Use of nested loops.

[Neptune]

Hello.

I am working my way through Zhang's "Teach yourself C in 24 hrs (2e)"
(Sam's series), and for nested loops, he writes (p116) "It's often
necessary to create a loop even when you are already in a loop." Then he
goes on to portray a contrived example that doesn't tell me under what
conditions a nested loop might be favoured as a solution? i.e. what are
nested loops useful for? What kinds of algorithms are served by nested
loops? etc. Is any of this making sense?

Anyway - thoughts welcomed.

- Andy

 

[Ben Pfaff]

I am working my way through Zhang's "Teach yourself C in 24 hrs (2e)"
(Sam's series), and for nested loops, he writes (p116) "It's often
necessary to create a loop even when you are already in a loop." Then
he goes on to portray a contrived example that doesn't tell me under
what conditions a nested loop might be favoured as a solution?
i.e. what are nested loops useful for? What kinds of algorithms are
served by nested loops? etc. Is any of this making sense?

A typical beginner's example is printing a bar graph,
e.g. (warning: poorly proofread):

#include <stdio.h>

int main (void)
{
int bars[] = {1, 6, 2, 4, 9};
int i;

for (i = 0; i < sizeof bars / sizeof *bars; i++) {
int j;

printf ("%3d ", bars);
for (j = 0; j < bars; j++)
putchar ('*');
putchar ('\n');
}
return 0;
}

 

[Neptune]

Neptune writes:

I am working my way through Zhang's "Teach yourself C in 24 hrs (2e)"
(Sam's series), and for nested loops, he writes (p116) "It's often
necessary to create a loop even when you are already in a loop." Then he
goes on to portray a contrived example that doesn't tell me under what
conditions a nested loop might be favoured as a solution? i.e. what are
nested loops useful for? What kinds of algorithms are served by nested
loops? etc. Is any of this making sense?

One case that might have some natural appeal is to compute the sum of all
the elements of a two-dimensional array. C only simulates (emulates?
whatever) a two-dimensional array, but the simulation is pretty effective.

Something like:

double sum = 0.0;
for(i=0; i<10; i++)
for(j=0; j<25; j++)
sum = sum + a[j];

It is often helpful to hide the nesting. This can be accomplished very
effectively by calling a function. It lets one focus on the thing of most
immediate interest. EG: sum_array() calls sum_columns().

Thanks for replying:
1. I get the idea in terms of the structure of a nested for loop and its
output (e.g. 2 lists of integers from 0 to 10 and 0 to 25) ... but
what does the 'a' stand-for in "sum = sum + a[j];"?
2. This is due to my ignorance, but under what conditions would one use
it? In this instance, why would I want a two-dimensional array, anyway?
What would I use it for? That was more to my question.



--
"Today a young man on acid realised that all matter was really energy
condensed to a slow vibration, that we are all one consciousness
experiencing itself subjectively, there's no such thing as death,
life is only a dream, and we're the imaginations of ourselves.
Here's Tom with the weather ..." - Bill Hicks.

 

[Neptune]

I am working my way through Zhang's "Teach yourself C in 24 hrs (2e)"
(Sam's series), and for nested loops, he writes (p116) "It's often
necessary to create a loop even when you are already in a loop." Then
he goes on to portray a contrived example that doesn't tell me under
what conditions a nested loop might be favoured as a solution?
i.e. what are nested loops useful for? What kinds of algorithms are
served by nested loops? etc. Is any of this making sense?

A typical beginner's example is printing a bar graph,
e.g. (warning: poorly proofread):

#include <stdio.h>

int main (void)
{
int bars[] = {1, 6, 2, 4, 9};
int i;

for (i = 0; i < sizeof bars / sizeof *bars; i++) {
int j;

printf ("%3d ", bars);
for (j = 0; j < bars; j++)
putchar ('*');
putchar ('\n');
}
return 0;
}

Cheers Ben
That was helpful. Gives me an idea of its usage outside of text book
example. I haven't come across "sizeof" before. I presume that it is
some standard way of referencing the length of the array?
Thanks


--
"Today a young man on acid realised that all matter was really energy
condensed to a slow vibration, that we are all one consciousness
experiencing itself subjectively, there's no such thing as death,
life is only a dream, and we're the imaginations of ourselves.
Here's Tom with the weather ..." - Bill Hicks.

