Seriously struggling with C

[RG]

Greetings friends,

This semester I have started a course in C programming. I was moving
along fine until I reached to the topic of loops (feeling embarrassed
among you elite programmers). My prof. would post program questions
and the solutions online. For practice I would try to do the problems.
I would reach to a certain point in the code, for example as far as
error trapping, but when the loop arrives, like knowing whether to use
for, while do, how to properly use the increment and decrements, and
counters,I am just not proficient in it and the class is moving ahead.
Eventually i would have to look at the solution and wondering to
myself, the reason i could not think of it. What ticks me off is that
other kids are getting this stuff easily, while I am having a hard
time.Kindly advise me on what actions I shoul take. I would
particularly like to have an idea of the thought process to engage in
when given the programme to write.
Thanks for your time and consideration.

RG

 

[priyam.trivedi]

Hi!
I am not an expert in programming but I do like programming as it is
"mentally stimulating." The first thing that I will tell you is don't
compare yourselves with others, once you start doing that, you are
going to feel worse.
Try seeing programming like a game. First, try and build your own
solution from a mathematical perspective, without taking the
constraints of C programming. I wont say that it will be easy, but give
it a shot. Try to write an alogorithm, I dont use them much, but it
does help to understand the direction in which the program flows. So
once you have your solution try writing it in your own words. And then
in the end use the syntax of the programming language and try it out in
your editor.
I dont how much this will help you. I am a freshman myself, and I am
just telling you the way I see it. Just take it as a challenge and I am
sure you will be fine.
Take care and Good Luck.
Priyam

 

[Nick Keighley]

This semester I have started a course in C programming. I was moving
along fine until I reached to the topic of loops (feeling embarrassed
among you elite programmers). My prof. would post program questions
and the solutions online. For practice I would try to do the problems.
I would reach to a certain point in the code, for example as far as
error trapping, but when the loop arrives, like knowing whether to use
for, while do, how to properly use the increment and decrements, and
counters,

one cause of your problems may be that there is no definitive right or
wrong answer. Anything you can do with a while() you can do with a
for(),
and the functionality of a for() can be duplicated with a while() and a

couple of other statements. C's for() is very powerful (some might
argue,
too powerful). I tend to classify loops into two main types.

- a fixed known number of iterations (times around the loop), for these
I
use for()
- an iterate until some condition is met

// loop n times
for (i = 0; i < n; i++)
do_something ();

I'd just learn this as a piece of boilerplate code to drop in when you
need a
known number of iterations.

// loop while some condition holds
while (condition)
do_something();

You can pretty much do anything with those two.
There is also a while at the end:-

do {
do_something()
}
while (condition);

but it practice this is far less useful. note it always does at least
one iteration.

A C idiom for a loop that goes "forever"

for (; // never stops
{
do_something();
if (exit_condition)
break;
do_something_else();
}

this can sometimes be useful, I'd avoid it until you are comfortable
with the first two forms.

Where C can be confusing is when the "cleverness" of the for() is
exploited.

// walk a linked list
for (i = head; i != NULL; i = i->next)
flop_node (i->data);

If this confuses you, use a while()

for (i = 1, j = 1; i < k, j < l; i++, j + 2)
something_scary (i, j);

if this confuses you... chop the program listing into little pieces and
feed it
to the author of the code

Keep It Simple is the first lore of programming

I am just not proficient in it and the class is moving ahead.
Eventually i would have to look at the solution and wondering to
myself, the reason i could not think of it. What ticks me off is that
other kids are getting this stuff easily, while I am having a hard
time.Kindly advise me on what actions I shoul take. I would
particularly like to have an idea of the thought process to engage in
when given the programme to write.

