{"id":6779,"date":"2025-01-25T00:01:14","date_gmt":"2025-01-25T08:01:14","guid":{"rendered":"https:\/\/c-for-dummies.com\/blog\/?p=6779"},"modified":"2025-02-01T12:10:27","modified_gmt":"2025-02-01T20:10:27","slug":"using-the-bsearch-function-to-find-strings","status":"publish","type":"post","link":"https:\/\/c-for-dummies.com\/blog\/?p=6779","title":{"rendered":"Using the bsearch()<\/em> Function to Find Strings"},"content":{"rendered":"

When learning the bsearch()<\/em> function, it helps to start with integers, as demonstrated in last week’s Lesson<\/a>. When the data is more complex, however, additional programming kung-fu is required to sort and search. The first hill to climb in this adventure is to hunt down a string.
\n
\nBefore setting out, remember that to find one string inside another you use the strstr()<\/em> function<\/a>. For finding a single string in an array of strings, the bsearch()<\/em> function works well.<\/p>\n

The issue with an array of strings is that it’s a cleverly disguised array of pointers: <\/p>\n

char *fruit[] = {
\n    \"cantalope\", \"grapefruit\", \"fig\", \"melon\",
\n    \"banana\", \"apple\", \"kiwi\", \"lemon\"
\n};<\/code><\/p>\n

The fruit[]<\/code> array holds the addresses of the strings it references. These addresses are character pointers. This data type is what’s referenced in the qsort()<\/em> function, which sorts the array, as well as the bsearch()<\/em> function, which searches for a matching string in the array.<\/p>\n

Here’s the qsort()<\/em> function’s format:<\/p>\n

qsort( fruit, size, sizeof(char *), compare);<\/code><\/p>\n

The array and its size value come first (size<\/code> is the number of elements). The next argument is the size of each element, which is not a string length. No, it’s the size of a character pointer, char *<\/code>. Finally comes the compare()<\/em> function’s name.<\/p>\n

Here are the arguments for the bsearch()<\/em> function:<\/p>\n

bsearch( &key, fruit, size, sizeof(char *), compare );<\/code><\/p>\n

The address of string key<\/code> is what the bsearch()<\/em> function searches for. The rest of the arguments are the same as for the qsort()<\/em> function.<\/p>\n

Both functions can share the same compare()<\/em> function. For this operation the strcmp()<\/em> function compares strings. But remember that the compare()<\/em> function is sent two void<\/em> pointers. These must be typecast to strings and properly addressed as pointer-pointers for the comparison to work:<\/p>\n

int compare(const void *a, const void *b)
\n{
\n    return( strcmp(*(const char **)a,*(const char **)b) );
\n}<\/code><\/p>\n

The (const char **)<\/code> typecast identifies each variable as a pointer-pointer, or the address of a string. (I find “address of a string” to be a better way to describe the pointer-pointer thing.) This value is an address, which is why the cast is prefixed with the *<\/code> to dereference the address and represent the string itself. These two values are compared in the strcmp()<\/em> function, with the result returned. This operation applies to both the qsort()<\/em> and bsearch()<\/em> functions to sort and search the array of strings.<\/p>\n

Here is sample code:<\/p>\n

2025_01_25-Lesson.c<\/a><\/h3>\n
\r\n#include <stdio.h>\r\n#include <stdlib.h>\r\n#include <string.h>\r\n\r\nint compare(const void *a, const void *b)\r\n{\r\n    return( strcmp(*(const char **)a,*(const char **)b) );\r\n}\r\n\r\nint main()\r\n{\r\n    char *fruit[] = {\r\n        \"cantalope\", \"grapefruit\", \"fig\", \"melon\",\r\n        \"banana\", \"apple\", \"kiwi\", \"lemon\"\r\n    };\r\n    char *key;    \/* input buffer must be allocated *\/<\/span>\r\n    char **r;\r\n    int size;\r\n\r\n    \/* obtain array size *\/<\/span>\r\n    size = sizeof(fruit)\/sizeof(char *);\r\n\r\n    \/* locate a value *\/<\/span>\r\n    key = malloc(32);\r\n    if (key==NULL) exit(1);\r\n    printf(\"Fruit: \");\r\n    scanf(\"%s\",key);\r\n\r\n    \/* sort the array *\/<\/span>\r\n    qsort( fruit, size, sizeof(char *), compare);\r\n\r\n    \/* locate a value *\/<\/span>\r\n    r = bsearch( &key, fruit, size, sizeof(char *), compare );\r\n    if( r!=NULL )\r\n        printf(\"Key %s found!\\n\",*r);\r\n    else\r\n        printf(\"Key %s not found.\\n\",key);\r\n\r\n    return 0;\r\n}<\/pre>\n
The program’s output:<\/p>\n
Fruit: banana
\nKey banana found!<\/code><\/p>\n
Just as with sorting complex data, the tricky part is to properly code the compare()<\/em> function. For next week’s Lesson<\/a>, I apply this insanity to sorting and searching an array of structures.<\/p>\n","protected":false},"excerpt":{"rendered":"
The deal with finding strings is that darned double-asterisk pointer notation! 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-6779","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\/6779","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=6779"}],"version-history":[{"count":6,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/6779\/revisions"}],"predecessor-version":[{"id":6827,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/6779\/revisions\/6827"}],"wp:attachment":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=6779"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=6779"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=6779"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}