{"id":5068,"date":"2021-11-27T00:01:11","date_gmt":"2021-11-27T08:01:11","guid":{"rendered":"https:\/\/c-for-dummies.com\/blog\/?p=5068"},"modified":"2021-12-04T09:11:27","modified_gmt":"2021-12-04T17:11:27","slug":"the-string-span-functions","status":"publish","type":"post","link":"https:\/\/c-for-dummies.com\/blog\/?p=5068","title":{"rendered":"The String Span Functions"},"content":{"rendered":"

When I scour the C for functions to assist my code, if often overlook a few that seem dull or confusing. One of those I’ve always glossed over is the strspn()<\/em> function, as well as its counterpart, strcspn()<\/em>.
\n
\nThese functions scan a string for the first occurrence of a given character — or for not of a given character.<\/p>\n

For example, if you want to discover where the first comma in a string is located, you use this statement:<\/p>\n

r = strcspn(\"Hello, world!\",\",\");<\/code><\/p>\n

The return value r<\/code> contains 5, which is the offset (starting with zero) where the comma is found in the string (char<\/em> array).<\/p>\n

Here are the two functions’ prototypes:<\/p>\n

size_t strspn(const char *s, const char *charset);
\nsize_t strcspn(const char *s, const char *charset);<\/code><\/p>\n

The first argument, s<\/code>, is the string to search. The second argument, charset<\/code>, is a string containing search characters. The return value is the offset within s<\/code> where one of the characters in charset<\/code> is either not found for strspn()<\/em> or not found for strcspn()<\/em>.<\/p>\n

Both functions are prototyped in the string.h<\/code> header file.<\/p>\n

The strspn()<\/em> function keeps looking through the first string as long as characters in the second string are found. Once a character in the first string doesn’t exist in the second, its offset is returned. The following code demonstrates:<\/p>\n

2021_11_27-Lesson-a.c<\/a><\/h3>\n
\r\n#include <stdio.h>\r\n#include <string.h>\r\n\r\nint main()\r\n{\r\n    char a[] = \"HELLO, WORLD!\";\r\n    char b[] = \"ABCDEFGHIJKLMNOPQRSTUVWXYZ\";\r\n    size_t r;\r\n\r\n    r = strspn(a,b);\r\n    printf(\"%zu = '%c'\\n\",r,a[r]);\r\n\r\n    return(0);\r\n}<\/pre>\n
At Line 10, the strspn()<\/em> function scans string a[]<\/code>. As long as it finds characters in string b[]<\/code> (the uppercase alphabet), it’s happy. When a non-matching character is encountered, its offset (starting at zero) is returned in variable r<\/code>. This value is output, along with the offending character:<\/p>\n
5 = ','<\/code><\/p>\n
The strcspn()<\/em> function does the opposite of strspn()<\/em>: It scans the first string until it finds a character available in the second:<\/p>\n
2021_11_27-Lesson-b.c<\/a><\/h3>\n
\r\n#include <stdio.h>\r\n#include <string.h>\r\n\r\nint main()\r\n{\r\n    char a[] = \"Hello, world!\";\r\n    char b[] = \" ,!\";\r\n    size_t r;\r\n\r\n    r = strcspn(a,b);\r\n    printf(\"%zu = '%c'\\n\",r,a[r]);\r\n\r\n    return(0);\r\n}<\/pre>\n
Above, string b[]<\/code> contains the non-alphabetic characters from string a[]<\/code>. The strcspn() <\/em>function at Line 10 searchings string a[]<\/code> for the first occurrence of any of these characters. Here’s the output:<\/p>\n
5 = ','<\/code><\/p>\n
The output is the same as the first example, though in this case the purpose of string b[]<\/code> is changed: It contains characters that stop the scan. Array element a[5]<\/code> is the comma, which stops the scan after the “o” in “Hello.”<\/p>\n
I can think of a few practical uses for both the strspn()<\/em> and strcspn()<\/em> function, though I’m more attracted to the strcspn()<\/em> function. In next week’s Lesson<\/a> I explore one way to use this function to plow through a string and pluck out words.<\/p>\n","protected":false},"excerpt":{"rendered":"
Standard library functions strspn<\/em> and strcspn<\/em> are oddballs that you may find useful. 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-5068","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\/5068","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=5068"}],"version-history":[{"count":4,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/5068\/revisions"}],"predecessor-version":[{"id":5095,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/5068\/revisions\/5095"}],"wp:attachment":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=5068"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=5068"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=5068"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}