{"id":7585,"date":"2026-05-23T00:01:32","date_gmt":"2026-05-23T07:01:32","guid":{"rendered":"https:\/\/c-for-dummies.com\/blog\/?p=7585"},"modified":"2026-05-16T20:25:41","modified_gmt":"2026-05-17T03:25:41","slug":"ctype-functions-isalnum-isalpha-and-isdigit","status":"publish","type":"post","link":"https:\/\/c-for-dummies.com\/blog\/?p=7585","title":{"rendered":"Ctype Functions: isalnum()<\/em>, isalpha()<\/em> and isdigit()<\/em>"},"content":{"rendered":"

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\nThe ctype functions isalnum()<\/em>, isalpha()<\/em>, and isdigit()<\/em> examine the alphanumeric ASCII characters. Each of these functions are related, so I cover them together in this post that continues my exploration of the ctype functions<\/a>.
\n
\nThe relationship between these functions works because the positive results of isalpha()<\/em> and idigit()<\/em> are useful in crafting the isalnum()<\/em> function: If a character is both alphabetic (isalpha()<\/em>) and a digit (isdigit()<\/em>), then it’s also alphanumeric (isalnum()<\/em>).<\/p>\n

The following code outputs the printable ASCII characters, from the space (code 32 or 0x20) through the DEL (code 127 or 0x7F). ASCII code 127 is “DEL” or delete, which doesn’t print but isn’t the same as a space. (More on this condition in another Lesson.) For each line, text is output to describe whether the character is alphanumeric and if so whether it’s alphabetic or a digit.<\/p>\n

