Exercise 3: Formatted Printing

Keep that Makefile around since it'll help you spot errors, and we'll be adding to it when we need to automate more things.

Many programming languages use the C way of formatting output, so let's try it:

View Source file ex3.c Only

#include <stdio.h>

int main()
{
    int age = 10;
    int height = 72;

    printf("I am %d years old.\n", age);
    printf("I am %d inches tall.\n", height);

    return 0;
}

Once you've finished that, do the usual make ex3 to build and run it. Make sure you fix all warnings.

This exercise has a whole lot going on in a small amount of code, so let's break it down:

• First we're including another header file called stdio.h. This tells the compiler that you're going to use the standard Input/Output functions. One of those is printf.

• Then we're using a variable named age and setting it to 10.

• Next we're using a variable height and setting it to 72.

• Then we're adding the printf function to print the age and height of the tallest 10-year-old on the planet.

• In printf, you'll notice we're including a format string, as seen in many other languages.

• After this format string, we're putting in the variables that should be "replaced" into the format string by printf.

The result is giving printf some variables and it's constructing a new string and then printing it to the terminal.

What You Should See

When you do the whole build, you should see something like this:

View Source file ex3.sh-session Only

$ make ex3
cc -Wall -g    ex3.c   -o ex3
$ ./ex3
I am 10 years old.
I am 72 inches tall.
$

Pretty soon I'm going to stop telling you to run make and what the build looks like, so please make sure you're getting this right and that it's working.

External Research

In the Extra Credit section of each exercise, you may have you go find information on your own and figure things out. This is an important part of being a self-sufficient programmer. If you're constantly running to ask someone a question before trying to figure things out yourself, then you'll never learn how to solve problems independently. You'll never build confidence in your skills and will always need someone else around to do your work.

The way to break this habit is to force yourself to try to answer your own question first, and then confirm that your answer is right. You do this by trying to break things, experimenting with your answer, and doing your own research.

For this exercise, I want you to go online and find out all of the printf escape codes and format sequences. Escape codes are \n or \t that let you print a newline or tab, respectively. Format sequences are the %s or %d that let you print a string or integer. Find them all, learn how to modify them, and see what kind of "precisions" and widths you can do.

From now on, these kinds of tasks will be in the Extra Credit, and you should do them.

How to Break It

Try a few of these ways to break this program, which may or may not cause it to crash on your computer:

• Take the age variable out of the first printf call, then recompile. You should get a couple of warnings.

• Run this new program and it will either crash or print out a really crazy age.

• Put the printf back the way it was, and then don't set age to an initial value by changing that line to int age;, and then rebuild it and run it again.

View Source file ex3.bad.sh-session Only

# edit ex3.c to break printf
$ make ex3
cc -Wall -g    ex3.c   -o ex3
ex3.c: In function 'main':
ex3.c:8: warning: too few arguments for format
ex3.c:5: warning: unused variable 'age'
$ ./ex3
I am -919092456 years old.
I am 72 inches tall.
# edit ex3.c again to fix printf, but don't init age
$ make ex3
cc -Wall -g    ex3.c   -o ex3
ex3.c: In function 'main':
ex3.c:8: warning: 'age' is used uninitialized in this function
$ ./ex3
I am 0 years old.
I am 72 inches tall.
$

Extra Credit

• Find as many other ways to break ex3.c as you can.

• Run man 3 printf and read about the other % format characters you can use. These should look familiar if you used them in other languages (they come from printf).

• Add ex3 the all list in your Makefile. Use this to make clean all and build all of your exercises thus far.

• Add ex3 to your clean list in yourMakefile as well. Use make clean to remove it when you need to.