--
Nick Keighley

The fscanf equivalent of fgets is so simple
that it can be used inline whenever needed:-
char s[NN + 1] = "", c;
int rc = fscanf(fp, "%NN[^\n]%1[\n]", s, &c);
if (rc == 1) fscanf("%*[^\n]%*c);
if (rc == 0) getc(fp);

 

[August Karlstrom]

Greetings friends,

This semester I have started a course in C programming. I was moving
along fine until I reached to the topic of loops (feeling embarrassed
among you elite programmers). My prof. would post program questions
and the solutions online. For practice I would try to do the problems.
I would reach to a certain point in the code, for example as far as
error trapping, but when the loop arrives, like knowing whether to use
for, while do, how to properly use the increment and decrements, and
counters,I am just not proficient in it and the class is moving ahead.
Eventually i would have to look at the solution and wondering to
myself, the reason i could not think of it. What ticks me off is that
other kids are getting this stuff easily, while I am having a hard
time.Kindly advise me on what actions I shoul take. I would
particularly like to have an idea of the thought process to engage in
when given the programme to write.
Thanks for your time and consideration.

Please, provide an example of something you don't grasp. The while loop
is probably the easiest loop construct to understand.

Example:

/* Show the contents of the array a of length len. */
k = 0;
while (k < len) {
printf("%d\n", a[k]);
k++;
}
/* Just by looking at the loop guard, we know that k >= len here
(actually k == len). */


August

 

[Rod Pemberton]

Greetings friends,

This semester I have started a course in C programming. I was moving
along fine until I reached to the topic of loops

like knowing whether to use
for, while do, how to properly use the increment and decrements, and
counters,

I would
particularly like to have an idea of the thought process to engage in
when given the programme to write.

This is my thought process on loops:
1) while - never use
2) for - use as much as possible
3) do while - only use when necessary

I'll explain. The while loop only checks for a single statement being true.
It is very simple. So simple in fact that it really is useless. It
duplicates functionality that exists in a for loop. Therefore, why use it?
Many C programmers don't. while(1) becomes for(;. while (a<b) becomes
for(;a<b .The for loop is a very powerful loop construct that can
implement almost any loop. The only time you don't want to use a for loop
is when the block of code in the loop needs to execute at least once. When
the code 'needs to execute at least once,' you use the do while loop. The
last flow control constructs are procedures, functions, and switches.
Procedures and functions aren't usually used for looping. The switch
statement is useful any time the data or a counter you've created doesn't or
won't increment or decrement by one.

The next step is to 'see' how a for loop works when it is written as a while
loop:

i=0;
while(i<10)
{
/* continue starts here for while() */
/* ...code body... */
/* continue starts here for for() */
i++;
}

That is the same as this for loop:

for (i=0; i<10; i++)
{
/* ...code body... */
}

The continue statement in a while continues execution at the top of the
while. But, for the for loop, execution continues before the third
expression (i++).


Rod Pemberton

 

[billh40]

Greetings friends,

This semester I have started a course in C programming. I was moving
along fine until I reached to the topic of loops (feeling embarrassed
among you elite programmers). My prof. would post program questions
and the solutions online. For practice I would try to do the problems.
I would reach to a certain point in the code, for example as far as
error trapping, but when the loop arrives, like knowing whether to use
for, while do, how to properly use the increment and decrements, and
counters,I am just not proficient in it and the class is moving ahead.
Eventually i would have to look at the solution and wondering to
myself, the reason i could not think of it. What ticks me off is that
other kids are getting this stuff easily, while I am having a hard
time.Kindly advise me on what actions I shoul take. I would
particularly like to have an idea of the thought process to engage in
when given the programme to write.
Thanks for your time and consideration.

RG

Go to www.bloodshed.net and download their free C/C++ compiler.
Then use it to test out your code to learn how to program in C/C++.

Bill Hanna

 

[Keith Thompson]

This is my thought process on loops:
1) while - never use
2) for - use as much as possible
3) do while - only use when necessary

I'll explain. The while loop only checks for a single statement being true.

Condition, not statement.

It is very simple. So simple in fact that it really is useless. It
duplicates functionality that exists in a for loop. Therefore, why use it?

That's bad advice. Use while() whenever it's the most natural
construct to use. Arbitrarily transforming "while (condition)" into
"for (;condition" is foolish.

 

[Keith Thompson]

Go to www.bloodshed.net and download their free C/C++ compiler.
Then use it to test out your code to learn how to program in C/C++.

