Finding the Peak number of an array in C

Question

I am new to coding in C. I am trying to write a code that finds the peak number in an array, that is the numbers that are greater than the numbers before and after said number.

This is my code. It runs without error but no output so I know I am doing something wrong.

#include <stdio.h>
int main() {
 int nums[14] = {1, 2, 3, 3, 2, 4, 1, 5, 6, 3, 1, 10, 2, 8};
  int peaks[4];

 for(int i = 0; i < nums[i]; i++){
     if(nums[i] > nums[i-1] && nums[i] > nums[i+1]){
         peaks == nums[i];
     }
     return peaks;
 }

 printf("Peak numbers are %d",peaks);
}

How can I get it to output this as the result: [4, 6, 10, 8]

Answer 1

Enable your compiler's warnigs!!! You should get something like this:

prog.c: In function 'main':
prog.c:8:16: warning: comparison between pointer and integer
          peaks == nums[i];
                ^~
prog.c:8:16: warning: statement with no effect [-Wunused-value]
          peaks == nums[i];
          ~~~~~~^~~~~~~~~~
prog.c:10:13: warning: returning 'int *' from a function with return type 'int' makes integer from pointer without a cast [-Wint-conversion]
      return peaks;
             ^~~~~
prog.c:10:13: warning: function returns address of local variable [-Wreturn-local-addr]
prog.c:13:28: warning: format '%d' expects argument of type 'int', but argument 2 has type 'int *' [-Wformat=]
  printf("Peak numbers are %d",peaks);
                           ~^  ~~~~~
                           %ls

So, change this:

 peaks == nums[i];

to this:

 peaks[j] = nums[i];

where j is another counter you could use.

Then you return peaks, why? It's not a custom function, so you shouldn't return.

Moreover, you try to print an array like:

printf("Peak numbers are %d",peaks);

which is not possible, you need to loop over the elements of the array, like this:

for(i = 0; i < 4; ++i)
    printf("%d\n", peaks[i]);

---

After correcting your warnings, you are ready to actually work on yuor logic. Your for loop will not iterate over all numbers of the array. So change this:

for(i = 0; i < nums[i]; i++)

to this:

for(i = 0; i < 14; i++)

Moreover your if statement will cause Undefined Behavior:

if(nums[i] > nums[i-1] && nums[i] > nums[i+1])

when i is 0 and when i is 13.

Answer 2

Here you have a corrected version of the program, your mistakes are in the comments:

#include <stdio.h>
int main() {
  int nums[14] = {1, 2, 3, 3, 2, 4, 1, 5, 6, 3, 1, 10, 2, 8};
  int peaks[4];
  int ctr=0;          // counter to count the amount of peaks

  for(int i = 1; i < 13; i++){   // start with one and end with 12, so you don't access elements outside of your array (-1 and 14)
                                 // You need to implement special cases for the first and the last element
    if(nums[i] > nums[i-1] && nums[i] > nums[i+1]){
      peaks[ctr] = nums[i];   // use an assignment instead of a comparison
      ctr++;                  // increment counter
    }
                              // No return here, it will end your main and the print will never be reached
  }

  printf("Peak numbers are ");
  for(int i = 0; i<ctr; i++)
    printf("%d ", peaks[i]);   // Print all the peaks you found, you need to print every element separate 

  return 0;  // main needs a return value
}

Try it online

I am trying to write a code that finds the peak number in an array, that is the numbers that are greater than the numbers before and after said number

I think the first and the last number can't fulfill this requirement, because they have only one neighbor.

Answer 3

This program works for any number of elements in the nums array and it takes into account the first and the last element correctly.

[sizeof(nums)/sizeof(nums[0]) is the actual number of elements in the nums array, therefore yolu can put any number of elements into the nums array and the program will always work correctly

Also the number of elements of peaks is no longer hard coded as 4 but the same as the number of elements in num, so we can insure that there won't be any index out of bounds problems. However as the maximum number of peaks of an array of size n is n/2 + 1 (not quite sure), we can probably write int peaks[sizeof(nums)/sizeof(nums[0])/2+1]

#include <stdio.h>

int main() {
  int nums[] = { 1, 2, 3, 3, 2, 4, 1, 5, 6, 3, 1, 10, 2, 8 };
  int peaks[sizeof(nums)/sizeof(nums[0])];
  int pi = 0;

  for (int i = 0; i < sizeof(nums)/sizeof(nums[0]); i++) {
    if (
        (i == 0 && nums[i + 1] < nums[i]) ||   /* first element */
        (i == sizeof(nums) / sizeof(nums[0]) && nums[i - 1] < nums[i]) || /* last element */
        (nums[i] > nums[i - 1] && nums[i] > nums[i + 1]) /* elements in the middle */
      )
    {
      peaks[pi++] = nums[i];
    }
  }

  for (int i = 0; i < pi; i++)
    printf("%d ", peaks[i]);
}

However the condition in the if statement can possibly be written in a more elegant manner.

Answer 4

bro write printf statement in if loop printf("%d",peak) or remove comparison operator ==in single = from peak==array[i] beacuse you are not comaring peak with them you are assigning value of array to peak when your condition satisfied