String reverse function/ global array modification help

[Tyler]

For starters here is the homework question:

read a string and print it in reverse (25)
reverseString function doesn't return anything
use one parameter
read a set of integers save them into an array and print the set in
reverse (25)
reverseInteger
use two parameters (array and size)
maximum array size is 100
use a loop to read values, stop when a user enters 'y'
Do Not print in reverse functions
Bonus
If you use pointers to swap values in reverse functions (10)
if you implemented with multiple files and Makefile (10)

essentially, I think my teacher wants us to modify the array/strings
globally but I'm not really sure how to do that and I'm getting some
funky errors with my code. However I think I'm really close.

Here's my code:

main.c

/*
* main.c
* hw_L9
*
* Created by Charles Cothren on 4/8/12.
* Copyright 2012 Syracuse University. All rights reserved.
*
*/

#include "reverseop.c"
#include <stdio.h>

int main ()
{
char part1string[100] = {0}, stop;
int part2array[100] = {0}, i = 0;


//Part 1


printf("Enter the string you would lik to reverse: ");
gets(part1string);

reverseString(part1string);

printf("Your string in reverse: %s", part1string);


//Part 2


printf("Integer Set Reverser\n");

while( stop /= 'y')
{
printf("Enter an the next integer: ");
scanf(" %d", part2array);
i++;
printf("Do you wish to stop? (y/n): ");
scanf(" %c", &stop);
}


reverseInteger(part2array, i);

printf("Your integers in reverse: %s", part2array);



return 0;
}

reverseop.c


#include <stdio.h>

void reverseString(char *strng);
void reverseInteger(int *arrayset[], int size);

void reverseString(char *strng)
{
int q = 0, i;
char strng2[strlen(strng)];


for (i = (strlen(strng)-1); i >= 0; i--)
{
strng2[q] = strng;
q++;
}

for (i = 0; i < (strlen(strng)-1); i++)
{
strng = strng2;
}

}

void reverseInteger(arrayset[], size)
{
int q = 0, i;
char arrayset2[size];

for (i = (size-1); i >= 0; i--)
{
arrayset2[q] = arrayset;
q++;
}


for (i = 0; i < (size-1); i++)
{
arrayset = arrayset2;
}
}

I get the error expected ) before ] token on this line and I don't
understand why

void reverseInteger(arrayset[], size)

 

[Stefan Ram]

read a string and print it in reverse (25) (...)
maximum array size is 100

If the length of the input string is not limited, but the
size of arrays is limited to 100, then complicated
constructions have to be used to cope with long inputs.

For example, when the string read has 2500 charactes, we
might use an array of 50 pointers to arrays of 50 chars
each to store the string.

When the string read has 125000 characters, one can use an
array of 50 pointers to arrays of 50 pointers to arrays of
50 chars each.

And so on.

This is getting more complicated by the fact that the length
of input is not known, when one starts to read it, so when
one starts with an array of 50 chars, the structure of the
input storage needs to be changed while reading more
characters.

But this is typical for programming: I/O and memory
management take all the effort, while the actual
»computation« (here: reversal) is not very much effort
in comparison.

SCNR

 

[Stefan Ram]

If the length of the input string is not limited, but the
size of arrays is limited to 100, then complicated
constructions have to be used to cope with long inputs.

That is, of course, unless one is using something like:

#include <stdio.h>

void reverse( void )
{ int const c = getchar(); if( c != EOF ){ reverse(); putchar( c ); }}

int main( void ){ reverse(); putchar( '\n' ); }

 

[Eric Sosman]

For starters here is the homework question:

read a string and print it in reverse (25)
reverseString function doesn't return anything
use one parameter
read a set of integers save them into an array and print the set in
reverse (25)
reverseInteger
use two parameters (array and size)
maximum array size is 100
use a loop to read values, stop when a user enters 'y'
Do Not print in reverse functions
Bonus
If you use pointers to swap values in reverse functions (10)
if you implemented with multiple files and Makefile (10)

essentially, I think my teacher wants us to modify the array/strings
globally but I'm not really sure how to do that and I'm getting some
funky errors with my code. However I think I'm really close.

It's really funky that you're feeling close.

Here's my code:

main.c

/*
* main.c
* hw_L9
*
* Created by Charles Cothren on 4/8/12.
* Copyright 2012 Syracuse University. All rights reserved.
*
*/

#include "reverseop.c"

Although this is technically legal, it's really not a good way
to do things. The purpose of a header file is not usually to contain
the code that implements functions, but to "publish" the functions'
"public faces." The header tells a prospective caller what he needs
in order to use a function: Its name, its arguments, and its return
value. The implementation would be in yet another file, the idea
being that you could apply a fix or improvement to the function code
without recompiling every module that calls it: As long as the name,
arguments, and return value remain unchanged, the callers need not
know about the details of how the function goes about its business.

