Possible mode error - c

Question

I've made this program that computes the mean, the median and the mode from an array. Although I've tested with some examples, I found out there might be a case that I have forgotten as for many of the inputs I've tested it works but the testing program that my teacher is using gave me an error for a certain test, but I was not presented with its input. Maybe someone can have a look and see if I am making a mistake at the mode point of the code:
#include <stdio.h>
#include <stdlib.h>
#include <limits.h>
void *safeMalloc(int n) {
void *p = malloc(n);
if (p == NULL) {
printf("Error: malloc(%d) failed. Out of memory?\n", n);
exit(EXIT_FAILURE);
}
return p;
}
int main(int argc, char *argv[]) {
int n, i;
scanf("%d", &n);
int *array = safeMalloc(n * sizeof(int));
for (i = 0; i < n; i++) {
int value;
scanf("%d", &value);
array[i] = value;
}
//mean
double mean;
double sum = 0;
for (i = 0; i < n; i++) {
sum = sum + (double)array[i];
}
mean = sum / n;
printf("mean: %.2f\n", mean);
//median
float temp;
int j;
for (i = 0; i < n; i++)
for (j = i + 1; j < n; j++) {
if (array[i] > array[j]) {
temp = array[j];
array[j] = array[i];
array[i] = temp;
}
}
printf("median: %d\n", array[n / 2]);
//mode
int val = array[0], noOfRepetitions = 1, valMax = array[0], maxRepetitions = 1, possibleMax = 1;
for (i = 1; i < n; i++) {
if (array[i] == val) {
noOfRepetitions++;
}
if (array[i] != val) {
val = array[i];
noOfRepetitions = 1;
}
if (noOfRepetitions == possibleMax) {
maxRepetitions = 1;
continue;
}
if (noOfRepetitions > maxRepetitions) {
valMax = val;
maxRepetitions = noOfRepetitions;
possibleMax = maxRepetitions;
}
}
if (maxRepetitions > 1) {
printf("mode: %d\n", valMax);
} else {
printf("mode: NONE\n");
}
return 0;
}
My idea for mode was because the numbers are sorted when just transverse it. If the next element is the same as the previous one, increase the noOfRepetitions. If the noOfRepetition is bigger than the maxRepetitions until now, replace with that. Also store the last maximum val needed if we have for example more than 2 numbers with the same number of repetitions.
EDIT: The mode of an array should return the number with the maximum number of occurrences in the array.If we have 2 or more number with the same number of maximum occurrences , there isn't a mode on that array.

Answer 1

I've discovered my mistake. I didn't think of the case when I have numbers with same maximum frequency and after that came one with lower frequency but still bigger than others. For example : 1 1 1 2 2 2 3 3 4 5 6.With my code , the result would have been 3 . I just needed to change the comparison of noOfRepetitions with oldMaxRepetition.

Answer 2

There seems to be no purpose for the variable possibleMax. You should just remove these lines:
if(noOfRepetitions==possibleMax){
maxRepetitions=1;
continue;
}
They cause maxRepetitions to be reset erroneously.
You could detect if the distribution is multimodal and print all mode values:
#include <stdio.h>
#include <stdlib.h>
#include <limits.h>
void *safeMalloc(int n) {
void *p = malloc(n);
if (p == NULL) {
printf("Error: malloc(%d) failed. Out of memory?\n", n);
exit(EXIT_FAILURE);
}
return p;
}
int main(int argc, char *argv[]) {
int n, i;
if (scanf("%d", &n) != 1 || n <= 0)
return 1;
int *array = safeMalloc(n * sizeof(int));
for (i = 0; i < n; i++) {
if (scanf("%d", &array[i]) != 1)
return 1;
}
//mean
double sum = 0;
for (i = 0; i < n; i++) {
sum = sum + (double)array[i];
}
printf("mean: %.2f\n", sum / n);
//median
int j;
for (i = 0; i < n; i++) {
for (j = i + 1; j < n; j++) {
if (array[i] > array[j]) {
int temp = array[j];
array[j] = array[i];
array[i] = temp;
}
}
}
printf("median: %d\n", array[n / 2]);
//mode
int val = array[0], noOfRepetitions = 1, valMax = array[0], maxRepetitions = 1;
for (i = 1; i < n; i++) {
if (array[i] == val) {
noOfRepetitions++;
if (noOfRepetitions > maxRepetitions) {
valMax = val;
maxRepetitions = noOfRepetitions;
}
} else {
val = array[i];
noOfRepetitions = 1;
}
}
if (maxRepetitions == 1) {
printf("mode: NONE\n");
} else {
printf("mode: %d", valMax);
val = array[0];
noOfRepetitions = 1;
for (i = 1; i < n; i++) {
if (array[i] == val) {
noOfRepetitions++;
} else {
if (noOfRepetition == maxRepetitions && val != valMax) {
printf(", %d", val);
}
val = array[i];
noOfRepetitions = 1;
}
}
if (noOfRepetition == maxRepetitions && val != valMax) {
printf(", %d", val);
}
printf("\n");
}
return 0;
}

Answer 3

Your code to search a mode seems too complicated. Compare this:
//mode
int val = array[0], noOfRepetitions = 1,
valMax = array[0], maxRepetitions = 1;
for (i = 1; i < n; i++) {
if (array[i] == val) {
if (++noOfRepetitions > maxRepetitions) {
valMax = val;
maxRepetitions = noOfRepetitions;
}
}
else
{
val = array[i];
noOfRepetitions = 1;
}
}
It's probably the simplest code to do what you need, but it overwrites maxVal and maxRepetitions much too often.
The following version overwrites the two 'max' variables only once per each new maximum found – at the cost of duplicating some part of code:
//mode
int val = array[0], noOfRepetitions = 1,
valMax = array[0], maxRepetitions = 1;
for (i = 1; i < n; i++) {
if (array[i] == val) {
++noOfRepetitions;
}
else
{
if (noOfRepetitions > maxRepetitions) {
valMax = val;
maxRepetitions = noOfRepetitions;
}
val = array[i];
noOfRepetitions = 1;
}
}
if (noOfRepetitions > maxRepetitions) {
valMax = val;
maxRepetitions = noOfRepetitions;
}