{"id":5949,"date":"2023-07-15T00:01:41","date_gmt":"2023-07-15T07:01:41","guid":{"rendered":"https:\/\/c-for-dummies.com\/blog\/?p=5949"},"modified":"2023-07-22T09:11:16","modified_gmt":"2023-07-22T16:11:16","slug":"using-scanf-to-build-a-string-part-ii","status":"publish","type":"post","link":"https:\/\/c-for-dummies.com\/blog\/?p=5949","title":{"rendered":"Using scanf()<\/em> to Build a String – Part II"},"content":{"rendered":"

Trying to salve my frustration with the scanf()<\/em> function, I decided in last week’s Lesson<\/a> to try to use scanf()<\/em> to build a string. Because the function terminates standard input at the first whitespace character (space, tab, newline), the strings input must be stored and the string built in memory.
\n
\nThe first change I made is to get rid of the buffer[]<\/code> array and replace it with a pointer. This move may seem redundant, as pointer s<\/code> is already used in the code to store the string. But I want to build a string. Therefore having one buffer for input and another for creating the string makes sense.<\/p>\n

2023_07_15-Lesson-a.c<\/a><\/h3>\n
\r\n#include <stdio.h>\r\n#include <stdlib.h>\r\n#include <string.h>\r\n\r\n#define SIZE 16\r\n\r\nint main()\r\n{\r\n    char *b,*s;\r\n\r\n    \/* allocate\/initalize buffers *\/<\/span>\r\n    b = malloc( SIZE * sizeof(char) );    \/* input *\/<\/span>\r\n    s = malloc( SIZE * sizeof(char) );    \/* string *\/<\/span>\r\n    if( b==NULL || s==NULL )\r\n    {\r\n        fprintf(stderr,\"Memory allocation error\\n\");\r\n        exit(1);\r\n    }\r\n    \/* initialize string storage *\/<\/span>\r\n    *b = *s = '\\0';\r\n\r\n    \/* fetch input *\/<\/span>\r\n    printf(\"Word: \");\r\n    scanf(\"%s\",b);\r\n    \/* copy the word *\/<\/span>\r\n    strcpy(s,b);\r\n\r\n    \/* output results *\/<\/span>\r\n    puts(s);\r\n\r\n    return(0);\r\n}<\/pre>\n
The code allocates storage for both pointers b<\/code> and s<\/code>, setting the size to SIZE<\/code> or 16 characters (15 characters plus one for the null character). Remember: the scanf()<\/em> function does not do bounds checking! Don’t input a long string just to see what happens and don’t ever use scanf()<\/em> in this manner in code you plan on releasing or even using non-experimentally.<\/p>\n
Both strings are tested for success: if( b==NULL || s==NULL )<\/code>. If either string is NULL<\/code>, the program mercifully stops.<\/p>\n
The strings are initialized, setting each to a null string<\/a>: *b = *s = '\\0';<\/code> This step may not be necessary, though I want to ensure that *b<\/code> and *s<\/code> are “real” strings.<\/p>\n
The rest of the code works like the original shown in last week’s Lesson. I added a printf()<\/em> statement to output a prompt. Here’s a sample run:<\/p>\n
Word: hello
\nhello<\/code><\/p>\n
The next step is to add a loop to collect words and build the string. This improvement requires a major change: Pointer s<\/code> must be reallocated each iteration of the loop to append the new word input:<\/p>\n
2023_07_15-Lesson-b.c<\/a><\/h3>\n
\r\n#include <stdio.h>\r\n#include <stdlib.h>\r\n#include <string.h>\r\n\r\n#define SIZE 16\r\n\r\nint main()\r\n{\r\n    char *b,*s;\r\n\r\n    \/* allocate\/initialize buffers *\/<\/span>\r\n    b = malloc( SIZE * sizeof(char) );    \/* input *\/<\/span>\r\n    s = malloc( sizeof(char) );            \/* string *\/<\/span>\r\n    if( b==NULL || s==NULL )\r\n    {\r\n        fprintf(stderr,\"Memory allocation error\\n\");\r\n        exit(1);\r\n    }\r\n    \/* initialize string storage *\/<\/span>\r\n    *b = *s = '\\0';\r\n\r\n    while(1)\r\n    {\r\n        \/* fetch input *\/<\/span>\r\n        printf(\"Word: \");\r\n        scanf(\"%s\",b);\r\n        \/* copy the word *\/<\/span>\r\n            \/* add two: space and null char *\/<\/span>\r\n        s = realloc(s,strlen(s) + strlen(b) + 2);\r\n        if( s==NULL )\r\n        {\r\n            fprintf(stderr,\"Reallocation error\\n\");\r\n            exit(1);\r\n        }\r\n        strcat(s,b);\r\n        strcat(s,\" \");\r\n    }\r\n\r\n    \/* output results *\/<\/span>\r\n    puts(s);\r\n\r\n    return(0);\r\n}<\/pre>\n
Pointer s<\/code> is allocated only a single character of storage for the null character: s = malloc( sizeof(char) );<\/code>  This improvement builds a short string, but it comes in handy when storage is reallocated later.<\/p>\n
And endless while<\/em> loop builds the string: while(1)<\/code><\/p>\n
Within the loop, the scanf()<\/em> function fetches input. The next statement reallocates storage for pointer s<\/code>:<\/p>\n
s = realloc(s,strlen(s) + strlen(b) + 2);<\/code><\/p>\n
The pointer’s new size s<\/code> is is based on its current size (strlen(s)<\/code>) plus the length of the string input (strlen(b)<\/code>) plus two. The two extra characters are for a space to separate the words and the null character.<\/p>\n
The ralloc()<\/em> function retains the original memory buffer contents. The result is a buffer that contains the original string plus storage for the new string input.<\/p>\n
Two strcat()<\/em> functions build the string. The first adds the input word. The second appends a space character.<\/p>\n
Output doesn’t happen, however, as the loop is endless. Further, why always append a space? Strings are more than words and spaces. I address these issues in next week’s Lesson<\/a>.<\/p>\n","protected":false},"excerpt":{"rendered":"
Building a string with the scanf()<\/em> function requires allocating memory on the fly. 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-5949","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\/5949","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=5949"}],"version-history":[{"count":7,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/5949\/revisions"}],"predecessor-version":[{"id":5971,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/5949\/revisions\/5971"}],"wp:attachment":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=5949"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=5949"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=5949"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}