So: Technically speaking, this satisfies the "multiple files"
requirement -- but don't expect to get full marks.

#include<stdio.h>

int main ()
{
char part1string[100] = {0}, stop;
int part2array[100] = {0}, i = 0;


//Part 1


printf("Enter the string you would lik to reverse: ");
gets(part1string);

Never use gets(). Not ever. It is an accident that isn't even
waiting to happen: It's already happened, twenty-four years ago.
(Look up "Morris Worm.") If you found gets() in a book, take a red
pencil and draw a big red X through that section. If your instructor
told you to use gets(), take an axe and draw a big red X through the
instructor.

reverseString(part1string);

printf("Your string in reverse: %s", part1string);

A smallish matter: Make sure there's a newline '\n' character
at the end of each line you output, including the final one. If
you don't, you might never see the final line.

//Part 2


printf("Integer Set Reverser\n");

while( stop /= 'y')

Two problems here: First, you almost certainly mean '!='
rather than '/='. Both are perfectly legal C operators, and both
(by ill chance) happen to be syntactically legal in this context,
but their meanings are wildly different.

Second, `stop' has not been given any value at all the first
time your program makes this test. That means you can't rely on
it having any particular value; specifically, you can't rely on it
having a value that isn't equal to 'y' ...

{
printf("Enter an the next integer: ");
scanf(" %d", part2array);
i++;
printf("Do you wish to stop? (y/n): ");
scanf(" %c",&stop);
}


reverseInteger(part2array, i);

printf("Your integers in reverse: %s", part2array);

No, for two reasons. First, `part2array' is not a string, and
thus cannot be printed with "%s". Second, only the first `i' elements
of `part2array' are of interest; the others are extraneous zeroes
that you don't want to print.

return 0;
}

reverseop.c


#include<stdio.h>

You're not doing any input/output in this file, so <stdio.h>
is unnecessary.

On the other hand, you *are* using string functions in this

void reverseString(char *strng);
void reverseInteger(int *arrayset[], int size);

The type of the `arrayset' parameter doesn't match the argument
the function is called with. (See above, and also see below.)

void reverseString(char *strng)
{
int q = 0, i;
char strng2[strlen(strng)];


for (i = (strlen(strng)-1); i>= 0; i--)
{
strng2[q] = strng;
q++;
}

for (i = 0; i< (strlen(strng)-1); i++)
{
strng = strng2;
}

This should work, I think, but it's working harder than it
needs to. Imagine that Alice, Barbara, and Clarisse are sitting
side by side in three chairs, and they decide to reverse their
order. Your solution is equivalent to setting out three more
chairs, moving all three girls to the new chairs (in a different
order), and moving them all back again: Three extra chairs, and
six total moves. Even if we suppose that only one girl can be on
her feet at any moment, can you think of a way to do the task with
just one extra chair and three moves? Can you generalize your new
method so that Alice,Barbara,Clarisse,...,Xena,Yolanda,Zerbinetta
can change places using one extra chair (and thirty-nine moves)
instead of twenty-six extra chairs (and fifty-two moves)?

}

void reverseInteger(arrayset[], size)

What are the types of these parameters?

{
int q = 0, i;
char arrayset2[size];

Why `char'?

for (i = (size-1); i>= 0; i--)
{
arrayset2[q] = arrayset;
q++;
}


for (i = 0; i< (size-1); i++)
{
arrayset = arrayset2;
}
}

Again, you're working too hard: The girls can reverse their
seating order without so much extra furniture.

I get the error expected ) before ] token on this line and I don't
understand why

void reverseInteger(arrayset[], size)

See above.

 

[Eric Sosman]

[...]
That is, of course, unless one is using something like:

#include<stdio.h>

void reverse( void )
{ int const c = getchar(); if( c != EOF ){ reverse(); putchar( c ); }}

Forbidden by the instructions for the exercise:

 

[Stefan Ram]

Just for fun I wrote the next program. It does not print in the
reverse function but admitedly does also not fulfill all requirements
since it might allocate an array with more than 100 components.
But at least it does not contain any hardcoded size limitations.