 

[Mark A. Odell]

That was helpful. Gives me an idea of its usage outside of text book
example. I haven't come across "sizeof" before. I presume that it is
some standard way of referencing the length of the array?
Thanks

Sizeof is a nice operator (it's not a function) that tells you the size of
any "object".

E.g.

sizeof (int);
- tells you the size of an int on your platform (in bytes). For types you
must use the parenthesis.


struct foo
{
int a;
double d;
long *p;
} fooVar;

sizeof fooVar
- tells you the size of the variable fooVar (a struct) in bytes. You don't
need parenthesis for a variable when uses with the sizeof operator.


int var[64];

sizeof var;
- tells you the size of the var in bytes.

The sizeof operator is calculated at compile time so it has no run-time
performance overhead.

 

[Keith Thompson]

That was helpful. Gives me an idea of its usage outside of text book
example. I haven't come across "sizeof" before. I presume that it is
some standard way of referencing the length of the array?
Thanks

"sizeof" should be explained in that text book of yours. If it isn't,
you need a better text book.

BTW, another example of a nested loop might be reading lines from a
file, and processing each character in each line. You might use a
triple-nested loop to open each of the files named on the command
line, reading each line from each file, and processing each character
on each line.

 

[Ben Pfaff]

double sum = 0.0;
for(i=0; i<10; i++)
for(j=0; j<25; j++)
sum = sum + a[j];

Thanks for replying:
1. I get the idea in terms of the structure of a nested for loop and
its output (e.g. 2 lists of integers from 0 to 10 and 0 to 25) ... but
what does the 'a' stand-for in "sum = sum + a[j];"?

It's just the name of an array, perhaps declared as
double a[10][25];

2. This is due to my ignorance, but under what conditions would one
use it? In this instance, why would I want a two-dimensional array,
anyway? What would I use it for? That was more to my question.

Suppose you're calculating a statistical crosstabulation;
e.g. you have a bunch of survey responses from several people,
and the survey includes two questions, one of which has 10
possible answers and another of which has 25 possible answers.
Then it may be interesting to figure out how often each possible
combination of responses (25 * 10 = 250 possibilities) was given
by respondents. A two-dimensional array with cell values
corresponding to a count of people is a natural way to do this.
The sum of all of the cell values is then the total number of
survey respondents.

There are of course many other possibilities, too.

 

[Ben Pfaff]

int bars[] = {1, 6, 2, 4, 9};
int i;
for (i = 0; i < sizeof bars / sizeof *bars; i++) {

That was helpful. Gives me an idea of its usage outside of text book
example. I haven't come across "sizeof" before. I presume that it is
some standard way of referencing the length of the array?

sizeof yields the number of bytes in its operand. `sizeof bars'
is the number of bytes in array bars[]; `sizeof *bars' is the
number of bytes in a single element of bars[]. Thus, the
quotient of those two expressions is the number of elements in
bars[]; in this case, 5.

 

[osmium]

I am working my way through Zhang's "Teach yourself C in 24 hrs (2e)"
(Sam's series), and for nested loops, he writes (p116) "It's often
necessary to create a loop even when you are already in a loop." Then he
goes on to portray a contrived example that doesn't tell me under what
conditions a nested loop might be favoured as a solution? i.e. what are
nested loops useful for? What kinds of algorithms are served by nested
loops? etc. Is any of this making sense?

One case that might have some natural appeal is to compute the sum of all
the elements of a two-dimensional array. C only simulates (emulates?
whatever) a two-dimensional array, but the simulation is pretty effective.

Something like:

double sum = 0.0;
for(i=0; i<10; i++)
for(j=0; j<25; j++)
sum = sum + a[j];

It is often helpful to hide the nesting. This can be accomplished very
effectively by calling a function. It lets one focus on the thing of most
immediate interest. EG: sum_array() calls sum_columns().