2026_05_23-Lesson-a.c<\/a><\/h3>\n
\r\n#include <stdio.h>\r\n#include <ctype.h>\r\n\r\n#define DEL 127\r\n\r\nint main()\r\n{\r\n    int ch;\r\n\r\n    for( ch=' '; ch<=DEL; ch++ )\r\n    {\r\n        printf(\"%3d %2X '%c' is\",ch,ch,ch);\r\n        if( isalnum(ch) )\r\n        {\r\n            printf(\" alphanumeric\");\r\n            if( isalpha(ch) )\r\n                printf(\" (alphabetic)\");\r\n            if( isdigit(ch) )\r\n                printf(\" (digit)\");\r\n        }\r\n        else\r\n        {\r\n            printf(\" non-alphanumeric\");\r\n        }\r\n        putchar('\\n');\r\n    }\r\n\r\n    return 0;\r\n}<\/pre>\n
The code uses the three ctype functions: isalnum()<\/em>, isalpha()<\/em>, and isdigit()<\/em>. A for<\/em> loop plows through characters ' '<\/code> through '~'<\/code>. Various printf()<\/em> statements output the qualities of these characters, specifically tagging alphanumeric and non-alphanumeric, which alphabetic and digit characters called out separately.<\/p>\n
I’m unhappy with the nested if<\/em> statements, but given the condition that the character is either alphabetic or digit, it works. And, yes, it could be an if-else<\/em> statement but that would remove the isdigit()<\/em> test.<\/p>\n
Here’s abbreviated output:<\/p>\n
 32 20 ' ' is non-alphanumeric\r\n 33 21 '!' is non-alphanumeric\r\n 34 22 '\"' is non-alphanumeric\r\n 35 23 '#' is non-alphanumeric\r\n...\r\n 46 2E '.' is non-alphanumeric\r\n 47 2F '\/' is non-alphanumeric\r\n 48 30 '0' is alphanumeric (digit)\r\n 49 31 '1' is alphanumeric (digit)\r\n 50 32 '2' is alphanumeric (digit)\r\n 51 33 '3' is alphanumeric (digit)\r\n 52 34 '4' is alphanumeric (digit)\r\n 53 35 '5' is alphanumeric (digit)\r\n 54 36 '6' is alphanumeric (digit)\r\n 55 37 '7' is alphanumeric (digit)\r\n 56 38 '8' is alphanumeric (digit)\r\n 57 39 '9' is alphanumeric (digit)\r\n 58 3A ':' is non-alphanumeric\r\n 59 3B ';' is non-alphanumeric\r\n...\r\n 63 3F '?' is non-alphanumeric\r\n 64 40 '@' is non-alphanumeric\r\n 65 41 'A' is alphanumeric (alphabetic)\r\n 66 42 'B' is alphanumeric (alphabetic)\r\n 67 43 'C' is alphanumeric (alphabetic)\r\n...\r\n 89 59 'Y' is alphanumeric (alphabetic)\r\n 90 5A 'Z' is alphanumeric (alphabetic)\r\n 91 5B '[' is non-alphanumeric\r\n 92 5C '\\' is non-alphanumeric\r\n 93 5D ']' is non-alphanumeric\r\n 94 5E '^' is non-alphanumeric\r\n 95 5F '_' is non-alphanumeric\r\n 96 60 '`' is non-alphanumeric\r\n 97 61 'a' is alphanumeric (alphabetic)\r\n 98 62 'b' is alphanumeric (alphabetic)\r\n...\r\n121 79 'y' is alphanumeric (alphabetic)\r\n122 7A 'z' is alphanumeric (alphabetic)\r\n123 7B '{' is non-alphanumeric\r\n124 7C '|' is non-alphanumeric\r\n125 7D '}' is non-alphanumeric\r\n126 7E '~' is non-alphanumeric\r\n127 7F '' is non-alphanumeric<\/pre>\n
Coding your own version of these ctype functions requires some basic comparisons, as shown in the following code. Note that I use the results of both isalpha()<\/em> and isdigit()<\/em> to craft my isalnum()<\/em> function:<\/p>\n
2026_05_23-Lesson-b.c<\/a><\/h3>\n
\r\n#include <stdio.h>\r\n\r\n#define TRUE 1\r\n#define FALSE 0\r\n#define DEL 127\r\n\r\nint isalpha(int c)\r\n{\r\n    if( (c>='A' && c<='Z') || (c>='a' && c<='z') )\r\n            return TRUE;\r\n    return FALSE;\r\n}\r\n\r\nint isdigit(int c)\r\n{\r\n    if( c>='0' && c<='9' )\r\n        return TRUE;\r\n    return FALSE;\r\n}\r\n\r\nint isalnum(int c)\r\n{\r\n    if( isalpha(c) || isdigit(c) )\r\n        return TRUE;\r\n    return FALSE;\r\n}\r\n\r\nint main()\r\n{\r\n    int ch;\r\n\r\n    for( ch=' '; ch<=DEL; ch++ )\r\n    {\r\n        printf(\"%3d %2X '%c' is\",ch,ch,ch);\r\n        if( isalnum(ch) )\r\n        {\r\n            printf(\" alphanumeric\");\r\n            if( isalpha(ch) )\r\n                printf(\" (alphabetic)\");\r\n            if( isdigit(ch) )\r\n                printf(\" (digit)\");\r\n        }\r\n        else\r\n        {\r\n            printf(\" non-alphanumeric\");\r\n        }\r\n        putchar('\\n');\r\n    }\r\n\r\n    return 0;\r\n}<\/pre>\n
The isalpha()<\/em> function perform a complex comparison to ensure that the character is in the range 'A'<\/code> through 'Z'<\/code> or 'a'<\/code> through 'z'<\/code>. When either condition is true, the character is alphabetic, TRUE is returned.<\/p>\n
The isdigit()<\/em> function returns TRUE when the character is in the range of ‘0’ through ‘9’.<\/p>\n
Finally, the isalnum()<\/em> function determines whether the character is alphabetic (isalpha()<\/em>) or numeric (isdigit()<\/em>) and returns TRUE is so.<\/p>\n
The program’s output is the same.<\/p>\n
In next week’s Lesson I cover the is isgraph()<\/em> and isprint()<\/em> ctype functions.<\/p>\n","protected":false},"excerpt":{"rendered":"
These ctype testing functions work on letters of the alphabet and numeric digits. 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-7585","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\/7585","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=7585"}],"version-history":[{"count":5,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/7585\/revisions"}],"predecessor-version":[{"id":7599,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/7585\/revisions\/7599"}],"wp:attachment":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=7585"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=7585"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=7585"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}