How is that useful? Since the OP is taking a course in C programming,
it's safe to assume he already has access to a C compiler. If there's
something about the bloodshed compiler that makes it particularly
useful for beginners, you might want to say so.

BTW, what do you mean by "C/C++ compiler"? There is no "C/C++" language.

 

[Rod Pemberton]

true.

Condition, not statement.

Expression, not condition? I doubt the OP is familiar with any language or
definitions of ISO/ANSI specifications, so please don't criticize me for
phrasing to his level of understanding.

it?

That's bad advice. Use while() whenever it's the most natural
construct to use. Arbitrarily transforming "while (condition)" into
"for (;condition" is foolish.

Not using the braces of a compound-statement for while's and if-else's when
there is a single statement is also the most natural construct to use. But,
of course, as we've seen many times here, that leads to later problems.
People forget to add the braces when a statement is converted to a
compound-statement. The same holds true for the while statement. A while
will frequently be converted to a for sometime in the future. At which
point, the programmer is likely to introduce an error. Why not just do it
manner which reduces the chance that an error, now or in the future, is
introduced? I've seen you suggest this on many occasions. I find it odd
that you'd take a contrary position now.

Rod Pemberton

 

[August Karlstrom]

That's bad advice. Use while() whenever it's the most natural
construct to use. Arbitrarily transforming "while (condition)" into
"for (;condition" is foolish.

I agree. Moreover, the advantage of the "while" statement over the C
"for" statement is its clear semantics. But hey, how many C programmers
care about that ;-).


August

 

[Keith Thompson]

Expression, not condition? I doubt the OP is familiar with any language or
definitions of ISO/ANSI specifications, so please don't criticize me for
phrasing to his level of understanding.

Yes, the standard uses the word "expression", and a search of the
standard indicates that it doesn't refer to the expression in a while
statement as a "condition". But "condition" is a perfectly reasonable
term to use in this context. "Statement" is not; it's simply
incorrect. I see no basis for your assumption that the the word
"statement" is going to be clearer to the OP.

Not using the braces of a compound-statement for while's and if-else's when
there is a single statement is also the most natural construct to use. But,
of course, as we've seen many times here, that leads to later problems.
People forget to add the braces when a statement is converted to a
compound-statement. The same holds true for the while statement. A while
will frequently be converted to a for sometime in the future. At which
point, the programmer is likely to introduce an error. Why not just do it
manner which reduces the chance that an error, now or in the future, is
introduced? I've seen you suggest this on many occasions. I find it odd
that you'd take a contrary position now.

Personally, I always use braces on compound statements (except in rare
cases where it's clearer to put the whole thing on a single line).

I don't believe it's particularly likely that any particular while
loop will be changed to a for loop in the future. Consider the common
while ((c = getchar()) != EOF) { ... }
Do you really think that this:
for (;(c = getchar()) != EOF { putchar(c); }
is an improvement?

Just use whichever form is clearer. Very often, that's going to be a
while loop.

 

[August Karlstrom]

How is that useful? Since the OP is taking a course in C programming,
it's safe to assume he already has access to a C compiler. If there's
something about the bloodshed compiler that makes it particularly
useful for beginners, you might want to say so.

BTW, what do you mean by "C/C++ compiler"? There is no "C/C++" language.

He probably means "a compiler that can compile C or C++ source texts".


August

 

[Ian Collins]

Not using the braces of a compound-statement for while's and if-else's when
there is a single statement is also the most natural construct to use. But,
of course, as we've seen many times here, that leads to later problems.
People forget to add the braces when a statement is converted to a
compound-statement. The same holds true for the while statement. A while
will frequently be converted to a for sometime in the future. At which
point, the programmer is likely to introduce an error. Why not just do it
manner which reduces the chance that an error, now or in the future, is
introduced? I've seen you suggest this on many occasions. I find it odd
that you'd take a contrary position now.

Will it? I can't recall ever doing that. I've gone the other way and
used while in place of for where incrementing the loop counter became
conditional. I often find splitting the termination condition adds clarity.

Would you use for when doing something like reading to the end of a file?

