Checking the frequency of numbers throughout an array - c

Question

I have the following code which I'm trying to use to show how many times a certain number appears throughout the array. I want it to print out how many times it counted a specific number. (all numbers range from 0-20, with an index of 0 - 99).
void count_frequency(int *number) {
int i;
int j;
int len = sizeof number / sizeof(int);
printf("%i\n", len);
printf("reached here");
for(i = 0; i < 99; i++){ //index{
int c = 0;
for(j = 0; j < 99; j++){
if(number[j] == number[i]){
c++;
}else{
continue;
}
}
printf("%i\n", c);
//printf("%i\n", number[i]);
}
}
int main(){
int i;
int table[MAX];
int len = sizeof table / sizeof(int);
printf("%ireached before loop\n", len);
for(i = 0; i < len; i++){
table[i] = random_in_range(0, 20);
}
count_frequency(table);
//printf("%i", sizeof(table) / sizeof(int));
return 0;
}

Answer 1

int len = sizeof number / sizeof(int);
will not going to give you what you are expecting. number is a pointer variable not an array. Change your function to
void count_frequency(int *number, int len) {
int i;
int j;
int index[21] = {0};
printf("%i\n", len);
printf("reached here");
for(i = 0; i < len; i++){ //index{
index[number[i]%21]++;
}
for(i = 0; i < 21; i++)
printf("%d", index[i]);
}
and your function call should be
count_frequency(table, len);

Answer 2

Here you are. Honestly there was some magical thinking in your code. This works - it's not "the best" but an improvement on what you had. Let me know if this isn't clear. My definition of rand_in_range is not completely accurate - it will bias very slightly towards lower numbers.
include <stdio.h>
#include <stdlib.h>
#define MAX 100
#define random_in_range(a,b) ( rand() % ((b) - (a) + 1) + (a) )
void count_frequency(int *table, int *output, int n1, int n2) {
// n1 = number of elements in table
// n2 = number of elements in output
int i;
// int len = sizeof number / sizeof(int); <<<< meaningless!
// printf("%i\n", len);
// printf("reached here");
for(i = 0; i < n1; i++) { // loop over the elements in the table
output[table[i] %n 2]++; // using % n2 so we don't go outside bounds of array.
}
}
int main(){
int i;
int table[MAX];
int counts[21]={0}; // <<<< need a place to put the result
int len = sizeof table / sizeof(int);
for(i = 0; i < len; i++){
table[i] = random_in_range(0, 20);
}
count_frequency(table, counts, MAX, 21);
for(i = 0; i < 21; i++) {
printf("number %d - frequency %d\n", i, counts[i]);
}
return 0;
}
Output:
number 0 - frequency 3
number 1 - frequency 5
number 2 - frequency 3
number 3 - frequency 6
number 4 - frequency 6
number 5 - frequency 7
number 6 - frequency 3
number 7 - frequency 5
number 8 - frequency 6
number 9 - frequency 9
number 10 - frequency 4
number 11 - frequency 2
number 12 - frequency 6
number 13 - frequency 8
number 14 - frequency 3
number 15 - frequency 4
number 16 - frequency 5
number 17 - frequency 6
number 18 - frequency 6
number 19 - frequency 3
number 20 - frequency 0

Answer 3

So you're saying you have an array of 100 numbers, each ranging from 0-20, and you want to count the occurrences of a SINGLE number within that array? Then this should do:
size_t count_occurrences(int interesting_num, int* array, size_t array_size) {
size_t i, occurrences = 0;
for (i=0; i < array_size; ++i) {
if (array[array_size] == interesting_num) {
++occurrences;
}
}
return occurrences;
}
Technically, you could replace those "size_t"s with unsigned chars, for the number ranges you're talking about, but given the dynamic calculation of len that you attempted, and the use of "MAX", I suspect you may want to increase those ranges at some point. So, I'm erring on the side of a safer, more general implementation.