How to declare 2D array in bash

问题:

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;

回答1:

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<=num_rows;i++)) do
    for ((j=1;j<=num_columns;j++)) do
        matrix[$i,$j]=$RANDOM
    done
done

f1="%$((${#num_rows}+1))s"
f2=" %9s"

printf "$f1" ''
for ((i=1;i<=num_rows;i++)) do
    printf "$f2" $i
done
echo

for ((j=1;j<=num_columns;j++)) do
    printf "$f1" $j
    for ((i=1;i<=num_rows;i++)) do
        printf "$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]}

回答2:

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

回答3:

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).

回答4:

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 .

回答5:

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=$3
  IFS=' ' read -r -a tmp <<< "${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

回答6:

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 <v>:
##      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 \"$1\":"
        for ((i=1; i<=$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<=$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<=$#; i++)); do
    eval param="\${$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 "$params_0" ]; then
    for ((i=1; i<=$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 "$params2_0" ]; then
    echo "Printing the elements of the vector 'params2':"
    for ((i=1; i<=$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<=10; i++)); do
    VectorAddElement a 0 i
    for ((j=1; j<=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 "$a_0" ]; then
    for ((i=1; i<=$a_0; i++)); do
        eval current_vector_lenght=\$a\_$i\_0
        if [ -n "$current_vector_lenght" ]; then
            for ((j=1; j<=$current_vector_lenght; j++)); do
                eval value=\"\$a\_$i\_$j\"
                printf "$value "
            done
        fi
        printf "\n"
    done
fi

################
### MAIN END ###
################

回答7:

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<4; ++i )); do
  for (( j=0; j<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".

回答8:

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<10;i=i+1)); do
 for ((j=1;j<10;j=j+1)); do 
  matrix_write a $i $j $[$i*10+$j]
 done
done

for ((i=1;i<10;i=i+1)); do
 for ((j=1;j<10;j=j+1)); do 
  echo "a_"$i"_"$j"="$(matrix_read a $i $j)
 done
done

回答9:

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

回答10:

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 "Enter the row"
read r
echo "Enter the column"
read c
i=0
j=0
echo "Enter the elements"
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}]} " "
    j=`expr $j + 1`
  done
  echo ""
  i=`expr $i + 1`
done

回答11:

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