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Basics of the scanf() Function in C Programming

For the input of specific types of variables in the C programming language, you’ll find that the scanf() function comes in handy. It’s not a general-purpose input function, and it has some limitations, but it’s great for testing code or grabbing values.

In a way, you could argue that scanf() is the input version of the printf() function. For example, it uses the same conversion characters (the % placeholder-things). Because of that, scanf() is quite particular about how text is input. Here’s the format:

#include <stdio.h>
int scanf(const char *restrict format,...);

Scary, huh? Just ignore it for now. Here’s a less frightening version of the format:

scanf("placeholder",variable);

In this version, placeholder is a conversion character, and variable is a type of variable that matches the conversion character. Unless it’s a string (char array), the variable is prefixed by the & operator.

The scanf() function is prototyped in the stdio.h header file, so you must include that file when you use the function.

Here are some scanf() examples:

scanf("%d",&highscore);

The preceding statement reads an integer value into the variable highscore. This assumes that highscore is an int variable.

scanf("%f",&temperature);

The preceding scanf() statement waits for a floating-point value to be input, which is then stored in the temperature variable.

scanf("%c",&key);

In the preceding line, scanf() accepts the first character input and stores it in the key variable.

scanf("%s",firstname);

The %s placeholder is used to read in text, but only until the first white space character is encountered. So a space or a tab or the Enter key terminates the string. (That sucks.) Also, firstname is a char array, so it doesn’t need the & operator in the scanf() function.

How to read a string with scanf()

One of the most common ways to put the scanf() function to use is to read in a chunk of text from standard input. To meet that end, the %s conversion character is used — just like in printf(), but with input instead of output.

SCANF() SWALLOWS A STRING

#include <stdio.h>
int main()
{
  char firstname[15];
  printf("Type your first name: ");
  scanf("%s",firstname);
  printf("Pleased to meet you, %s.\n",firstname);
  return(0);
}

Exercise 1: Type the source code from scanf() Swallows a String into a new project, ex0712, in Code::Blocks. Build and run.

Line 5 declares a char array — a string variable — named firstname. The number in the brackets indicates the size of the array, or the total number of characters that can be stored there. The array isn’t assigned a value, so it’s created empty. Basically, the statement at Line 5 sets aside storage for up to 15 characters.

The scanf() function in Line 8 reads a string from standard input and stores it in the firstname array. The %s conversion character directs scanf() to look for a string as input, just as %s is a placeholder for strings in printf()’s output.

Exercise 2: Modify the source code from scanf() Swallows a String so that a second string is declared for the person’s last name. Prompt the user for their last name as well, and then display both names by using a single printf() function.

  • The number in the brackets (refer to Line 5) gives the size of the char array, or the length of the string, plus one.

  • When you create a char array, or string variable, ensure that you create it of a size large enough to hold the text. That size should be the maximum number of characters plus one.

  • The reason for increasing the char array size by one is that all strings in C end with a specific termination character. It’s the NULL character, which is written as \0. The compiler automatically adds the \0 to the end of string values you create in your source code, as well as text read by various text-input functions.

    You must remember to add room for that character when you set aside storage for string input.

How to read values with scanf()

The scanf() function can do more than read strings. It can read in any value specified by a conversion character.

SCANF() EATS AN INTEGER

#include <stdio.h>
int main()
{
  int fav;
  printf("What is your favorite number: ");
  scanf("%d",&fav);
  printf("%d is my favorite number, too!\n",fav);
  return(0);
}

In scanf() Eats an Integer, the scanf() function reads in an integer value. The %d conversion character is used, just like printf() — indeed, it’s used in Line 9. That character directs scanf() to look for an int value for variable fav.

Exercise 3: Create a project, ex0714, using the source code shown in scanf() Eats an Integer. Build and run. Test the program by typing various integer values, positive and negative.

Perhaps you’re wondering about the ampersand (&) in the scanf() function. The character is a C operator — specifically, the memory address operator. It’s one of the advanced features in C that’s related to pointers. An ampersand must prefix any variable specified in the scanf() function. The exception is an array, such as the firstname char array in scanf() Eats an Integer.

Try running the program again, but specify a decimal value, such as 41.9, or type text instead of a number.

The reason you see incorrect output is that scanf() is very specific. It fetches only the variable type specified by the conversion character. So if you want a floating-point value, you must specify a float variable and use the appropriate conversion character; %f, in that case.

Exercise 4: Modify the source code from scanf() Eats an Integer so that a floating-point number is requested, input, and displayed.

  • You don’t need to prefix a char array variable with an ampersand in the scanf() function; when using scanf() to read in a string, just specify the string variable name.

  • The scanf() function stops reading text input at the first white space character, space, tab, or Enter key.

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