{"id":4516,"date":"2020-12-26T00:01:56","date_gmt":"2020-12-26T08:01:56","guid":{"rendered":"https:\/\/c-for-dummies.com\/blog\/?p=4516"},"modified":"2021-01-02T08:26:15","modified_gmt":"2021-01-02T16:26:15","slug":"in-search-of-the-foreach-keyword","status":"publish","type":"post","link":"https:\/\/c-for-dummies.com\/blog\/?p=4516","title":{"rendered":"In Search of the foreach<\/em> Keyword"},"content":{"rendered":"

C has three looping keywords: do<\/em>, for<\/em>, and while<\/em>. These keywords construct a block of statements that repeat, hopefully but not necessarily with a terminating condition. Other programming languages offer additional looping keywords, including the popular and useful foreach<\/em>.
\n
\nForgetting about the goto<\/em> keyword, the foreach<\/em> keyword finds itself in languages that offer a richer variety of data constructions than C. In these languages, something I call a collection<\/em> exists. This data structure is where you often see foreach<\/em> used, though it can also be used with any array of data.<\/p>\n

Consider the following data set:<\/p>\n

$list = { \"Dr. No\", \"From Russia with Love\", \"Goldfinger\", \"Thunderball\" }<\/code><\/p>\n

One way a foreach<\/em> keyword can be used on this collection is:<\/p>\n

foreach( $item in $list )
\n    print $item<\/code><\/p>\n

The above code (assume it’s valid) outputs each string in the $list<\/code> collection, represented as variable $item<\/code> in the loop. A C language equivalent might be:<\/p>\n

for( i=0; i<items; i++ )
\n    printf(\"%d\\n\",list[i]);<\/code><\/p>\n

Unlike other types of loop, the foreach<\/em> statement lacks an iteration counter. Instead, its job is to process the collection (the second argument) individually, using the first argument to repeatedly represent the items in the list.<\/p>\n

A foreach<\/em> statement in C would operate on an array. Its loop count is the number of elements in an array. The C language lacks an operator (or method) to return an array’s size, so a specific calculation must be made to determine how many elements the array holds. This calculation is dependent upon the array’s data type.<\/p>\n

For example, in a character array, the number of elements is determined by dividing the memory size of the array by the size of the data type (char<\/em>). This process is done differently for each array data type: char<\/em>, int<\/em>, float<\/em>, pointers, and so on. The following code demonstrates:<\/p>\n

2020_12_26-Lesson-a.c<\/a><\/h3>\n
\r\n#include <stdio.h>\r\n\r\nint main()\r\n{\r\n    char c[] = { 'a', 'b', 'c', 'd' };\r\n    int i[] = { 10, 100 };\r\n    float f[] = { 1.4, 5.7, 9.0, 22.7, 88.8 };\r\n    char *s[3] = { \"this\", \"that\", \"the other\" };\r\n\r\n    puts(\"Array element count:\");\r\n    printf(\"c = %lu\\n\",sizeof(c)\/sizeof(char) );\r\n    printf(\"i = %lu\\n\",sizeof(i)\/sizeof(int) );\r\n    printf(\"f = %lu\\n\",sizeof(f)\/sizeof(float) );\r\n    printf(\"s = %lu\\n\",sizeof(s)\/sizeof(char*) );\r\n\r\n    return(0);\r\n}<\/pre>\n
In the source code, the expression sizeof(n)\/sizeof(type)<\/code> returns the number of elements in the array: n<\/code> is an array variable and type<\/code> is the variable’s data type. For the character pointer array, the data type is a char<\/em> pointer, as shown in Line 14 with sizeof(char*)<\/code>.<\/p>\n
Here’s the program’s output:<\/p>\n
Array element count:
\nc = 4
\ni = 2
\nf = 5
\ns = 3<\/code><\/p>\n
It’s possible to calculate the number of elements in the array, but what’s not obvious to the code is the array’s data type; the sizeof<\/em> operator returns the proper sizes, but it can’t guess at the type of data it’s examining. This difficulty must be overcome if a foreach<\/em> keyword is to be emulated in C.<\/p>\n
One trick is to use the size of an element in the array instead of the data type. For example, in an int<\/em> array, element a[0]<\/code> is the size of an integer. Likewise, in a char<\/em> array, a[0]<\/code> is the size of a char<\/em>. Using this trick is demonstrated here:<\/p>\n
2020_12_26-Lesson-b.c<\/a><\/h3>\n
\r\n#include <stdio.h>\r\n\r\nint main()\r\n{\r\n    char c[] = { 'a', 'b', 'c', 'd' };\r\n    int i[] = { 10, 100 };\r\n    float f[] = { 1.4, 5.7, 9.0, 22.7, 88.8 };\r\n    char *s[3] = { \"this\", \"that\", \"the other\" };\r\n\r\n    puts(\"Array element count:\");\r\n    printf(\"c = %lu\\n\",sizeof(c)\/sizeof(c[0]) );\r\n    printf(\"i = %lu\\n\",sizeof(i)\/sizeof(i[0]) );\r\n    printf(\"f = %lu\\n\",sizeof(f)\/sizeof(f[0]) );\r\n    printf(\"s = %lu\\n\",sizeof(s)\/sizeof(s[0]) );\r\n\r\n    return(0);\r\n}<\/pre>\n
This solution represents the first step to emulating the foreach<\/em> keyword, as the array size is obtained. I continue to explore this concept in next week’s Lesson<\/a>.<\/p>\n","protected":false},"excerpt":{"rendered":"
The foreach<\/em> looping keyword is found in many of the newer, trendier programming languages. Why not emulate it for C? 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-4516","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\/4516","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=4516"}],"version-history":[{"count":11,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/4516\/revisions"}],"predecessor-version":[{"id":4558,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/4516\/revisions\/4558"}],"wp:attachment":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=4516"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=4516"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=4516"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}