How to declare 2D array in bash

Question

I m wondering how to declare a 2D array in bash and then initialize to 0   In C it looks like this   int a 4  5     0     And how do I assign a value to an element  As in C   a 2  3    3

User · Answer

Bash does not support multidimensional arrays.

You can simulate it though by using indirect expansion:

#!/bin/bash
declare -a a0=(1 2 3 4)
declare -a a1=(5 6 7 8)
var="a1[1]"
echo ${!var}  # outputs 6

Assignments are also possible with this method:

let $var=55
echo ${a1[1]}  # outputs 55

Edit 1: To read such an array from a file, with each row on a line, and values delimited by space, use this:

idx=0
while read -a a$idx; do
    let idx++;
done </tmp/some_file

Edit 2: To declare and initialize a0..a3[0..4] to 0, you could run:

for i in {0..3}; do
    eval "declare -a a$i=( $(for j in {0..4}; do echo 0; done) )"
done

User · Answer

2D array can be achieved in bash by declaring 1D array and then elements can be accessed using  r   col size    c   Below logic delcares 1D array  str 2d arr  and prints as 2D array   col size 3 str 2d arr    str 2d arr    abc   200   xyz   str 2d arr    def   300   ccc   str 2d arr    aaa   400   ddd    echo  Print 2D array  col count 0 for elem in   str 2d arr      do     if     col count  -eq   col size     then         echo            col count 0     fi     echo -e   elem  c        col count     done echo      Output is  Print 2D array abc 200 xyz  def 300 ccc aaa 400 ddd   Below logic is very useful to get each row from the above declared 1D array str 2d arr     Get nth row and update to 2nd arg get row n         row  1     local -n a  2     start idx    row   col size       for   i   0  i  lt    col size   i      do         idx    start idx   i           a     str 2d arr   idx         done    arr    get row n 0 arr echo  Row 0  for e in   arr      do     echo -e   e  c  done echo      Output is  Row 0 abc 200 xyz

User · Answer

Another approach is you can represent each row as a string  i e  mapping the 2D array into an 1D array  Then  all you need to do is unpack and repack the row s string whenever you make an edit     Init a 4x5 matrix a   00 01 02 03 04   10 11 12 13 14   20 21 22 23 24   30 31 32 33 34    aset       row  1   col  2   value  3   IFS     read -r -a tmp  lt  lt  lt     a  row      tmp  col   value   a  row     tmp           Set a 2  3    9999 aset 2 3 9999    Show result for r in    a       do   echo  r done   Outputs   00 01 02 03 04 10 11 12 13 14 20 21 22 9999 24 30 31 32 33 34