 

[David Rubin]

double sum = 0.0;
for(i=0; i<10; i++)
for(j=0; j<25; j++)
sum = sum + a[j];

[snip]
Thanks for replying:
1. I get the idea in terms of the structure of a nested for loop and its
output (e.g. 2 lists of integers from 0 to 10 and 0 to 25) ... but

In this example, a is a (two-dimensional) array, likely defined as

double a[10][25];

what does the 'a' stand-for in "sum = sum + a[j];"?
2. This is due to my ignorance, but under what conditions would one use
it? In this instance, why would I want a two-dimensional array, anyway?
What would I use it for? That was more to my question.

Two-dimensional arrays are useful in many circumstances; numerical
programming is an obvious one. For example, this is how you would do
matrix addition, transposition, multiplication by a scalar, etc.

/david

 

[Neptune]

It's just the name of an array, perhaps declared as
double a[10][25];

OK - thought as much, but wanted to confirm.

Suppose you're calculating a statistical crosstabulation;
e.g. you have a bunch of survey responses from several people,
and the survey includes two questions, one of which has 10
possible answers and another of which has 25 possible answers.
Then it may be interesting to figure out how often each possible
combination of responses (25 * 10 = 250 possibilities) was given
by respondents. A two-dimensional array with cell values
corresponding to a count of people is a natural way to do this.
The sum of all of the cell values is then the total number of
survey respondents.

There are of course many other possibilities, too.

This gels it for me. Thanks Ben. Very useful.

--
"Today a young man on acid realised that all matter was really energy
condensed to a slow vibration, that we are all one consciousness
experiencing itself subjectively, there's no such thing as death,
life is only a dream, and we're the imaginations of ourselves.
Here's Tom with the weather ..." - Bill Hicks.

 

[Neptune]

Two-dimensional arrays are useful in many circumstances; numerical
programming is an obvious one. For example, this is how you would do
matrix addition, transposition, multiplication by a scalar, etc.

Thanks David. I wouldn't know where to start with the examples you list,
but I get the basic idea. Cheers.

--
"Today a young man on acid realised that all matter was really energy
condensed to a slow vibration, that we are all one consciousness
experiencing itself subjectively, there's no such thing as death,
life is only a dream, and we're the imaginations of ourselves.
Here's Tom with the weather ..." - Bill Hicks.

 

[Neptune]

"sizeof" should be explained in that text book of yours. If it isn't,
you need a better text book.

ooops - it is. Right in the next section under "Using conditional
operators".

BTW, another example of a nested loop might be reading lines from a
file, and processing each character in each line. You might use a
triple-nested loop to open each of the files named on the command
line, reading each line from each file, and processing each character
on each line.

Thanks

--
"Today a young man on acid realised that all matter was really energy
condensed to a slow vibration, that we are all one consciousness
experiencing itself subjectively, there's no such thing as death,
life is only a dream, and we're the imaginations of ourselves.
Here's Tom with the weather ..." - Bill Hicks.

 

[Neptune]

Sizeof is a nice operator (it's not a function) that tells you the size of
any "object".

E.g.

sizeof (int);
- tells you the size of an int on your platform (in bytes). For types you
must use the parenthesis.


struct foo
{
int a;
double d;
long *p;
} fooVar;

sizeof fooVar
- tells you the size of the variable fooVar (a struct) in bytes. You don't
need parenthesis for a variable when uses with the sizeof operator.


int var[64];

sizeof var;
- tells you the size of the var in bytes.

The sizeof operator is calculated at compile time so it has no run-time
performance overhead.

That was a useful summary - thanks. As it so happens it looks like that
is the next section in this book, so I'll keep your precise in mind when
reading.

--
"Today a young man on acid realised that all matter was really energy
condensed to a slow vibration, that we are all one consciousness
experiencing itself subjectively, there's no such thing as death,
life is only a dream, and we're the imaginations of ourselves.
Here's Tom with the weather ..." - Bill Hicks.

 

[Neptune]

sizeof yields the number of bytes in its operand. `sizeof bars'
is the number of bytes in array bars[]; `sizeof *bars' is the
number of bytes in a single element of bars[]. Thus, the
quotient of those two expressions is the number of elements in
bars[]; in this case, 5.

Cheers Ben. Your explanation coupled with that given by Mark will be
very useful when I tackle the next section in my book. Much obliged.

