Swapping array elements using pointer addressing

Hi, I have created a 1d array of 25 elements that I'd like to manipulate using pointers. So far I have the array printing out as if it were a [5][5] 2d array. I would like to swap the first and fourth column of the array. I'd appreciate any help on finding and correcting my mistake.
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 void swap_column(char *arr)
 {
     const int SIZE = 25;
     
     for(size_t i = 0; i < SIZE; i++)
     {
        
         char temp_array = *arr;
         *arr = *(arr + i);
         *(arr + i) = temp_array;
         
         
     }
     cout<< "Columns Swapped!"<< endl;
 }


output currently looks like:


e w e s t
h e a r t
e g r i t 
c l o n e 
o d o r s


should look like:

s s w e e
t h e a r
t e g r i 
t c l o n 
e o d o r
Last edited on
*arr is arr[0], which is the first element of the array. So you're always swapping with the first element.

There are 5 elements per column, so you'd only need to swap 5 pairs of elements.
You can access a (row, column) element using arr[row * 5 + column]

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// Example program
#include <iostream>

// column number starts from 0. Feel free to add 1.
void swap_columns_5x5(char arr[], int column_A, int column_B)
{
    for (size_t row = 0; row < 5; row++)
    {
        size_t index_A = 5 * row + column_A;
        size_t index_B = 5 * row + column_B;
        
        char temp_char = arr[index_A]; // (it's a single char, not an array)
        arr[index_A]   = arr[index_B];
        arr[index_B]   = temp_char;
    }
}

void print_5x5(char arr[]) // like your other thread
{
    for (int i = 0; i < 5; i++)
    {
        for (int j = 0; j < 5; j++)
        {
            std::cout << arr[5 * i + j] << ' ';   
        }
        std::cout << '\n';
    }
}

int main()
{
    char arr[] = "abcde12345vwxyzABCDE!@#$%";
    
    print_5x5(arr);
    std::cout << '\n';
    swap_columns_5x5(arr, 0, 3);
    print_5x5(arr);
}

a b c d e 
1 2 3 4 5 
v w x y z 
A B C D E 
! @ # $ % 

d b c a e 
4 2 3 1 5 
y w x v z 
D B C A E 
$ @ # ! % 


Also, instead of
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        char temp_char = arr[index_A];
        arr[index_A]   = arr[index_B];
        arr[index_B]   = temp_char;

you can use std::swap

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#include <algorithm>

// ...

std::swap(arr[index_A], arr[index_B]);
Last edited on
I would like to swap the first and fourth column of the array


Current:

e w e s t
h e a r t
e g r i t 
c l o n e 
o d o r s


Required:

s s w e e
t h e a r
t e g r i 
t c l o n 
e o d o r


The required does not follow from swapping first and forth column. Swapping first and forth column would give:


s w e e t
r e a h t
i g r e t
n l o c e
r d o o s


Does not compute ????

I ended up using two constant variables, one to let the for loop know when the end of a row was, and the other was used to control the swap.

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 void swap_column(char *arr)
 {
     const int SIZE = 25;
     const int ROW = 5;
     const int COL = 3;
     
     for(size_t i = 0; i<SIZE; i+=ROW)
     {
        char temp = *(arr + i);
        *(arr + i) = *(arr + i+COL);
        *(arr + i+ COL) = temp; 
         
     } 
   
 }
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