[Courses] C Programming For Absolute Beginners, Lesson 4: Getting Looped

[Carla Schroder]

Hello again! Today we dive into loops. Loops are essential to programming 
because they save typing out squillions of lines of code. Let's take a look at 
this short example:

============

// loopy, getting looped in C!

#include <stdio.h>

int main ()
{
  int a = 0;
  int total = 0;

for ( a = 1; a <= 50; a = a + 1 )
    total = total + a;
  printf ( "The sum of this loop is %i\n", total );
  return 0;
}

============

This results in:

$ loopy
The sum of this loop is 1275

Let's add a few lines that expand the loop so we can see what it is doing:

============

#include <stdio.h>

int main ()
{
  int a = 0;
  int total = 0;
  int longway = 0;

for ( a = 1; a <= 50; a = a + 1 )
    total = total + a;
    printf ( "The sum of this loop is %i\n", total );

   longway = 
1+2+3+4+5+6+7+8+9+10+11+12+13+14+15+16+17+18+19+20+21+22+23+24+25+26+27+28+29+30+31+32+33+34+35+36+37+38+39+40+41+42+43+44+45+46+47+48+49+50;

   printf ("The sum of adding the loop the long way is %i\n", longway );
   return 0;
}

===========

$ loopy
The sum of this loop is 1275
The sum of adding the loop the long way is 1275

You can play with this by changing the values of a in the loop statement, like 
this:

for ( a = 5; a <= 33; a = a + 3 )

Let's dissect this so we know what the heck is going on. We start with our 
standard comment with the program name, and then the preprocessor statement:

// loopy, getting looped in C!

#include <stdio.h>

Then our main function, which is required in C programs:

int main ()
{

Then we declare and initialize our variables. The declaration is 'int a', and 
initializing means assigning it a value, like'int a = 0'. We already learned 
that initializing variables when they are declared is a good practice for 
preventing unpredictable results.

  int a = 0;
  int total = 0;

In the olden days before the C99 standard it was a requirement to declare 
variables at the beginning of a statement block (the code between pairs of 
curly braces.) C99 allows us to mix declarations and statements in whatever 
chaotic ways we want to. For the sake of readability and maintainability let's 
stick with the old way. 

Now we come to the engine of this little program, the for statement. 

for ( a = 1; a <= 50; a = a + 1 )

The basic structure of a for statement is like this:

for ( variable_initialization; loop_condition; loop_expression )
        code that executes as long as the condition is true, our program 
statement

variable_initialization, loop_condition, and loop_expression are not any kind 
of official terminology; they are descriptive terms that I made up.

So 'a = 1' initializes a to a value of 1, its value before the loop begins to 
run. 

 'a <= 50' means "keep running as long as a is less than or equal to 50." You 
could also say 'a < 51'.  

'a = a + 1' performs the calculations of the for statement, and the results of 
these calculations are tested against the loop_condition. As long as the test 
returns True, which means 'a = a +1' is less than or equal to 50, the program 
keeps running. When it returns False, which is 51 or greater, then the program 
stops.

One way of capturing the results of the loop is with our program statement:

   total = total + a;

This passes the value of 'a' into 'total', and then we print it in the way we 
learned in our previous lessons:

  printf ( "The sum of this loop is %i\n", total );

But that only prints the final result. We can see the progress of the loop by 
printing 'a' directly:

printf ( "The sum of this loop is %i\n", a );

So this example:

========

#include <stdio.h>

int main ()
{
  int a = 0;

  for ( a = 12; a <= 30; a = a + 4 )
  printf ( "The sum of this loop is %i\n", a );
  return 0;
}

=========

Does this:

$ loopy
The sum of this loop is 12
The sum of this loop is 16
The sum of this loop is 20
The sum of this loop is 24
The sum of this loop is 28

So if you're a little wobbly with your arithmetic and have to work things out 
the long way, this helps you see what your program is doing.

printf has many wonderful formatting options that we can take advantage of. I 
place a high value on readability.  So we can make our little example look all 
cool:

=================

#include <stdio.h>

int main ()
{

  int a = 0;

printf ( "This is my awesome loopy program\n\n" );
printf ( " See the value of a in each\n loop in a nice table\n" );
printf ( "________ \n" ); 
  
for ( a = 12; a <= 30; a = a + 4 )
  printf ( "   %i\n",a );
  return 0;
}

==================

And it produces this:

$ loopy
This is my awesome loopy program

 See the value of a in each
 loop in a nice table
________ 
   12
   16
   20
   24
   28

Note how spacing and line breaks are handled. 

HOMEWORK

Modify the last example to display two columns of output that use the 'a' and  
'total' variables.  Play around with spacing and line breaks.

Make a box around your program, like

+++++++++++++++++++++++++++++
+This is my awesome loopy program  +
+                                                                          +
+ See the value of a in each                    +
+ loop in a nice table                                  +
+________                                                       +
+   12                                                                  +
+  16                                                                   +
+   20                                                                  +
+   24                                                                  +
+   28                                                                  +
++++++++++++++++++++++++++++++


Next week we shall take a deep dive into functions. We've been using them all 
along, so it's high time to start understanding them in more depth.


-- 
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
Carla Schroder
ace Linux nerd
author of Linux Cookbook,
Linux Networking Cookbook,
Book of Audacity
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~