{"id":2183,"date":"2016-10-29T00:01:49","date_gmt":"2016-10-29T07:01:49","guid":{"rendered":"http:\/\/c-for-dummies.com\/blog\/?p=2183"},"modified":"2016-10-15T12:23:31","modified_gmt":"2016-10-15T19:23:31","slug":"a-scanf-string-trick","status":"publish","type":"post","link":"https:\/\/c-for-dummies.com\/blog\/?p=2183","title":{"rendered":"A scanf()<\/em> String Trick"},"content":{"rendered":"

I’m not a fan of the scanf()<\/em> function, despite how handy it can be. It’s a great teaching tool, but for a program that accepts string input, you need to use fgets()<\/em> instead. Still, scanf()<\/em> can be used to read a string, providing that you know the whitespace requirements beforehand.
\n
\nAs a review, the scanf()<\/em> function fetches specific type of information from standard input. The format for desired input is specified as the function’s first argument, a string. That string typically contains a single placeholder character, such as %d<\/code> for an int<\/em> value.<\/p>\n

After the formatting string, scanf()<\/em>‘s second argument is pointer variable; the &<\/code> operator is prefixed to the variable if it’s not already a pointer. The goal is to pass the variable’s address to the function so that the value fetched from standard input is stored in that variable.<\/p>\n

Here’s a sample:<\/p>\n

scanf(\"%c\",&ch);<\/code><\/p>\n

A single character, represented by %c<\/code>, is fetched from the standard input stream and saved into char<\/em> variable ch<\/code>.<\/p>\n

What frustrates me about scanf()<\/em> and strings is the function’s fixation with whitespace characters: At the sign of the first space, tab, or newline character, the function stops reading input. So scanf()<\/em> isn’t a string-input function; it’s more of a word-input function.<\/p>\n

You can take advantage of scanf()<\/em>‘s word-input aspect to read multiple words in a string, but only when you’re certain of the exact number of words.<\/p>\n

As an example, the following program asks the user to type in their first, middle, and last names — three character chunks separated by whitespace:<\/p>\n

\r\n#include <stdio.h>\r\n\r\nint main()\r\n{\r\n    char first[24], middle[24], last[24];\r\n\r\n    printf(\"Type your name (first middle last): \");\r\n    scanf(\"%23s\",first);\r\n    scanf(\"%23s\",middle);\r\n    scanf(\"%23s\",last);\r\n    printf(\"Glad to meet you, %s %s %s\\n\",\r\n            first,\r\n            middle,\r\n            last);\r\n\r\n    return(0);\r\n}<\/pre>\n
I set the buffer size to 24 characters for each of the char<\/em> arrays (strings). Further, the formatting string for the scanf()<\/em> functions uses 23 to help truncate input. (The 24th character is the null, \\0<\/code>, at the end of the string.) Here’s a sample run:<\/p>\n
Type your name (first middle last): Arthur Templeton Grockmeister\r\nGlad to meet you, Arthur Templeton Grockmeister<\/code><\/pre>\n
The three scanf()<\/em> functions read each work chunk from the input stream: The scanf()<\/em> function at Line 8 stops reading after the first space. The next scanf()<\/em> function at Line 9 stops after the second space. The final scanf()<\/em> function (Line 10) stops at the newline character.<\/p>\n
If you type a name with more than 23 characters, input is cropped, thanks to the %23s<\/code> string format placeholder:<\/p>\n
Type your name (first middle last): Art T Grockalockfickafockameister\r\nGlad to meet you, Art T Grockalockfickafockamei<\/code><\/pre>\n
If the user types a fewer than three words, then the program continues to wait until all three scanf()<\/em> functions are fed. That’s not good program design, of course, which is why I instead prefer a single string-swallowing function like fgets()<\/em>.<\/p>\n","protected":false},"excerpt":{"rendered":"
The scanf()<\/em> function can’t read strings with whitespace in them, so here’s a clever trick. 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-2183","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\/2183","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=2183"}],"version-history":[{"count":3,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/2183\/revisions"}],"predecessor-version":[{"id":2190,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/2183\/revisions\/2190"}],"wp:attachment":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=2183"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=2183"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=2183"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}