reverse array in range in C - arrays

Question

I have to solve it in C language. I have arrays with n integers. L and U are lower and upper bound. I have to reverse numbers in array which is in [L,U]. I tried it by this way, but in some cases the answer is wrong. What mist be changed in the code? Or is there any other logic to complete the task?
#include <stdio.h>
int main() {
int x, arr[100], n, l, u, a, temp, temp1;
scanf("%d%d%d", &n, &l, &u);
for (int i = 0; i < n; i++) {
scanf("%d", &x); // read elements of an array
arr[i] = x;
}
a = n / 2;
for (int i = 0; i < a; i++) {
for (int j = a; j < n; j++) {
if (arr[i] >= l && arr[i] <= u) {
if (arr[j] >=l && arr[j] < u) {
temp = arr[j];
temp1 = arr[i];
arr[i] = temp;
arr[j] = temp1;
}
}
}
}
for (int i = 0; i < n; i++) {
printf("%d ", arr[i]);
}
}
sample input:
10(number of integers) -7(lower bound) 5(upper bound)
-10 -9 5 -2 -3 7 10 6 -8 -5
sample output:
-10 -9 -5 -3 -2 7 10 6 -8 5
my output:
-10 -9 -5 -2 -3 7 10 6 -8 5

Answer 1

There is an O(N) solution that does not require nesting of loops.
First, with the code as you as you have it, declare an additional array and some other helper variables that keeps track of what indices need to be swapped.
int left, right;
int swaplist[100] = {0};
int swapcount = 0;
Your can keep your initial intake loop exactly as you have it, but amended to append the index of the newly scanned value to the swaplist array if the value is between the lower and upper bounds.
for (int i = 0; i < n; i++) {
scanf("%d", &x); // read elements of an array
arr[i] = x;
if ((x >= l) && (x <= u)) {
swaplist[swapcount++] = i;
}
}
Then a single loop to iterate over "swaplist" and do the swaps against the original array.
left = 0;
right = swapcount-1;
while (left < right) {
int leftindex = table[left];
int rightindex = table[right];
int tmp = arr[leftindex];
arr[leftindex] = arr[rightindex];
arr[rightindex] = tmp;
left++; right--;
}

Answer 2

You made a valiant attempt. Your nested for() loops are appropriate for some kinds of sorting algorithms, but not for what seems to be the purpose of this task.
From the sample input and desired output, you really want to establish a 'bracket' at either end of the array, then shift both toward the centre, swapping elements whose value happens to satisfy low <= n <= high value. (In this case, -7 <= n <= 5).
Here's a solution:
#include <stdio.h>
int swap( int arr[], size_t l, size_t r ) { // conventional swap algorithm
int t = arr[l];
arr[l] = arr[r];
arr[r] = t;
return 1;
}
int main() {
int arr[] = { -10, -9, 5, -2, -3, 7, 10, 6, -8, -5, }; // your data
size_t i, sz = sizeof arr/sizeof arr[0];
for( i = 0; i < sz; i++ ) // showing original version
printf( "%d ", arr[i] );
putchar( '\n' );
#define inRange( x ) ( -7 <= arr[x] && arr[x] <= 5 ) // a good time for a macro
size_t L = 0, R = sz - 1; // 'L'eft and 'R'ight "brackets"
do {
while( L < R && !inRange( L ) ) L++; // scan from left to find a target
while( L < R && !inRange( R ) ) R--; // scan from right to find a target
} while( L < R && swap( arr, L, R ) && (L+=1) > 0 && (R-=1) > 0 );
for( i = 0; i < sz; i++ ) // showing results
printf( "%d ", arr[i] );
putchar( '\n' );
return 0;
}
-10 -9 5 -2 -3 7 10 6 -8 -5
-10 -9 -5 -3 -2 7 10 6 -8 5

