Basics of Formatted Input/Output in C


Output with printf


Conversion Specifiers

A conversion specifier is a symbol that is used as a placeholder in a formatting string. For integer output (for example), %d is the specifier that holds the place for integers.

Here are some commonly used conversion specifiers (not a comprehensive list):

  %d    int (signed decimal integer) 
  %u    unsigned decimal integer 
  %f    floating point values (fixed notation) - float, double 
  %e    floating point values (exponential notation) 
  %s    string 
  %c    character  

Printing Integers

Printing Floating-point numbers

Printing characters and strings

output.c -- contains all of the above sample outputs. Try running it yourself




input1.c -- linked here
#include <stdio.h>

int main()
 int i;
 float f;
 char c;

 printf("Enter an integer and a float, then Y or N\n> ");
 scanf("%d%f%c", &i, &f, &c);

 printf("You entered:\n");
 printf("i = %d, f = %f, c = %c\n", i, f, c);


Sample run #1

User input underlined, to distinguish it from program output
Enter an integer and a float, then Y or N
> 34 45.6Y
You entered:
i = 34, f = 45.600, c = Y

Sample Run #2

Enter an integer and a float, then Y or N
> 12 34.5678 N
You entered:
i = 12, f = 34.568, c =  
Note that in this sample run, the character that was read was NOT the letter 'N'. It was the space. (Remember, white space not skipped on character reads).

This can be accounted for. Consider if the scanf line looked like this:

 scanf("%d%f %c", &i, &f, &c);
There's a space betwen the %f and the %c in the format string. This allows the user to type a space. Suppose this is the typed input:
 12 34.5678 N
Then the character variable c will now contain the 'N'.

input2.c -- a version of the example with this change is linked here

Interactive Input

You can make input more interactive by prompting the user more carefully. This can be tedious in some places, but in many occasions, it makes programs more user-friendly. Example:
  int age;
  double gpa;
  char answer;

  printf("Please enter your age: ");
  scanf("%d", &age);
  printf("Please enter your gpa: ");
  scanf("%lf", %gpa);
  printf("Do you like pie (Y/N)? ");
  scanf("%c", %answer); 

A good way to learn more about scanf is to try various inputs in various combinations, and type in test cases -- see what happens!

printf/scanf with C-strings

An entire C-style string can be easily printed, by using the %s formatting symbol, along with the name of the char array storing the string (as the argument filling in that position):

  char greeting[] = "Hello";

  printf("%s", greeting);     // prints the word "Hello"
Be careful to only use this on char arrays that are being used as C-style strings. (This means, only if the null character is present as a terminator).

Similarly, you can read a string into a char array with scanf. The following call allows the entry of a word (up to 19 characters and a terminating null character) from the keyboard, which is stored in the array word1:

  char word1[20];

  scanf("%s", word1);

Characters are read from the keyboard until the first "white space" (space, tab, newline) character is encountered.  The input is stored in the character array and the null character is automatically appended.

Note also that the & was not needed in the scanf call (word1 was used, instead of &word1). This is because the name of the array by itself (with no index) actually IS a variable that stores an address (a pointer).