User · Answer

A way to simulate arrays in bash  it can be adapted for any number of dimensions of an array       bin bash     The following functions implement vectors  arrays  operations in bash     Definition of a vector  lt v gt           v 0 - variable that stores the number of elements of the vector         v 1  v n  where n v 0 - variables that store the values of the vector elements  VectorAddElementNext        Vector Add Element Next   Adds the string contained in variable  2 in the next element position  vector length   1  in vector  1      local elem value     local vector length     local elem name      eval elem value      2       eval vector length    1  0     if   -z   vector length     then         vector length    0       fi      vector length     vector length   1        elem name  1  vector length      eval  elem name     elem value       eval  1 0  vector length    VectorAddElementDVNext        Vector Add Element Direct Value Next   Adds the string  2 in the next element position  vector length   1  in vector  1      local elem value     local vector length     local elem name      eval elem value   2      eval vector length    1  0     if   -z   vector length     then         vector length    0       fi      vector length     vector length   1        elem name  1  vector length      eval  elem name     elem value       eval  1 0  vector length    VectorAddElement        Vector Add Element   Adds the string contained in the variable  3 in the position contained in  2  variable or direct value  in the vector  1      local elem value     local elem position     local vector length     local elem name      eval elem value      3       elem position     2       eval vector length    1  0     if   -z   vector length     then         vector length    0       fi      if    elem position -ge  vector length    then         vector length  elem position     fi      elem name  1  elem position      eval  elem name     elem value       if      elem position -eq 0    then         eval  1 0  vector length     fi    VectorAddElementDV        Vector Add Element   Adds the string  3 in the position  2  variable or direct value  in the vector  1      local elem value     local elem position     local vector length     local elem name      eval elem value   3      elem position     2       eval vector length    1  0     if   -z   vector length     then         vector length    0       fi      if    elem position -ge  vector length    then         vector length  elem position     fi      elem name  1  elem position      eval  elem name     elem value       if      elem position -eq 0    then         eval  1 0  vector length     fi    VectorPrint        Vector Print   Prints all the elements names and values of the vector  1 on sepparate lines      local vector length      vector length     1 0       if     vector length     0     then         echo  Vector    1   is empty       else         echo  Vector    1             for   i 1  i lt   vector length  i      do             eval echo     i          1   i                      OR  eval printf     s   n        i          1   i               done     fi    VectorDestroy        Vector Destroy   Empties all the elements values of the vector  1      local vector length      vector length     1 0       if       vector length     0     then         for   i 1  i lt   vector length  i      do             unset  1  i         done         unset  1 0     fi                           MAIN START                            Setting vector  params  with all the parameters received by the script  for   i 1  i lt      i      do     eval param      i       VectorAddElementNext params param done    Printing the vector  params   VectorPrint params  read temp     Setting vector  params2  with the elements of the vector  params  in reversed order  if   -n   params 0     then     for   i 1  i lt   params 0  i      do         count    params 0-i 1           VectorAddElement params2 count params  i     done fi    Printing the vector  params2   VectorPrint params2  read temp     Getting the values of  params2  s elements and printing them  if   -n   params2 0     then     echo  Printing the elements of the vector  params2        for   i 1  i lt   params2 0  i      do         eval current elem value     params2   i           echo  params2  i    current elem value        done else     echo  Vector  params2  is empty   fi  read temp     Creating a two dimensional array   a    for   i 1  i lt  10  i      do     VectorAddElement a 0 i     for   j 1  j lt  8  j      do         value     8     i - 1     j            VectorAddElementDV a  i  j  value     done done     Manually printing the two dimensional array   a    echo  Printing the two-dimensional array  a    if   -n   a 0     then     for   i 1  i lt   a 0  i      do         eval current vector lenght   a   i  0         if   -n   current vector lenght     then             for   j 1  j lt   current vector lenght  j      do                 eval value     a   i   j                   printf   value               done         fi         printf   n      done fi                       MAIN END

User · Answer

You can simulate them for example with hashes  but need care about the leading zeroes and many other things  The next demonstration works  but it is far from optimal solution      bin bash declare -A matrix num rows 4 num columns 5  for   i 1 i lt  num rows i     do     for   j 1 j lt  num columns j     do         matrix  i  j   RANDOM     done done  f1         num rows  1  s  f2    9s   printf   f1     for   i 1 i lt  num rows i     do     printf   f2   i done echo  for   j 1 j lt  num columns j     do     printf   f1   j     for   i 1 i lt  num rows i     do         printf   f2    matrix  i  j       done     echo done   the above example creates a 4x5 matrix with random numbers and print it transposed  with the example result             1         2         3         4  1     18006     31193     16110     23297  2     26229     19869      1140     19837  3      8192      2181     25512      2318  4      3269     25516     18701      7977  5     31775     17358      4468     30345   The principle is  Creating one associative array where the index is an string like 3 4  The benefits    it s possible to use for any-dimension arrays    like  30 40 2 for 3 dimensional  the syntax is close to  C  like arrays    matrix 2 3

User · Answer

If each row of the matrix is the same size  then you can simply use a linear array and multiplication   That is   a    for    i 0  i lt 4    i     do   for    j 0  j lt 5    j     do      a i 5 j  0   done done   Then your a 2  3    3 becomes   a 2 5 3    3   This approach might be worth turning into a set of functions  but since you can t pass arrays to or return arrays from functions  you would have to use pass-by-name and sometimes eval  So I tend to file multidimensional arrays under  things bash is simply Not Meant To Do