#include <stdio.h> /* puts, putchar */
#include <stdlib.h> /* malloc, free */

char * buff = 0;
size_t n;

void reverse( size_t const i )
{ int const c = getchar();
size_t const j = i + 1;
if( c != EOF )
{ reverse( j );
if( buff )buff[ n - j ]=( char )c; }
else
{ buff = malloc( j );
n = i;
if( buff )buff[ i ]= 0; }}

int main( void )
{ reverse( 0 );
if( buff ){ puts( buff ); free( buff ); buff = 0; putchar( '\n' ); }}

 

[Barry Schwarz]

For starters here is the homework question:

read a string and print it in reverse (25)
reverseString function doesn't return anything
use one parameter
read a set of integers save them into an array and print the set in
reverse (25)
reverseInteger
use two parameters (array and size)
maximum array size is 100
use a loop to read values, stop when a user enters 'y'
Do Not print in reverse functions
Bonus
If you use pointers to swap values in reverse functions (10)
if you implemented with multiple files and Makefile (10)

essentially, I think my teacher wants us to modify the array/strings
globally but I'm not really sure how to do that and I'm getting some
funky errors with my code. However I think I'm really close.

Why would you think that when your code doesn't even compile?

Here's my code:

main.c

/*
* main.c
* hw_L9
*
* Created by Charles Cothren on 4/8/12.

Since your name is Tyler, who is Charles and why are you posting his
homework?

* Copyright 2012 Syracuse University. All rights reserved.
*
*/

#include "reverseop.c"
#include <stdio.h>

int main ()
{
char part1string[100] = {0}, stop;
int part2array[100] = {0}, i = 0;


//Part 1


printf("Enter the string you would lik to reverse: ");
gets(part1string);

How will you insure the user does not enter more than 100 characters?

reverseString(part1string);

Where is the prototype for this function?

printf("Your string in reverse: %s", part1string);

It is normally advisable to end a printf with a \n.

//Part 2


printf("Integer Set Reverser\n");

while( stop /= 'y')

Why would you want to divide the value in stop by the value 'y'? Where
is stop defined?

{
printf("Enter an the next integer: ");
scanf(" %d", part2array);

Where is i defined?

i++;
printf("Do you wish to stop? (y/n): ");
scanf(" %c", &stop);
}


reverseInteger(part2array, i);

printf("Your integers in reverse: %s", part2array);

The %s promises printf that the corresponding argument is a char*.
Does the expression part2array evaluate to anything with type pointer
to char?

Do you get paid by the line? While some vertical white space makes
the code easier to understand, excessive blank lines just force people
to scroll back and forth endlessly.

return 0;
}

reverseop.c


#include <stdio.h>

void reverseString(char *strng);
void reverseInteger(int *arrayset[], int size);

This says that the first parameter is an array of pointers.

void reverseString(char *strng)

Let's play computer and process "abc".

{
int q = 0, i;
char strng2[strlen(strng)];

Where is the prototype for strlen? Of course this isn't legal since
the dimension of an array definition must be a compile time constant.

for (i = (strlen(strng)-1); i >= 0; i--)
{
strng2[q] = strng;
q++;
}

So strng2 contains 'c', 'b', 'a'. It is not a string since it does
not contain a terminating '\0'.

for (i = 0; i < (strlen(strng)-1); i++)

strlen(strng) evaluates to 3. strlen(strng)-1 evaluates to 2. You
iterate this loop for i with values 0 and 1. You never iterate for i
set to 2.

{
strng = strng2;

As a result, strng now contains "cbc" since you never copied the 'a'
over from strng2[2].

}

}

void reverseInteger(arrayset[], size)

Don't you think the parameters should have a type? This line will
generate so many compiler diagnostics the compiler will just give up.

{
int q = 0, i;
char arrayset2[size];

Why is this an array of char when you are supposed to be processing an
array of int.

for (i = (size-1); i >= 0; i--)
{
arrayset2[q] = arrayset;
q++;
}

You will save yourself a whole lot of aggravation if you learn to
indent consistently.

for (i = 0; i < (size-1); i++)
{
arrayset = arrayset2;

Again, you don't process the last element.

}
}

I get the error expected ) before ] token on this line and I don't
understand why

void reverseInteger(arrayset[], size)

Because it is gibberish. Look at the definition of reverseString for
a hint about something omitted.

In addition to the error you describe, my compiler produced nine other
diagnostics, some of which are fatal to your effort. If your compiler
doesn't, then you need to up the warning level and turn off
non-standard extensions.

 

[James Kuyper]

On Sun, 8 Apr 2012 19:38:58 -0700 (PDT), Tyler


Do you get paid by the line? While some vertical white space makes
the code easier to understand, excessive blank lines just force people
to scroll back and forth endlessly.

I think you mean "needlessly". If it is indeed "endlessly", you've just
discovered a new alternative to the life imprisonment.

....

{
int q = 0, i;
char strng2[strlen(strng)];

Where is the prototype for strlen? Of course this isn't legal since
the dimension of an array definition must be a compile time constant.

Not in C99. An automatic array with a dimension that is not a constant
expression defines a VLA. Do you have any reason to believe that he's
restricted to using C90?