\n
Pointer variable **blorf<\/code> is declared at Line 10. It’s a pointer-to-a-pointer variable, as shown by the dang double *<\/code> operators.<\/p>\n
In Line 12, variable blorf<\/code> is initialized to variable months<\/code>. Both variables are of the same type — **blorf<\/code> and **months<\/code>, two char<\/em> pointer-to-pointer variables — so the &<\/code> (address-of) unary operator isn’t needed. (Both variable already hold memory locations.) After that, blorf<\/code> can substitute for months<\/code>, which it does at Line 14 to display the array of strings.<\/p>\n
Confused? Probably. Don’t sweat it. This kind of notation is rare. Despite that rarity, I stubbornly continue this pointless pointer-to-pointer discussion in next week’s Lesson<\/a>.<\/p>\n","protected":false},"excerpt":{"rendered":"
Welcome to pointers from beyond the first dimension! If the *ptr<\/em> thing scares you, then **ptr<\/em> can turn you to stone. 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-441","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\/441","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=441"}],"version-history":[{"count":13,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/441\/revisions"}],"predecessor-version":[{"id":714,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/441\/revisions\/714"}],"wp:attachment":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=441"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=441"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=441"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}