insertion sort using dynamic memory - c

Question

I do not know how to pass parameters to insertSort. I am also not certain if I am using scanf correctly.
I want the dynamic array like this:
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
void insertSort(int LENGTH, int *arr)
{
int temp;
for(int i=0; i<LENGTH; i++)
{
temp = arr[i];
for(int j=i+1; j<LENGTH; j++)
{
if( temp > arr[j] )
{
arr[i] = arr[j];
arr[j] = temp;
}
}
}
}
int main()
{
int LENGTH = 10;
int *arr = malloc(sizeof(int)*LENGTH); //create dynamic memory
memset(arr, 0, LENGTH); //initialize the space
printf("please enter nums length : %d\n", LENGTH);
for(int i=0; i<LENGTH; i++) //put numbers to the dynamic space
{
printf("%d index : ", i);
scanf("%d", &arr[i]);
}
for(int i=0; i<LENGTH; i++) //print those space
printf("%3d", arr[i]);
insertSort(&LENGTH, &arr); //SORT !
for(int i=0; i<LENGTH; i++)
printf("%3d", arr[i]);
}

Answer 1

Let's take this one piece at a time. For starters, your insertSort is not an insertion sort. An insertion sort implementation looks like:
void insertSort(int LENGTH, int *arr)
{
for (int i = 1; i < LENGTH; i++)
{
int tmp = arr[i], j;
for (j = i; j >= 1 && tmp < arr[j-1]; j--)
arr[j] = arr[j-1];
arr[j] = tmp;
}
}
Next, when you are testing functions, etc.., forego the user input and just test your function with a fixed set of data as a test-case, e.g.
int main(void) {
int arr[] = { 10, 2, 8, 5, 4, 6, 7, 3, 9, 1 },
nmemb = (int)(sizeof arr/sizeof *arr);
insertSort (nmemb, arr);
for (int i = 0; i < nmemb; i++)
printf ("%3d", arr[i]);
putchar ('\n');
}
Lastly, scanf. You cannot begin to use scanf correctly unless you validate its return. Further, in your case, what happens if the user types "ten" instead of the number 10 -- try it...
Any time you are taking user-input, you must account for each character that remains in the input buffer (stdin here). This is especially true when taking input with scanf (or family) due to the way scanf handles input or matching failures. When either occurs, no further characters are read, and any offending characters are left unread in the input buffer -- just waiting to bite you again on your next attempted read (generally resulting in an infinite loop if you are taking input within a loop)
(this is one of the primary reason a line-oriented function such as fgets or POSIX getline are recommended for taking user input)
scanf can be used, if used correctly. This means you are responsible for checking the return of scanf every time. You must handle three conditions
(return == EOF) the user canceling input by generating a manual EOF by pressing Ctrl+d (or on windows Ctrl+z, but see CTRL+Z does not generate EOF in Windows 10);
(return == expected No. of conversions) indicating a successful read -- it is then up to you to check whether the input meets any additional criteria (e.g. positive integer, positive floating-point, etc..); and
otherwise, you must handle the matching or input failure and you must account for every character that may be left in your input buffer. (generally you will scan forward in the input buffer until a '\n' or EOF is found discarding any extraneous characters that remain)
If you do your job, you can successfully use scanf as needed.