User · Answer

The below code will definitely work provided if you are working on a Mac you have bash version 4  Not only can you declare 0 but this is more of a universal approach to dynamically accepting values  2D Array declare -A arr echo  quot Enter the row quot  read r echo  quot Enter the column quot  read c i 0 j 0 echo  quot Enter the elements quot  while    i -lt  r   do   j 0   while    j -lt  c     do     echo  i  j     read m     arr   i    j    m     j  expr  j   1    done   i  expr  i   1  done  i 0 j 0 while    i -lt  r   do   j 0   while    j -lt  c     do     echo -n   arr   i    j     quot   quot      j  expr  j   1    done   echo  quot  quot    i  expr  i   1  done

User · Answer

Bash doesn t have multi-dimensional array  But you can simulate a somewhat similar effect with associative arrays  The following is an example of associative array pretending to be used as multi-dimensional array   declare -A arr arr 0 0  0 arr 0 1  1 arr 1 0  2 arr 1 1  3 echo    arr 0 0     arr 0 1      will print 0 1   If you don t declare the array as associative  with -A   the above won t work  For example  if you omit the declare -A arr line  the echo will print 2 3 instead of 0 1  because 0 0  1 0 and such will be taken as arithmetic expression and evaluated to 0  the value to the right of the comma operator

User · Answer

You can also approach this in a much less smarter fashion   q    q    1-2   q    a-b    for set in   q      do echo   set  -   echo   set   -  done   of course a 22 line solution or indirection is probably the better way to go and why not sprinkle eval every where to

User · Answer

One can simply define two functions to write   4 is the assigned value  and read a matrix with arbitrary name   1  and indexes   2 and  3  exploiting eval and indirect referencing      bin bash  matrix write       eval  1    2    3  4    aux  1    2    3            Alternative way    let  aux  4                 ---    matrix read       aux  1    2    3  echo    aux     for   i 1 i lt 10 i i 1    do  for   j 1 j lt 10 j j 1    do    matrix write a  i  j    i 10  j   done done  for   i 1 i lt 10 i i 1    do  for   j 1 j lt 10 j j 1    do    echo  a   i    j     matrix read a  i  j   done done

User · Answer

Mark Reed suggested a very good solution for 2D arrays  matrix   They always can be converted in a 1D array  vector   Although Bash doesn t have a native support for 2D arrays  it s not that hard to create a simple ADT around the mentioned principle   Here is a barebone example with no argument checks  etc  just to keep the solution clear  the array s size is set as two first elements in the instance  documentation for the Bash module that implements a matrix ADT    https   github com vorakl bash-libs blob master src docs content pages matrix rst       bin bash  matrix init           matrix init instance x y data          declare -n self  1                                                               declare -i width  2 height  3                                                     shift 3                                                                           self    width    height                                                                                                                                         matrix get                                                                             matrix get instance x y      declare -n self  1                                                               declare -i x  2 y  3                                                             declare -i width   self 0   height   self 1                                        echo    self 2 y width x                                                                                                                                       matrix set                                                                             matrix set instance x y data      declare -n self  1                                                               declare -i x  2 y  3                                                             declare data   4                                                                 declare -i width   self 0   height   self 1                                        self 2 y width x     data                                                                                                                                      matrix destroy                                                                                matrix destroy instance      declare -n self  1                                                               unset self                                                                        my matrix 3  2     one  two  three     1 1   2 2   3 3     matrix init my matrix                                                                    3 2                                                                          one two three                                                                 1 1   2 2   3 3     print my matrix 2  0  matrix get my matrix 2 0    print my matrix 1  1  matrix get my matrix 1 1    my matrix 1  1   4 4 4  matrix set my matrix 1 1  4 4 4                                                     print my matrix 1  1  matrix get my matrix 1 1                                                            remove my matrix matrix destroy my matrix

User · Answer

For simulating a 2-dimensional array  I first load the first n-elements  the elements of the first column   local pano array       i 0  for line in   grep   filename    file   do    url   extract url from xml  line    pano array i    url    i    i 1   done   To add the second column  I define the size of the first column and calculate the values in an offset variable  array len     pano array       i 0  while     i -lt  array len    do   url    pano array  i      offset     array len i      found file   get file  url    pano array  offset   found file    i    i 1   done

[linux] How to declare 2D array in bash

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