Transposing a matrix represented as a string?

Question

I have a 25 character long string where every subsequent 5 characters represent a row in the matrix.

String matrix="qwsedrhendtedksiehwnehsbt";

I am trying to transpose it using a function

public String transpose(String str) {

    if (str == null || str.length() == 1 || str.length() == 2) {
        return str;
    } else {
        return str.substring(0, 1) + str.substring(str.length() -1, str.length()) + transpose(str.substring(1, str.length() -1) );
    }
}

produces

result="qtwbssehdernhwehneditsekd"

this isnt the expected result. the result must be

expectedResult="qrtiewheehsedhsenkwbddsnt"

I am not able to understand where I am going wrong

Answer 1

This method works for strings representing square matrices.

static String transpose(String s) {
    int n = s.length();
    int m = (int) Math.sqrt(n);
    if (m * m != n)
        throw new IllegalArgumentException();
    StringBuilder sb = new StringBuilder();
    for (int i = 0; i < n - 1; i++)
        sb.append(s.charAt(i * m % (n - 1)));
    sb.append(s.charAt(n - 1));
    return sb.toString();
}

This more general version works for strings representing rectangular matrices.

static String transpose(String s, int rows, int columns) {
    int n = s.length();
    if (rows * columns != n)
        throw new IllegalArgumentException();
    StringBuilder sb = new StringBuilder();
    for (int i = 0; i < n - 1; i++)
        sb.append(s.charAt(i * columns % (n - 1)));
    sb.append(s.charAt(n - 1));
    return sb.toString();
}

Answer 2

This transposes a nxn char matrix stored in a string of length n².

static String transpose( String s ){
    char[] c = s.toCharArray();
    int n = (int)Math.sqrt(s.length());
    for( int i = 0; i < n; ++i ){
        for( int j = i+1; j < n; ++j ){
            char h = c[i*n+j];
            c[i*n+j] = c[j*n+i];
            c[j*n+i] = h;
        }
    }
    return new String( c );
}

Answer 3

I have an c++ implementation

#include <stdio.h>
#include <string.h>
#include <stdlib.h>


char* transpose (const char* matString, unsigned int col, unsigned int row)
{
    char *answer = (char*)calloc(strlen(matString), sizeof(char));

    unsigned int index = 0;

    for(unsigned int i=0;i<col;i++)
   {
        for(unsigned int j=0;j<row;j++)
    {
        answer[index++] = matString[j * col + i];
    }
 }

    return answer;
}

int main(int argc, char **argv)
{
printf("%s", transpose("ABCDEF", 3, 2));

    return 0;
}

It gives output ADBECF

for the program the input matrix was

A-B-C
D-E-F

After transposition,.it's row will be column & columns will be row

A-D
B-E
C-F

& the string representation will look like ADBECF . For a quick explanation of the matrix transposition can have a look inside Matrix transposition

this is more generalized implementation of your problem specifies. It takes an row & col input for further generalization. Hope it helps you.