 

[Pedro Graca]

This semester I have started a course in C programming. I was moving
along fine until I reached to the topic of loops (feeling embarrassed
among you elite programmers). My prof. would post program questions
and the solutions online. For practice I would try to do the problems.
I would reach to a certain point in the code, for example as far as
error trapping, but when the loop arrives, like knowing whether to use
for, while do, how to properly use the increment and decrements, and
counters,I am just not proficient in it and the class is moving ahead.
Eventually i would have to look at the solution and wondering to
myself, the reason i could not think of it. What ticks me off is that
other kids are getting this stuff easily, while I am having a hard
time.Kindly advise me on what actions I shoul take. I would
particularly like to have an idea of the thought process to engage in
when given the programme to write.

"Practice makes perfect."

Do your textbook examples, get used to the various loop constructs, try
all of them for every example you see. After a while you'll have
developed a feeling for your preferred way to do it ... and, of course,
you can always change the loop to some other construct later.


Allow me, as a fellow newbie, to provide a few examples:


********
** Write a program that reads numbers from the keyboard
** until 0 is entered, and then outputs their sum.

I'd use a while(), as in

int getnumber(void); /* implementation left out for brevity */
int main(void) {
int sum = 0;
int number;

while ((number = getnumber()) != 0) {
sum += number;
}
printf("The sum is %d.\n", sum);
return 0;
}


********
** Write a program that reads ten numbers from the keyboard
** and then outputs their sum.

Here, I might try a for() loop (even though the control variable isn't
used inside the loop)

int getnumber(void); /* implementation left out for brevity */
int main(void) {
int n, sum = 0;

for (n = 0; n < 10; ++n) {
sum += getnumber();
}
printf("The sum is %d.\n", sum);
return 0;
}


********
** Write a function that takes an array and a number of elements
** to add, and returns their sum.

For this type of thing (where it doesn't matter if I start at the bottom
or at the top of the array) I really like to do it a bit more cryptic,
and avoid creating a new local variable:

int sum_elems(int array[], int parcels) {
int sum = 0;

while (parcels--) {
sum += array[parcels];
}
return sum; /* array[parcels-1] + ... + array[1] + array[0] */
}

compare with the for() version

int sum_elems(int array[], int parcels) {
int sum = 0;
int n;

for (n = 0; n < parcels; ++n) {
sum += array[n];
}
return sum; /* array[0] + array[1] + ... + array[parcels-1] */
}



As others have noted, the for() and the while() loop have (almost)
the same possibilities; the do ... while() loop is similar but with
the extra twitch that is executes at least once. I rarely use it,
and often code around it and use a while instead

do {
/* stuff that changes `done' */
} while (!done);

I might code as

done = 0;
while (!done) {
/* stuff that changes `done' */
}


But this is just me ... try all the different loop constructs for the
same problem, noticing which gave you more troubles in implementing so
that you can later choose the one you prefer more comfortably.

 

[Ben Bacarisse]

This semester I have started a course in C programming. I was moving along
fine until I reached to the topic of loops

I used to teach programming to beginners and the symptoms you describe are
not uncommon. I think the key remark you make is:

Eventually i would have to look at the solution and wondering to myself,
the reason i could not think of it.

First, don't beat yourself up about not getting the solution because it is
very unlikely that you would get (exactly) the instructor's solution.
Give yourself credit if you have had some ideas that were along the same
sort of lines -- even if you had used a different kind of loop and you got
all the details wrong! Programming is very precise, and that can mean
that your attempts may seem further away from being solutions than they
actually are.

Secondly, read and learn from the solution. It sounds like you follow
them but simply can't get there yet yourself. Provided you do follow
them, set yourself a series of problems to modify each one in a number of
ways to learn how to make new loops out of the ones you have seen (process
an array from the other end, sum every other element rather than them all
or whatever). If you can't think of variations, try posting an example
here and asking for some mini-problems based on it. This will let you
explore the possibilities of various loops without having to generate the
whole program.

Third, get some help locally. Whilst you can get some help here, you will
get more if you can talk things over with someone in real time. If the
instructor is not available, ask some of the other students. They are
likely to be flattered and, if they are grow up enough, will be happy to
try to help you.

