Confused about functions

[agentxx04]

Hi. Our assignment was to creat a program that can find the average, median
& mode of a #of integers. Here's my program:

#include<stdio. h>

int main()
{
int item[100];
int a, b, t, mode;
int median_index;
float median, avg;

int count;
printf("How Many numbers do you want to enter? ");
scanf("%d", &count);
for(a=0; a<count; a++){
scanf("%d", &item[a]);
}
for(a=1; a<count; a++)
for(b=count-1; b>=a; --b){
if(item[b-1] > item[b]){
t=item[b-1];
item[b-1] = item[b];
item[b]=t;
}
}
/*median*/

median = count/2;
if(count%2 ==1){
printf("There are odd sets of numbers.\n");
median = item[count/2];
}

else {
median_index = count/2;

median = (item[median_index]+item[median_index-1])/2.0;
}
printf("The Median is %.1f\n", median);

/*average*/

avg = 0;
for(a=0; a<count; a++){
avg = avg+item[a];}
printf("Average is: %.1f\n", avg/count);

/*mode*/

for(a=0; a<count; a++){
if(item[a]== item[a+1]){
mode=item[a];
}
else{
mode=0;}
}

printf("Mode is: %d\n", mode);
return 0;
}

The problem is that I need to have the average, median & mode as 3 separate
functions. I don't know how to set those functions up. If anyone can show me
how this is possible, I would greatly appreciate it.

Thank You. : )

P.S. - I notice that there is a fault in my mode section of my program. If
the user enters 2 modes (ex. 1,2,2,4,4,5) my program will only display the
first. How could I correct this?

[Michael Mair]

agentxx04 wrote:

Hi. Our assignment was to creat a program that can find the average, median
& mode of a #of integers. Here's my program:

#include<stdio. h>
insert
#define NUMITEMS (100) int main() make that int main (void) {
int item[100]; make this NUMITEMS; int a, b, t, mode;
int median_index;
float median, avg; C does not give any guarantees about the relative integer ranges
of floating point variables, but if sizeof(float)== sizeof(int)
then there exist ints (for example INT_MAX from <limits.h>)
which cannot represented exactly by floats. So, I would suggest
to use either double or long double for median and avg.

int count;
printf("How Many numbers do you want to enter? ");
scanf("%d", &count); Check whether count <=NUMITEMS -- otherwise I would
just try and enter NUMITEMS+1... for(a=0; a<count; a++){
scanf("%d", &item[a]); scanf() returns the number of input items read, so to be
sure you should check whether scanf returns 1 in your case. }
Sorting: You essentially need the information
_what_ to sort, that is the array item and the type of the
array elements,
_how_many_ elements there are to sort.
You expect no value from this for(a=1; a<count; a++)
for(b=count-1; b>=a; --b){
if(item[b-1] > item[b]){
t=item[b-1];
item[b-1] = item[b];
item[b]=t;
}
}
Determination of the median: You need a sorted array and
the size of the array (and have to know the array type.
You expect a float/double/long double value out of this. /*median*/

median = count/2; This is unnecessary. if(count%2 ==1){
printf("There are odd sets of numbers.\n");
median = item[count/2];
}

else {
median_index = count/2;

median = (item[median_index]+item[median_index-1])/2.0; count = 3 => median_index = 1 => you are looking at item[0]
and item[1]. Make it median_index+1. }
printf("The Median is %.1f\n", median);

Determination of average: You need array, type of array
members and number of array elements. array has not to be
sorted.
You expect a float/double/long double value out of this. /*average*/

avg = 0;
for(a=0; a<count; a++){
avg = avg+item[a];}
printf("Average is: %.1f\n", avg/count);
Mode: I do not understand in the least what mode should
be but I guess the above holds here, too. It seems that you
assume a sorted array. /*mode*/

for(a=0; a<count; a++){ You have an off-by-one error here: if a==count-1
then the next line will access item[count] which
is out of bounds. Make it run as long a<count-1
or start at a=1 and look at item[a-1] and item[a]. if(item[a]== item[a+1]){
mode=item[a];
}
else{
mode=0;}
} This effectively sets in every iteration of the
loop mode to either 0 or item[a]. If
item[count-1]==item[count] then mode=item[count-1],
otherwise 0 after the loop. This does not require a
loop at all.
printf("Mode is: %d\n", mode);
return 0;
}

The problem is that I need to have the average, median & mode as 3 separate
functions. I don't know how to set those functions up. If anyone can show me
how this is possible, I would greatly appreciate it.

Thank You. : )

P.S. - I notice that there is a fault in my mode section of my program. If
the user enters 2 modes (ex. 1,2,2,4,4,5) my program will only display the
first. How could I correct this?

