Finding mode of a sorted array - c

Question

I was hoping to get some input on how to correctly find the mode of a sorted array in C. Here's what I'm working with. First, my freqDistrib function:
void freqDistrib(int t[], int num) {
int i, j, x;
static int f[SIZE];
printf("\n\n");
printf("Part C:\n");
printf(" %-5s %-16s\n", "score", "frequency");
printf(" %-5s %-16s\n", "-----", "---------");
for (i = 1; i < num; i++) {
x = t[i];
j = i - 1;
while (j >= 0 && x > t[j]) {
t[j + 1] = t[j];
j = j - 1;
}
t[j + 1] = x;
} // sorts input in descending order
for (i = 0; i < num; i++) {
++f[t[i]];
if (t[i] != t[i + 1])
printf(" %3d %7d\n", t[i], f[t[i]]);
} // finds the frequency of each input and prints
}
and my mode function:
void modeScore(int t[], int num) {
int i, j, max, mode;
int f[SIZE];
for (i = 0; i < num; i++) {
f[t[i]]++;
}
mode = 0;
max = 0;
for (j = 0; j < num; j++) {
if (f[j] > max) {
max = f[j];
mode = j;
}
}
printf("\n\n");
printf("Part F:\n");
printf("%3d is mode\n", mode);
}
Output right now:
0 is mode
I realize that I need to compare the current frequency tally to a max, and if it is higher than set it equal to max. I just can't for the life of my figure out why this isn't working. Any input would be appreciated thanks.

Answer 1

As noted in a comment, the principal problem was that you did not initialize f in the function.
Here's working code which shows that was all the change that was necessary:
#include <stdio.h>
enum { SIZE = 100 };
static void modeScore(int t[], int num)
{
int f[SIZE] = { 0 };
for (int i = 0; i < num; i++)
f[t[i]]++;
int mode = 0;
int max = 0;
for (int j = 0; j < num; j++)
{
if (f[j] > max)
{
max = f[j];
mode = j;
}
}
printf("Part F: %3d is mode (count %d)\n", mode, max);
}
int main(void)
{
int data[] =
{
/* random -n 40 0 9 | commalist -b ' ' -l 38 */
7, 8, 3, 8, 5, 9, 5, 8, 9, 6,
8, 0, 1, 8, 2, 0, 3, 4, 3, 3,
3, 0, 2, 7, 5, 4, 6, 5, 9, 0,
1, 9, 1, 7, 8, 0, 5, 4, 0, 8,
};
enum { DATA_SIZE = sizeof(data) / sizeof(data[0]) };
modeScore(data, DATA_SIZE);
return 0;
}
For the given data, the output is:
Part F: 8 is mode (count 7)
(Funny coincidence: the first time I ran it — with slightly different data — the answer came back as Part F: 0 is mode (count 7). This was actually correct; I changed one of the zeros into an eight to produce the current result.)
Note that I would normally design the function to return the modal value, rather than have it do the printing. As it stands, it can't be reused because the printing is not material to most code that wants to determine the mode.
The code also does not check that the values in t fall in the range 0..SIZE-1 (but it should).
The parameter order you use is traditional, but there is at least nominally some advantage to using:
void modeScore(int num, int t[num])
to express explicitly that the array t has num elements in it. The size must precede its use in the array dimentson, though.

Answer 2

Since you say the array is sorted, you don't need to create a temporary array to count the values. Instead, you just need to traverse the input, keeping count of how many identical values you've seen in the current run. At the end of the run, then compare the length of the run to the previous longest run, and update if the new run is longer.
(Actually, we update whenever the current run is longer, so we don't forget to update if the mode is the very last value).
Notice that we no longer need SIZE to bound our values; we can now work with the full range of int if we wish.
#include <stdio.h>
static void modeScore(const int t[], int num)
{
int start = 0;
int mode = t[0];
int score = 0;
for (int i = 1; i < num; ++i) {
if (i - start > score) {
// a longer run than the previous candidate
score = i - start;
mode = t[start];
}
if (t[start] != t[i])
// we've reached a change in value
start = i;
}
printf("Part F: %3d is mode (count %d)\n", mode, score);
}
You didn't provide a main(), so I adapted Jonathan Leffler's:
int main(void)
{
int data[] =
{
/* random -n 40 0 9 | commalist -b ' ' -l 38 */
0, 0, 0, 0, 0, 0, 1, 1, 1, 2,
2, 3, 3, 3, 3, 3, 4, 4, 4, 5,
5, 5, 5, 5, 6, 6, 7, 7, 7, 8,
8, 8, 8, 8, 8, 8, 9, 9, 9, 9,
};
modeScore(data, sizeof data / sizeof data[0]);
return 0;
}