 

[Ben Bacarisse]

As others have noted, the for() and the while() loop have (almost) the
same possibilities; the do ... while() loop is similar but with the extra
twitch that is executes at least once. I rarely use it,

It has been widely noted (though not so widely that I could give you a
reference if you asked!) that "while" is more useful than "do" because it
handles degenerate cases naturally. It is unfortunate than the example of
prompting for input (much discussed in an earlier thread) is both a
natural use of "do" and beloved of introductory programming course
designers. It gives a false impression to beginners of how likely they
are to havce need of it.

 

[John Bode]

Greetings friends,

This semester I have started a course in C programming. I was moving
along fine until I reached to the topic of loops (feeling embarrassed
among you elite programmers). My prof. would post program questions
and the solutions online. For practice I would try to do the problems.
I would reach to a certain point in the code, for example as far as
error trapping, but when the loop arrives, like knowing whether to use
for, while do, how to properly use the increment and decrements, and
counters,I am just not proficient in it and the class is moving ahead.
Eventually i would have to look at the solution and wondering to
myself, the reason i could not think of it. What ticks me off is that
other kids are getting this stuff easily, while I am having a hard
time.Kindly advise me on what actions I shoul take. I would
particularly like to have an idea of the thought process to engage in
when given the programme to write.
Thanks for your time and consideration.

RG

It takes different people different amounts of time to learn certain
concepts. I know it's frustrating, *especially* when everyone else
seems to get it so easily, but keep plugging at it and eventually it
*will* make sense. I've been there; it took me a full two weeks longer
to grok the idea of a linked list than any of my classmates (not to
mention figuring out how to implement it in Fortran...). C's not the
easiest language to learn for novice programmers, anyway.

As for loops...

Generally, if you're iterating over a finite set of values or items
(either repeating an operation X times, or walking over an array or
other sequential structure), you would use a for loop. If you're
repeating an operation while some condition is true (or false), you'd
use a while loop. A do-while loop is a special case of a while loop,
where the operations in the loop are executed at least once.

 

[Richard G. Riley]

This is my thought process on loops:
1) while - never use

so you disagree with the most famous c line in history?

while(*dest++=*src++);

2) for - use as much as possible
3) do while - only use when necessary

I'll explain. The while loop only checks for a single statement being true.
It is very simple. So simple in fact that it really is useless. It

This is totally ridiculous. Many algorithms and program flows are
based on a single condition.

duplicates functionality that exists in a for loop. Therefore, why use it?
Many C programmers don't. while(1) becomes for(;. while (a<b) becomes
for(;a<b .The for loop is a very powerful loop construct that can
implement almost any loop. The only time you don't want to use a
for loop

And, as such, can be overkill for simple solutions. What can be easier
to read than

while(numThingsToDo--)

????


There is nothing big and clever by using "for" to obfuscate your code
where the power of the "for" loop is not required.

 

[pete]

so you disagree with the most famous c line in history?

while(*dest++=*src++);


This is totally ridiculous.

I can't make any sense out of that either.

Many algorithms and program flows are
based on a single condition.


And, as such, can be overkill for simple solutions. What can be easier
to read than

while(numThingsToDo--)

????

There is nothing big and clever by using "for" to obfuscate your code
where the power of the "for" loop is not required.

K&R2
3.5 Loops--While and For
"Whether to use while or for
is largely a matter of personal preference."

Most of the loops I write are while loops.
I write do loops whenever I can,
but the appropriate situations don't come up as often
as they do for the while loops and for loops.

When I want to increment a variable from one value to another,
I use a for loop,
but a substantial portion of the for loops that I've written,
have been condensed from while loops.

 

[Ed Jensen]

I don't believe it's particularly likely that any particular while
loop will be changed to a for loop in the future. Consider the common
while ((c = getchar()) != EOF) { ... }
Do you really think that this:
for (;(c = getchar()) != EOF { putchar(c); }
is an improvement?

Just use whichever form is clearer. Very often, that's going to be a
while loop.

How about this?

while (c = getchar(), c != EOF) { putchar(c); }

I meant this as a joke, but the more I look at it, the more I like it!