1.2. Input/Output (printf and scanf)

C’s printf function prints values to the terminal, and the scanf function reads in values entered by a user. The printf and scanf functions belong to C’s standard I/O library, which needs to be explicitly included at the top of any .c file that uses these functions using #include <stdio.h>. In this section, we introduce the basics of using printf and scanf in C programs. Chapter 2’s I/O section discusses C’s input and output functions in more detail.

1.2.1. printf

C’s printf function is very similar to formatted print in Python, where the caller specifies a format string to print. The format string often contains formatting specifiers such as special characters that will print tabs (\t) or newlines (\n), or placeholders for values in the output. Placeholders consist of % followed by a type specifier letter (e.g. %d represents a placeholder for an integer value). For each placeholder in the format string, printf expects an additional argument. Here’s an example program in Python and C with formatted output:

Table 1. Comparison of the syntax for printing in Python and C.
Python version C version 
# Python formatted print example


def main():

    print("Name: %s,  Info:" % "Vijay")
    print("\tAge: %d \t Ht: %g" %(20,5.9))
    print("\tYear: %d \t Dorm: %s" %(3, "Alice Paul"))

# call the main function:
main()
 
/* C printf example */
#include <stdio.h> // needed for printf

int main() {

    printf("Name: %s,  Info:\n", "Vijay");
    printf("\tAge: %d \t Ht: %g\n",20,5.9);
    printf("\tYear: %d \t Dorm: %s\n",
            3,"Alice Paul");

    return 0;
}

When run, both versions of this program produce identically formatted output:

Name: Vijay,  Info:
	Age: 20 	 Ht: 5.9
	Year: 3 	 Dorm: Alice Paul

The main difference between C’s printf and Python’s print functions are that the Python version implicitly prints a newline character at the end of the output string and the C version does not. As a result, the C format strings in the above have newline (\n) characters at the end to explicitly print a newline character. The syntax for listing the argument values for the placeholders in the format string is also slightly different in C’s printf and Python’s print functions.

C uses the same formatting placeholders as Python does for specifying different types of values. The example above demonstrates the following formatting placeholders:

%g:  placeholder for a float (or double) value
%d:  placeholder for a decimal value (int, short, char)
%s:  placeholder for a string value

C additionally supports the %c placeholder for printing out a character value. This placeholder is useful when a programmer wants to print out the ASCII character associated with a particular numeric encoding. Here’s an example C code snippet that prints out a char as its numeric value (%d) and as its character encoding (%c):

// example printing a char value as its decimal representation (%d)
// and as the ASCII character that its value encodes (%c)

char ch;

ch = 'A';
printf("ch value is %d which is the ASCII value of  %c\n", ch, ch);

ch = 99;
printf("ch value is %d which is the ASCII value of  %c\n", ch, ch);

When run, the program’s output looks like this:

ch value is 65 which is the ASCII value of  A
ch value is 99 which is the ASCII value of  c

1.2.2. scanf

C’s scanf function represents one method for reading in values entered by the user (via the keyboard) and storing them in program variables. The scanf function can be a bit picky about the exact format in which the user enters data, which means that it is not very robust to badly formed user input. In the I/O section, we discuss more robust ways of reading input values from the user. For now, remember that if your program gets into an infinite loop due to badly formed user input, you can always type CONTROL-C (simultaneously press and hold the Ctrl and c keys) to terminate it.

Reading in input is handled differently in Python and C: Python uses the input function to read in a value as a string, and then the program converts the string value to an int; and C uses scanf to read in an int value and to store it at the location in memory of an int program variable (e.g. &num1). Table 2 displays example programs for reading user input values in Python and C:

Table 2. Comparison of methods for reading input values in Python and C.
Python version C version 
# Python input example


def main():


    num1 = input("Enter a number:")
    num1 = int(num1)
    num2 = input("Enter another:")
    num2 = int(num2)

    print("%d + %d = %d" % (num1, num2, (num1+num2)))

# call the main function:
main()
 
/* C input (scanf) example */
#include <stdio.h>

int main() {
    int num1, num2;

    printf("Enter a number: ");
    scanf("%d", &num1);
    printf("Enter another: ");
    scanf("%d", &num2);

    printf("%d + %d = %d\n", num1, num2, (num1+num2));

    return 0;
}

When run, both programs read in two values (30, and 67 in this example):

Enter a number: 30
Enter another: 67
30 + 67 = 97

Like printf, scanf takes a format string that specifies the number and type of values to read in (e.g., "%d" specifies one int value). The scanf function skips over leading and trailing whitespace as it reads in a numeric value, so its format string only needs to contain a sequence of formatting placeholders, usually with no whitespace or other formatting characters between the placeholders in its format string. The arguments for the placeholders in the format string specify the locations of program variables into which the values read in will be stored. Prefixing the name of a variable with the & operator produces the location of that variable in the program’s memory — the memory address of the variable. The pointers section discusses the & operator in more detail. For now, we use it only in the context of the scanf function.

Here’s another scanf example, where the format string has placeholders for two values: the first an int, the second a float:

scanf_ex.c
int x;
float pi;

// read in an int value followed by a float value ("%d%g")
// store the int value at the memory location of x (&x)
// store the float value at the memory location of pi (&pi)
scanf("%d%g", &x, &pi);

When inputting data to a program via scanf, individual numeric input values must be separated by at least one whitespace character. However, because scanf skips over additional leading and trailing whitespace characters (e.g. spaces, tabs, newlines), a user could enter input values with any amount of space before or after each input value. For example, if a user enters the following for the call to scanf above, scanf will read in 8 and store it in the x variable, and read in 3.14 and store it in the pi variable:

          8                   3.14