Example: Sorting:
In: array, size
Out: Nothing
void sort_it (int size, int array[])
{
int a, b; /* loop counters, type like size */
int tmp; /* temp. storage, type like array[0] */

for (a=1; a<size; a++)
for (b=size-1; b>=a; --b) {
if (array[b-1] > array[b]) {
tmp = array[b-1];
array[b-1] = array[b];
array[b] = t;
}
}
}
Example: Median:
In: array, count
Out: float/double/long double; in keeping with your program,
I will use float but disadvise its use.
float array_median (int size, int array[])
{
int index; /* index, type like size */
float median; /* median variable, type like return type */

index = size/2;
if ( (size%2) == 1 ) {
printf("There are odd sets of numbers.\n");
median = array[index];
}
else {
median = (array[index]+array[index+1])/2.0; /* (*) Mark */
}

return median;
}

Rest left to reader as exercise.

Notes:
- For array indices, it is better to use the type size_t
(instead of int), as size_t ist guaranteed to work for all
possible indices. (If you have 16-Bit integers and 2GB
memory and use an array with more than pow(2,15)-1 elements
(e.g. 33000), then int cannot used any longer as index
whereas size_t can.)
- (*): If array[index]+array[index+1]>INT_MAX, you run into
trouble. If you demand that one of the two is converted to
double or long double prior to addition then the sum will
be done in (long) double:
median = ((double)array[index]+array[index+1])/2.0;
suffices.
- Your sorting algorithm is the worst possible. Look for
insertion sort or selection sort for (marginally) better
"simple" algorithms and for Shell sort or quicksort for
more sophisticated algorithms able to deal quickly (in
comparison with the former) with large arrays.
- There are some more issues but I suggest you work on
your code and come back then.
Cheers
Michael
--
E-Mail: Mine is an /at/ gmx /dot/ de address.

[Martin Ambuhl]

agentxx04 wrote:

Hi. Our assignment was to creat a program that can find the average, median
& mode of a #of integers. Here's my program:

Try something like this (but don't try turning this in unless you can
justify every line):

#include <stdio.h>
#include <stdlib.h>

int qsort_dblcmp(co nst void *, const void *);
int qsort_cntcmp(co nst void *, const void *);
double get_median(int n, double x[n]);
double get_average(int n, double x[n]);
void get_modes(int n, double x[n], int *nm, double m[n]);

int main(void)
{
double *item, *modes;
int ndx, nmode;
double median, average;

int count;
printf("How Many numbers do you want to enter? ");
scanf("%d", &count);
if (count < 1)
exit(EXIT_SUCCE SS);
if (!(item = malloc(count * sizeof *item))) {
fprintf(stderr, "I could not get space for %d numbers.\n"
"quitting ...\n", count);
exit(EXIT_FAILU RE);
}
if (!(modes = malloc(count * sizeof *modes))) {
fprintf(stderr, "I could not get space for %d possible modes.\n"
"quitting ...\n", count);
exit(EXIT_FAILU RE);
}
for (ndx = 0; ndx < count; ndx++) {
scanf("%lf", &item[ndx]);
}
qsort(item, count, sizeof *item, qsort_dblcmp);
median = get_median(coun t, item);
average = get_average(cou nt, item);
printf("The Median is %g\n", median);
printf("Average is: %g\n", average);
get_modes(count , item, &nmode, modes);
printf("There are %d nodes. They are: \n", nmode);
for (ndx = 0; ndx < nmode; ndx++)
printf("%g\n", modes[ndx]);

free(item);
free(modes);

return 0;
}

int qsort_dblcmp(co nst void *p1, const void *p2)
{
return *(double *) p1 - *(double *) p2;
}

double get_median(int n, double x[n])
{
if (n % 2)
return x[n / 2];
return (x[n / 2] + x[n / 2 - 1]) / 2;
}

double get_average(int n, double x[n])
{
double sum = 0;
int i;
for (i = 0; i < n; i++)
sum += x[i];
return sum / n;
}

typedef struct
{
double v;
int n;
} Cnt;

void get_modes(int n, double x[n], int *nm, double m[n])
{
int i, this = 0;
Cnt cnt[n];
if (n < 1) {
*nm = 0;
return;
}
for (i = 0; i < n; i++) {
cnt[i].v = 0;
cnt[i].n = 0;
}
cnt[0].v = x[0];
cnt[0].n = 1;
for (i = 1; i < n; i++) {
if (x[i] != cnt[this].v) {
this++;
cnt[this].v = x[i];
}
++cnt[this].n;
}
qsort(cnt, this + 1, sizeof *cnt, qsort_cntcmp);
*nm = 0;
m[0] = cnt[0].v;
for (i = 1; i <= this && cnt[i].n == cnt[i - 1].n; i++) {
++*nm;
m[*nm] = cnt[i].v;
}
++*nm;

}

int qsort_cntcmp(co nst void *p1, const void *p2)
{
return ((const Cnt *) p2)->n - ((const Cnt *) p1)->n;
}

[Method Man]

> double get_median(int n, double x[n]);

double get_average(int n, double x[n]);

I had a couple questions regarding the above function prototypes:

1. Is the 'n' in 'double x[n]' related to the first parameter 'int n'? I'm
assuming it's not.
2. Does adding 'n' to 'double x[n]' mean anything in a function declaration?
I'm assuming it doesn't.