--
"Today a young man on acid realised that all matter was really energy
condensed to a slow vibration, that we are all one consciousness
experiencing itself subjectively, there's no such thing as death,
life is only a dream, and we're the imaginations of ourselves.
Here's Tom with the weather ..." - Bill Hicks.

 

[Peter Pichler]

BTW, another example of a nested loop might be reading lines from a
file, and processing each character in each line. You might use a
triple-nested loop to open each of the files named on the command
line, reading each line from each file, and processing each character
on each line.

Neptune, in many cases you use multiple nested loops without even knowing
it.

Let's look at Keith's a modified example. Open a text file and read and
print
each line. You need a loop in which to read a line, test you have
successfully
read it and then print it. Inside that loop, you would probably use fgets
for
reading and puts or printf for printing. Each of these functions contains at
least one loop, possibly more than one. So, you may get nested loops simply
by
calling a function in a loop. Isn't programming fun

To answer your original question, nested loops are used whenever an
algorithm
asks for it. Processing more than one-dimensional entities is the most
common
example. My example is one of them: the line number could be considered the
vertical and the position of a character in a line the horizontal dimension.

 

[David Rubin]

Thanks David. I wouldn't know where to start with the examples you list,
but I get the basic idea. Cheers.

Start with Google. No problem.

/david

 

[Keith Thompson]

int bars[] = {1, 6, 2, 4, 9};
int i;
for (i = 0; i < sizeof bars / sizeof *bars; i++) {

That was helpful. Gives me an idea of its usage outside of text book
example. I haven't come across "sizeof" before. I presume that it is
some standard way of referencing the length of the array?

sizeof yields the number of bytes in its operand. `sizeof bars'
is the number of bytes in array bars[]; `sizeof *bars' is the
number of bytes in a single element of bars[]. Thus, the
quotient of those two expressions is the number of elements in
bars[]; in this case, 5.

This is, of course, quite correct.

One thing to watch out for is the distinction between arrays and
pointers. They are *not* the same thing (though some people might try
to tell you they are), but there are some contexts in which a pointer
name and an array name can be used in the same way. There are times
when you have to be very careful to know whether you're applying
sizeof to an array object or to a pointer.

Section 6 of the C FAQ at <http://www.eskimo.com/~scs/C-faq/top.html>
covers this well.

 

[Thomas Stegen]

Thanks David. I wouldn't know where to start with the examples you list,
but I get the basic idea. Cheers.

A simpler more concrete example. Consider an othello game
<http://www.ugateways.com/bof4.html>

You have a board which is made up of 8 by 8 squares. You decide
to represent the state of a squares as an integer.

#define UNOCCUPIED -1
#define BLACK 0
#define WHITE 1

Bonus question, why is it a good (and is it good?) idea to make
black 0 and white 1 instead of any other two numbers?

Now you need the board

#define SIZE 8

int board[SIZE][SIZE];

To start of the game you need some simple code
int i;
int j;

for(i = 0;i < SIZE;i++)
{
for(j = 0;j < SIZE;j++)
{
board[j] = UNNOCCUPIED;
}
}

board[3][3] = WHITE;
board[3][4] = BLACK;
board[4][3] = WHITE;
board[4][4] = BLACK;

Ok, You are now ready to start the game. (And that you can do
yourself

<OT>
I saw your signature. Tool fan? Or "just" Bill Hicks?
IMNSVHO they both rock!
</OT>

 

[Allan Bruce]

Hello.

I am working my way through Zhang's "Teach yourself C in 24 hrs (2e)"
(Sam's series), and for nested loops, he writes (p116) "It's often
necessary to create a loop even when you are already in a loop." Then he
goes on to portray a contrived example that doesn't tell me under what
conditions a nested loop might be favoured as a solution? i.e. what are
nested loops useful for? What kinds of algorithms are served by nested
loops? etc. Is any of this making sense?

Anyway - thoughts welcomed.

- Andy

One example you may come across in your book is a basic bubble-sort which
will sort an array of elements, e.g.

for (i=0; i<MAX-1; i++)
for (j=0; j<MAX-1-i; j++)
if (ELEMENT[j+1]>ELEMENT[j])
{
/*swap the elements*/
TEMP = ELEMENT[j+1];
ELEMENT[j+1] = ELEMENT[j];
ELEMENT[j] = TEMP;
}

HTH
Allan