Sorting C

Question

I have an array that is sorted by the bubble method.I need sorting by even indices. I understand that this needs to be done through for (i = 2; i <20; i + = 2) but nothing works. Please help
My code:

#include <stdio.h>

main() {
int arr[20]={-12,0,3,34,2,99,81,21,75,7,48,-5,31,95,77,43,-50,88,-4,46}, i, j, b;
for (i=0; i<20; i++) {
printf("%4d", arr[i]);
}
printf("\n");

for (i=0; i < 20-1; i++) {
for (j=0; j < 20-i-1; j++) {
if (arr[j] < arr[j+1]) {
b = arr[j];
arr[j] = arr[j+1];
arr[j+1] = b;
}
}
}

for (i=0; i<20; i++)
printf("%4d", arr[i]);
printf("\n");
}

This code sorts all elements of an array.

Answer 1

Alex well, I think I got it working, with a help from a friend. Check this code hope it suits what you want to do;

https://code.sololearn.com/c78fkfd5VuiM/?ref=app

Answer 2

"I don’t want to complicate anything. Your code has a strange output, not the same as needed"

It doesn't flex the mind too much if you move your attention from additional decoration to the core mechanics of these two guys and the way they work together

// Outer loop is a helper loop: since we are discarding odd indexes 20 steps to traverse the whole list will be (20 - 1) / 2 which is 9 steps
int half_range = (N - 1) / 2;
for (int i = 0, s; i < half_range; ++i)
{
int travers_till = N - 2 * (i + 1);
for (int j = 0; j < travers_till; j += 2)
{
if (arr[j] < arr[j + 2]) { /*SWAPPING*/ }
}
}

Process in details (consider even steps for j):
i = 0, travers_till = 18, j = {0, 2, 4, ..., 16}
i = 1, travers_till = 16, j = {0, 2, 4, ..., 14}
i = 2, travers_till = 14, j = {0, 2, 4, ..., 12}
i = 3, travers_till = 12, j = {0, 2, 4, ..., 10}
i = 4, travers_till = 10, j = {0, 2, 4, ..., 8}
i = 5, travers_till = 8, j = {0, 2, 4, 6}
i = 6, travers_till = 6, j = {0, 2, 4}
i = 7, travers_till = 4, j = {0, 2}
i = 8, travers_till = 2, j = {0}

Answer 3

Alex , You can copy just the sorting block from line 42 to 54 if you don't want the other parts. And replace the "N" in the array declaration, and in the for loops declaration with 20 (number of array elements). Basically, these are the most important part, the part that sorts the elements on even indices.

Ask again if you have any doubts ...

Answer 4

The output seems to be well sorted, each pair also are sorted, in descending order. Can you give a little example of how exactly the output should be? or maybe you have solved this and updated the code?

Answer 5

What happens with numbers greater than 81?
BTW, this is what I see, I put each pair in square brackets, as I see it, they are sorted, sorry but I don't understand what the problem here.

[99 95] [88 81] [77 75] [48 46] [43 34]
[31 21] [7 3] [2 0] [-4 -5] [-12 -50]

Answer 6

Do you mean to sort the elements on even indices, only elements on even indices are sorted? I'm confused ...

(Edit)
Also do you mean to ignore the elements on odd indices all together?

Answer 7

Then you will read, compare, and swap (if necessary) the items on index

[0], [2], [4], [6], ..., [18]

Looks pretty tough, I'll see if I can think of something, I'll get back to you later if I can.

I think you need to rephrase the original post so the problem can be understood easier by later viewers.

P.S. If you want, I can delete my posts from here, so it returns to "Unanswered" state (no response) to prevent others misunderstanding this may have been solved, and refrained ...

Answer 8

yes, I need sorting by even indices

Answer 9

Ipang ,this is not what I need, I need to modify my code, I don’t want to complicate anything. Your code has a strange output, not the same as needed

Answer 10

Ipang,ok,I'll try when I'm home and accomplish my goal, thanks

Answer 11

output should be:
81,77,75,48,31,3,2,-4,-50
(i.e [6]=81,77=[14],75=[8]..and so on)

Answer 12

my goal is to arrange items that are in pair positions in descending order

Answer 13

[2],[4],[6],[8],[10],[12]....[18]
but descending