Printing

We're going to briefly discuss how to print stuff to the terminal so you can start writing some C.

printf

Earlier we saw the puts() function which prints strings to the terminal. This function is really good for strings but does not work for any other data type. Instead, there is the printf() function which prints formatted text to the terminal. This allows you to print different data types to the terminal and control various aspects of the output.

Signature

The general signature of printf() is quite unique in C and how it achieves it is a bit of compiler magic in order to actually implement but you do not have to worry about it. printf() takes as its first argument a string that represents the output format of the printed text. The next argument is the .... This is called the ellipsis and it is used to represent a variable number of untyped function arguments. This allows you to pass ass many arguments as you want to printf() and it will print them all as long as there are an equivalent number of positional arguments in the format string. The variadic arguments are inserted in output string in the same order they are passed to printf() ie. there is now way to indicate in the format string which variadic argument to use at any given positional argument. The positional argument introducer character is the % followed by a modifier to indicate in incoming type.

printf(char* fmtstring, ...);

Note:

• Ignore the use of char* for now.
• printf() is different to puts() in that it doesn't pad the end if the output string with a newline so you will have to manually provide it. The newline character is '\n'. The backslash is a special character that indicates the proceeding character is "escaped". Escaped characters have special meanings for string and character data. If the format string doesn't have any positional arguments then printf() will just print the string like puts(). printf() is not able to print data of any kind without a format string ie. printf(10) would fail to compile.

Example

The following simple code snippet creates to variables num and dec and computes their sum. It then prints a string according to the format "%d + %f = %f", substituting num, dec and sum respectively.

#include <stdio.h>

int main()
{
    int num     = 4;
    double dec  = 3.54756;
    double sum  = num + dec;

    printf("%d + %f = %f", num, dec, sum);

    return 0;
}

Question: Notice how we used double for the type of sum. What would happen if sum type was int?

If you want to have a play with printf(), copy the following code snippet run it on your own device. The command line will output different varieties of 'Hello World!'.

#include <stdio.h>

int main() {
    printf("%30s\n", "Hello World!"); // Padding added
    printf("%40s%10s%20s%15s\n", "Hell", "o", "World", "!"); 
    printf("%10.7s\n", "Hello World!"); // Print only the first 7 characters with padding
    printf("%100c%c%c%c%c %c%c%c%c%c%c%c\n", 
         72, 101, 108, 108, 111, 32, 87, 111, 114, 108, 100, 33, '\n'); // Hex values
    return 0;
}

Formatting Specification

You'll notice we used a different character after the % for each argument. This is because printf() needs to know the type of the incoming arguments so that it can format the string appropriately. For example floating point types have to use a decimal point when transformed into a text format while integers do not.

C has a concise language for printf() format arguments with the general format for a positional argument specifier being:

_%\[flag\]\[width\]\[.precision\]\[length\]type-specifier_

There are a variety of different options for each part of the specification. Below is a series of tables breaking down the various options for each sub-specifier but note that only type-specifier is needed, the others are optional.

Type Specifiers

Flags

Width

Precision

Length