{"id":4313,"date":"2020-08-29T00:01:01","date_gmt":"2020-08-29T07:01:01","guid":{"rendered":"https:\/\/c-for-dummies.com\/blog\/?p=4313"},"modified":"2020-09-05T08:47:07","modified_gmt":"2020-09-05T15:47:07","slug":"unraveling-the-scanf-function","status":"publish","type":"post","link":"https:\/\/c-for-dummies.com\/blog\/?p=4313","title":{"rendered":"Unraveling the scanf()<\/em> Function"},"content":{"rendered":"

One of the things I dread in my books, is foisting the scanf()<\/em> function upon a beginner. I have no way around this frustration.
\n
\nThe scanf()<\/em> function is useful in that it cleanly gathers input, converting it into a value. It’s horrid for reading strings, but the function is vital for basic input and getting a beginner started. My issue is that the format is straight from the fiery pits of Hell, darkly illustrated in Figure 1.<\/p>\n

\"The

Figure 1. How the scanf()<\/em> function’s arguments are put together.<\/p><\/div>\n

Here you go, eager beginner! Good luck! But let me explain what’s going on.<\/p>\n

I believe beginners have a good handle on the function thing: They recognize that scanf()<\/em> is a function. They can even stomach the two arguments, separated by a comma. But these are the most wily arguments possible: \"%d\"<\/code> and &a<\/code>.<\/p>\n

The first mystery item is a format string. It uses cryptic placeholders to represent the type of data requested.<\/p>\n

The second mystery item is the address of a variable, a concept relying upon a knowledge of pointers. No need to mention that pointers are the most terrifying aspect of C or any programming language, a topic postponed well into adulthood and completely out of place at the start of any training material. Yet here it is.<\/p>\n

Like most cryptic things, scanf()<\/em> is powerful. Its flexibility allows you to comb standard input, setting whatever text flows in to a given variable. This variable must be passed as a pointer argument; it would subtract from scanf()<\/em>‘s usefulness to otherwise assign it a specific data type, say to instead return the input value. Therefore the function must be cryptic to a degree because the data type placeholder must match the variable’s data type, both of which must be flexible.<\/p>\n

Beginners can tolerate scanf()<\/em> as they learn, but I desire to dive deeper. Consider the format string. It’s pretty much the same as the printf()<\/em> format string, so must the input match exactly?<\/p>\n

2020_08_29_Lesson.c<\/a><\/h3>\n
\r\n#include <stdio.h>\r\n\r\nint main()\r\n{\r\n    int a;\r\n\r\n    printf(\"How old are you? \");\r\n    scanf(\"I am %d years old\",&a);\r\n    printf(\"I'm %d, too!\\n\",a);\r\n    \r\n    return(0);\r\n}<\/pre>\n
The scanf()<\/em> format string at Line 8 contains some text in addition to the placeholder. This specific format restricts input only to text that matches.<\/p>\n
For my first sample run, I just typed in a value:<\/p>\n
How old are you? 29
\nI'm 266395685, too!<\/code><\/p>\n
Oops. The format string demands more text input. Here’s my second sample run:<\/p>\n
How old are you? I am 29 years old
\nI'm 29, too!<\/code><\/p>\n
In this run, I typed the full text, I am 29 years old<\/code>. This is the input scanf()<\/em> is looking for. It translates the only value, but the rest of the text input is necessary for proper interpretation.<\/p>\n
Next, I tried to mess with the function by typing only the first part of the format string:<\/p>\n
How old are you? I am 29<\/p>\n
years old
\nI'm 29, too!<\/code><\/p>\n
After typing I am 29<\/code> I pressed Enter. Nothing. Enter again. Nothing. Only when I typed the rest of the format string did scanf()<\/em> finally process the input.<\/p>\n
This formatting string quirk isn’t the only oddity with scanf()<\/em>. Another way to abuse this function is covered in next week’s Lesson<\/a>.<\/p>\n","protected":false},"excerpt":{"rendered":"
One of the most despised functions in C, scanf()<\/em> is both useful and mysterious. 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-4313","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\/4313","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=4313"}],"version-history":[{"count":8,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/4313\/revisions"}],"predecessor-version":[{"id":4355,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=\/wp\/v2\/posts\/4313\/revisions\/4355"}],"wp:attachment":[{"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=4313"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=4313"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/c-for-dummies.com\/blog\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=4313"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}