Answer 3

If I have understood the assignment correctly you need to reverse elements of an array that satisfy some condition.
If so then these nested for loops
for (int i = 0; i < a; i++) {
for (int j = a; j < n; j++) {
if (arr[i] >= l && arr[i] <= u) {
if (arr[j] >=l && arr[j] < u) {
temp = arr[j];
temp1 = arr[i];
arr[i] = temp;
arr[j] = temp1;
}
}
}
}
do not make sense.
It is enough to use only one for loop as shown in the demonstration program below.
#include <stdio.h>
int main( void )
{
int a[] = { 1, 10, 2, 3, 20, 4, 30, 5, 40, 6, 7, 50, 9 };
const size_t N = sizeof( a ) / sizeof( *a );
for (size_t i = 0; i < N; i++)
{
printf( "%d ", a[i] );
}
putchar( '\n' );
int l = 10, u = 50;
for (size_t i = 0, j = N; i < j; i++ )
{
while (i < j && !( l <= a[i] && a[i] <= u )) ++i;
if (i < j)
{
while (i < --j && !( l <= a[j] && a[j] <= u ));
if (i < j)
{
int tmp = a[i];
a[i] = a[j];
a[j] = tmp;
}
}
}
for (size_t i = 0; i < N; i++)
{
printf( "%d ", a[i] );
}
putchar( '\n' );
}
The program output is
1 10 2 3 20 4 30 5 40 6 7 50 9
1 50 2 3 40 4 30 5 20 6 7 10 9
You could write a separate function as for example
#include <stdio.h>
void reverse_in_range( int a[], size_t n, int low, int upper )
{
for (size_t i = 0, j = n; i < j; )
{
while (i < j && !( low <= a[i] && a[i] <= upper )) ++i;
if (i < j)
{
while (i < --j && !( low <= a[j] && a[j] <= upper ));
if (i < j)
{
int tmp = a[i];
a[i] = a[j];
a[j] = tmp;
++i;
}
}
}
}
int main( void )
{
int a[] = { 1, 10, 2, 3, 20, 4, 30, 5, 40, 6, 7, 50, 9 };
const size_t N = sizeof( a ) / sizeof( *a );
for (size_t i = 0; i < N; i++)
{
printf( "%d ", a[i] );
}
putchar( '\n' );
reverse_in_range( a, N, 10,50 );
for (size_t i = 0; i < N; i++)
{
printf( "%d ", a[i] );
}
putchar( '\n' );
}

Answer 4

Thanks for everyone help. I read all of them, but I found another way to solve this problem. I will write it just in case. (some variable names are random, so in case of questions, comment).
#include <stdio.h>
int main() {
int x, main[100], n, l, u, a = 0, arr[100], temp, m = 0,f=0,c,d;
scanf("%d%d%d", &n, &l, &u);
for (int i = 0; i < n; i++) {
scanf("%d", &x); // read elements of an array
main[i] = x;
if (x >= l && x <= u) {
a++; //check if element is in range [l,u] and increasing a. later "a" will be used a length of the array "arr". this array cootains elements, which in in [u,l].
}
}
//add [u,l] elements in new array "arr"
for (int i = 0; i < n; i++) {
if (main[i] >= l && main[i] <= u) {
arr[m] = main[i];
m++; //index counter of "arr",
}
}
d=0;
for(int i=0;i<n;i++){
if(main[i]==arr[d]){
c=arr[a-d-1];
main[i]=c;
d++;
}
}
for(int i=0;i<n;i++){
printf("%d ",main[i]);
}
}

Answer 5

My best guess is that scanf is very annoying, on top of that, your format is ambiguous.
How will %d%d%d read 1234? Will it give you 12 3 and 4? 1 23 and 4? ...
try to do
scanf("%d %d %d" ...); // or
scanf("%d, %d, %d" ...);
something like that. Note that scanf is not recommended to be used, getc is a neat alternative, though also annoying when you want to read numbers with more than one digit, but you could create a function read_number, which, based on getc, will read a number as a string and return the int value with stoi.