murrymcintosh / ist-printf

ist-printf

The C programming language introduced the function printf to let programmers control the format of their output, including outputting the values of variables. The basic syntax of the printf function is:

printf("format string", variable_list, ...);

The Format String

First, a string is a specific set of characters that the C compiler will not treat as code. The specific ASCII bytes for each character are stored in the exact order as written. In C (and most other programming languages) strings are delimited by opening and closing quotation marks.

Example

"Murry"  // This string is stored in hex as 4D 75 72 72 79

The printf function will output whatever string is given as it's first argument. However, printf will also look at the format string for special sequences of characters. For example, when printf sees a '\n', it will insert a new line character. The format string may also contain special format specifiers that act as placeholders for variable data.

Format Specifiers

A format specifiers start with the % character, followed by different options. The only required option is the type modifer. The type modifier is a one character letter that tells printf what kind of data to expect. Here is a brief list of the important format specifier types.

• %i - Signed integer type in base 10
• %d - Also signed integer type in base 10
• %o - Integer represented in base 8 (octal)
• %x - Integer represented in base 16 (hexadecimal)
• %f - Floating point displayed with decimal point
• %e - Floating point displayed in scientific notation
• %c - Single character
• %s - String

When printf sees these format specifiers in the format string, it replaces each one with the value of a variable listed after the format string.

printf("1st = %i, 2nd = %f, 3rd = %c", a, b, c);

printf.c

Take a look at the contents of the printf.c file. Then, compile and run it.

cat printf.c
gcc printf.c -o printf
./printf

Now, modify this file so that there is only one printf statement that prints out each value. The order of the variables given to printf will be the order that the format specifiers are replaced. Using vim, change the three printf statements into this one:

printf("a = %i, f = %f, c = %c\n", a, f, c);

Recompile and run.

integers.c

Now let's take a look at some different ways to display integers. Make a copy of the printf.c file to create a new file integers.c

cp printf.c integers.c

Using vim, edit the integers.c file by deleting all references to the f and c variables. Then create 4 new printf statements outputting the value of a in different formats.

printf("a displayed as base 10 integer: %i\n", a);
printf("a displayed as base 8 integer: %o\n", a);
printf("a displayed as base 16 integer: %x\n", a);
printf("a displayed as base 16 integer: %X\n", a);
printf("a displayed as base 10 unsigned integer: %u\n", a);

Compile and run this new program.

gcc integers.c -o integers
./integers

As an added exercise, edit integers.c and change the value of a to a negative number. How does the output change when a is negative?