If my assumptions are correct, I'd think it's more clear to write 'double
x[]' or just 'double[]' as your parameter.

[Michael Mair]

Method Man wrote:

[Attribution inserted:] Martin Ambuhl wrote

double get_median(int n, double x[n]);
double get_average(int n, double x[n]);

I had a couple questions regarding the above function prototypes:

1. Is the 'n' in 'double x[n]' related to the first parameter 'int n'? I'm
assuming it's not.
2. Does adding 'n' to 'double x[n]' mean anything in a function declaration?
I'm assuming it doesn't.

If my assumptions are correct, I'd think it's more clear to write 'double
x[]' or just 'double[]' as your parameter.

Martin wrote C99 code; there, 1. is answered with yes and 2. is answered
with x is a VLA of n doubles.
Cheers
Michael
--
E-Mail: Mine is a gmx dot de address.

[pete]

Michael Mair wrote:

agentxx04 wrote:

Hi. Our assignment was to creat a program that can find the average, median
& mode of a #of integers. Here's my program:

#include<stdio. h>

insert
#define NUMITEMS (100)

int main()

make that int main (void)

{
int item[100];

make this NUMITEMS;

int a, b, t, mode;
int median_index;
float median, avg;

C does not give any guarantees about the relative integer ranges
of floating point variables, but if sizeof(float)== sizeof(int)
then there exist ints (for example INT_MAX from <limits.h>)
which cannot represented exactly by floats. So, I would suggest
to use either double or long double for median and avg.

I would suggest type int for the median.

--
pete

[Michael Mair]

pete wrote:

Michael Mair wrote:

agentxx04 wrote:

Hi. Our assignment was to creat a program that can find the average, median
& mode of a #of integers. Here's my program:

#include<std io.h>

insert
#define NUMITEMS (100)

int main()

make that int main (void)

{
int item[100];

make this NUMITEMS;

int a, b, t, mode;
int median_index;
float median, avg;

C does not give any guarantees about the relative integer ranges
of floating point variables, but if sizeof(float)== sizeof(int)
then there exist ints (for example INT_MAX from <limits.h>)
which cannot represented exactly by floats. So, I would suggest
to use either double or long double for median and avg.

I would suggest type int for the median.

The OP defines the median for an even number of array elements as
average of the two in the middle. For a difference of 1 between the
two we cannot find an int value to replace the average.

This is consistent with, e.g, the definition on
http://www.shodor.org/interactivate/dictionary/m.html
(note: this is the first I came across when googling for definition
of median) :
|"Middle value" of a list. The smallest number such that at least half
|the numbers in the list are no greater than it. If the list has an odd
|number of entries, the median is the middle entry in the list after
|sorting the list into increasing order. If the list has an even number
|of entries, the median is equal to the sum of the two middle (after
|sorting) numbers divided by two. The median can be estimated from a
|histogram by finding the smallest number such that the area under the
|histogram to the left of that number is 50% (cf Mean, Median and Mode
|Discussion).
so I think double is the right thing.

However, we are doing C, not statistics, so I leave it to the OP
which definition he wants :-)
Cheers
Michael
--
E-Mail: Mine is a gmx dot de address.

[pete]

Michael Mair wrote:

I would suggest type int for the median.
The OP defines the median for an even number of array elements as
average of the two in the middle. For a difference of 1 between the
two we cannot find an int value to replace the average.

However, we are doing C, not statistics, so I leave it to the OP
which definition he wants :-)

Sorry.
I realize my suggestion was off topic too.

--
pete

[osmium]

"agentxx04" writes:

Hi. Our assignment was to creat a program that can find the average,
median
& mode of a #of integers. Here's my program:

#include<stdio. h>

int main()
{
int item[100];
int a, b, t, mode;
int median_index;
float median, avg;

double mean(int* arr, int n);
double median(int* arr, int n);
int mode(int* arr, int n);

Write the matching functions. arr is item (it's already a pointer), and n
is the number of valid items in arr. I think the instructor asssumes that
there *is* a mode and that n is at least 3 or so. It is customary to use
double in C where common sense would lead you to use float. Float is
usually treated as a poor relation.
<snip>

[Old Wolf]

Michael Mair <Mi**********@i nvalid.invalid> wrote:

Method Man wrote:
Martin Ambuhl wrote

double get_median(int n, double x[n]);
double get_average(int n, double x[n]);

1. Is the 'n' in 'double x[n]' related to the first parameter
'int n'? I'm assuming it's not.
2. Does adding 'n' to 'double x[n]' mean anything in a
function declaration? I'm assuming it doesn't.

Martin wrote C99 code; there, 1. is answered with yes and 2. is answered
with x is a VLA of n doubles.

If so, then why would he have bothered with the first parameter?

The n in x[n] means exactly the same in C99 as it did in C89
(ie. nothing). You can't pass VLAs by value any more than
you can pass regular arrays by value. So the answers
are 'no' and 'no'.