{"id":4521,"date":"2021-01-02T00:01:08","date_gmt":"2021-01-02T08:01:08","guid":{"rendered":"https:\/\/c-for-dummies.com\/blog\/?p=4521"},"modified":"2021-01-02T08:27:49","modified_gmt":"2021-01-02T16:27:49","slug":"emulating-the-foreach-keyword","status":"publish","type":"post","link":"https:\/\/c-for-dummies.com\/blog\/?p=4521","title":{"rendered":"Emulating the foreach<\/em> Keyword"},"content":{"rendered":"

In last week’s Lesson<\/a>, I covered how to obtain the number of elements in an array. This process is the first step to emulating the foreach<\/em> keyword in the C language.
\n
\nThe best approach is to divide the array size in memory by the size of its first element, which presents a consistent way to obtain the element count. Otherwise, a separate foreach()<\/em> function must be crafted for each data type.<\/p>\n

Here is the expression: sizeof(a)\/sizeof(a[0])<\/code>, where a<\/code> is an array of any data type.<\/p>\n

To prove that this approach works, I created the following code. It identifies the size of array f[]<\/code> and spews out the elements based on the size<\/code> value calculated:<\/p>\n

2021_01_02-Lesson-a.c<\/a><\/h3>\n
\r\n#include <stdio.h>\r\n\r\nint main()\r\n{\r\n    float f[] = { 1.4, 5.7, 9.0, 22.7, 88.8 };\r\n    int size,x;\r\n\r\n    size = sizeof(f)\/sizeof(f[0]);\r\n    for( x=0; x<size; x++ )\r\n        printf(\"%.1f\\n\",f[x]);\r\n\r\n    return(0);\r\n}<\/pre>\n
The foreach<\/em> part of the code is handled by the for<\/em> loop at Line 9. Here is the output:<\/p>\n
1.4
\n5.7
\n9.0
\n22.7
\n88.8<\/code><\/p>\n
At this point, the goal is to create the foreach<\/em> equivalent. Unfortunately, it can’t be a function. That’s because passing an array to a function passes the array’s address, not the full array. So any sizeof<\/em> statement used within the function returns the size of the array’s address (a pointer, usually 8 bytes), not the size of the array in memory.<\/p>\n
No, to properly emulate a foreach()<\/em> keyword in C, you must create a macro. It combines both Lines 8 and 9 from the preceding example:<\/p>\n
#define foreach(a,b) for(int a=0;a<(sizeof(b)\/sizeof(b[0]));a++)<\/code><\/p>\n
Variable a<\/code> is declared as an int<\/em> within the macro. The rest of the assignment is a for<\/em> loop, with the array size calculation as its middle part. Variable b<\/code> represents the array.<\/p>\n
Obviously, this construction isn’t identical to a true foreach<\/em> statement, nor does it match any of the alternatives I’ve seen presented on various programming websites. Despite my best efforts, it works within its given parameters: The second argument must be an array and the first argument is an integer variable not otherwise used in the function:<\/p>\n
2021_01_02-Lesson-b.c<\/a><\/h3>\n
\r\n#include <stdio.h>\r\n\r\n#define foreach(a,b) for(int a=0;a<(sizeof(b)\/sizeof(b[0]));a++)\r\n\r\nint main()\r\n{\r\n    float f[] = { 1.4, 5.7, 9.0, 22.7, 88.8 };\r\n\r\n    foreach(x,f)\r\n        printf(\"%.1f\\n\",f[x]);\r\n\r\n    return(0);\r\n}<\/pre>\n
The output is identical to the first example.<\/p>\n
This approach also works for creating simple repeating loops. For example:<\/p>\n
2021_01_02-Lesson-c.c<\/a><\/h3>\n
\r\n#include <stdio.h>\r\n\r\n#define foreach(a,b) for(int a=0;a<(sizeof(b)\/sizeof(b[0]));a++)\r\n\r\nint main()\r\n{\r\n    char c[] = \"abcdefg\";\r\n\r\n    foreach(x,c)\r\n        puts(\"Whee!\");\r\n\r\n    return(0);\r\n}<\/pre>\n
You need not do anything with the variable; just let the loop repeat, as shown in this output:<\/p>\n
Whee!
\nWhee!
\nWhee!
\nWhee!
\nWhee!
\nWhee!
\nWhee!
\nWhee!<\/code><\/p>\n
I would hope that a future update to C — a significant update — might include such new keywords as foreach<\/em> and perhaps conveniences from other languages. Still, venerable C has survived without these additions. Obviously, anything you can think of can be programmed one way or the other. All you need is motivation and time.<\/p>\n","protected":false},"excerpt":{"rendered":"
The best way to create a foreach<\/em> keyword or function is to write a macro. 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-4521","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\/4521","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=4521"}],"version-history":[{"count":4,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/4521\/revisions"}],"predecessor-version":[{"id":4559,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/4521\/revisions\/4559"}],"wp:attachment":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=4521"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=4521"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=4521"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}