2.9. Basic Input / Output (I/O)

2.9.1. printf()

In the “Hello World” example we saw the use of printf() to display a simple character string to the screen. Here we will look at how to display the value of variables using printf().

printf(format, ...)

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

The string pointed to by format consists of two types of items. The first type is characters that will be printed. The second type contains format commands that define the way the other arguments are displayed. Variables, which are part of the output, are listed after the format string.

For example …

printf( "%s: your score for level %d was %f.\n", "Bill", 3, 89.3);

2.9.2. Format modifiers

Variable type specifier

Letter	Meaning
s	string
c	character
d	integer
u	unsigned integer
f	floating point
o	octal
x	hexadecimal

l

A long data type follows. It can be used with: d, o, u, x and f. lf is for type double.

L

Extra long data. ‘Lf’ for long double.

field width

An integer following the % sign indicates the field width. '%5d', right justifies an integer in a 5 character field. The default padding is spaces, which can be modified with a second character: '%05d', will print zeros to the left of the variable to fill out the 5 character space.

negative sign

left justify. '%-5d' means that the integer is left justified in a 5 character field.

precision

'%.2f' Will print exactly 2 digits of precision to the right of the decimal point.

2.9.3. Special formating symbols

Escape code	Meaning
\n	New line
\t	tab character
\\	A backslash character

2.9.4. Examples of printf()

#include <stdio.h>

int main(void)
{
    int i = 3;
    char *string = "Howdy";
    double pi = 3.141592653589793;
    float r = 20.5;

    printf( "%s\t%d %lf %f\n", string, i, pi, r );
    printf( "%9s%-4d\t%10.2lf %5.4f\n", string, i, pi, r );
}

pilgrim:~> gcc print1.c
pilgrim:~> ./a.out
Howdy 3 3.141593 20.500000
    Howdy3           3.14 20.5000

Note

This the point where you should complete homework assignment 1: Homework 1 - A simple equation.

2.9.5. scanf()

scanf()

The scanf() function is a general purpose input routine that reads from stdin, by default the keyboard, and stores the information read into the variables pointed to in its argument list.

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

The variables written to by scanf() are passed to it as pointers. This is because arguments to functions are passed to C functions by value, not by reference. A pointer provides a mechanism to effectively pass arguments to a function by reference.

Definition: A pointer is a memory address. When a program runs, each variable is stored at a certain location in the computer’s memory. So a pointer references, not to the value of the variable, but the place in memory where the variable is stored.

A pointer to a variable is generated by the & symbol in front of the variable name.

2.9.6. scanf() examples

int i;
float y;

scanf( "%d", &i );
scanf( "%f", &y );
scanf( "%d, %f", &i, &y );

Note that scanf()’s format string has three types of values:

• Format specifiers – %x, which follow the same syntax as format specifiers with printf().
• Non-whitespace characters – the comma in the third example is read by scanf() and discarded.
• Whitespace characters – spaces, tabs and even line returns are ignored by scanf() until it reads the desired number of variables.

Warning

scanf() is good for reading a single variable, but is not the best at reading a large number of variables. The problem relates to the user of the program. They may not understand exactly what data they are asked to provide. Therefore, for complex data input, scanf() is not recommended. Later, we will discuss a routine called ReadLine() that is more robust (String Example - ReadLine).

2.9.7. getchar / putchar Example

This example illustrates an alternative to scanf() and printf(). Here the data is read one character at time. Use of the fflush() function is to force the display of any data that may still be in the standard output buffer. At this point, this example is informational only.

#include <stdio.h>

int main(void)
{
    int c;

    while( (c=getchar()) != EOF )
        putchar(c);
    fflush(stdout);
    return 0;
}

Note

This is the point where you should complete homework assignment 2: Homework 2 - Prompt for input.