Printf and Hexadecimal Numbers

The printf function is very complex and can be used to display data in a number of different ways.

Display Width

When displaying numbers with printf, you can tell it how many characters to use by putting a number before the d in the %d.

For example:

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char *argv[]) {
    printf("%5d\n", 20);
    printf("%10d\n", 20);
    printf("%20d\n", 20);

    return EXIT_SUCCESS;
}

Would print the following:

   20
       20
                 20

Leading Zeroes

You can tell printf to fill the empty space before a value with 0s instead of blank space by putting a 0 before the number.

For example:

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char *argv[]) {
    printf("%05d\n", 20);
    printf("%010d\n", 20);
    printf("%020d\n", 20);

    return EXIT_SUCCESS;
}

Would print the following:

00020
0000000020
00000000000000000020

Hexadecimal

The computer and printf store all numbers in binary, but when you use %d in printf, you’re letting printf know that you want it to show you the number in decimal (base 10). You can also tell printf to show you numbers in hexadecimal (base 16) using %x or %X. %x will display the letters A, B, C, D, E, and F in lower-case when they appear in the number and %X will use upper-case. You can also use %#X and %#x to show the hexadecimal number starting with a 0x.

For example, the following program:

#include <stdio.h>
#include <stdlib.h>

int main(int argc, char *argv[]) {
    printf("%x\n", 170);
    printf("%X\n", 170);
    printf("%#x\n", 170);
    printf("%#X\n", 170);
    printf("\n");

    printf("%x\n", 48879);
    printf("%X\n", 48879);
    printf("%#x\n", 48879);
    printf("%#X\n", 48879);
    printf("\n");

    // Adding in width
    printf("%4x\n", 170);
    printf("%04x\n", 170);
    printf("%16x\n", 170);
    printf("%016x\n", 170);

    return EXIT_SUCCESS;
}

Would print:

aa
AA
0xaa
0XAA

beef
BEEF
0xbeef
0XBEEF

  aa
00aa
              aa
00000000000000aa

The Exercise

Create a program called addition2.c. This program should ask for two positive integers using the message Please enter two positive integers: then display the sum of the integers in hexadecimal as n + n = sum.

Make sure to replace the n with the numbers entered in the same order in hexadecimal and the sum with the sum of the two numbers in hexadecimal. Each hexadecimal number should be proceded by 0x and should have two hexadecimal digits.

Some Examples

$ ./addition2
Please enter two positive integers: 2 5
0x02 + 0x05 = 0x07

$ ./addition2
Please enter two positive integers: 3 10
0x03 + 0x0a = 0x0d

$ ./addition2
Please enter two positive integers: 10 27
0x0a + 0x1b = 0x25

To run some simple automated tests:

$ 1511 autotest addition2

To run Styl-o-matic:

$ 1511 stylomatic addition2.c
Looks good!

You’ll get advice if you need to make changes to your code.

Submit your work with the give command, like so:

$ give cs1511 wk03_addition2

Or, if you are working from home, upload the relevant file(s) to the wk03_addition2 activity on Give Online.