Associative arrays (Bash)

An associative array is an array where the index doesn't have to be a number, but can be symbolic. E.g.:

declare -A j

j[fruit]=apple
j[color]=blue

• You can use associative arrays to mimic multi-dimensional arrays, with emphasize on mimic
• Associative arrays are new in Bash 4. To verify which version of Bash you have: bash --version.

There are no multidimensional arrays

Bash doesn't have multidimensional arrays (as of 2022.09.29). Associative arrays aren't multidimensional arrays either, but they can emulate them. This has some limitations.

As an example:

unset j
declare -A j

j[0,0]="00"
j[0,1]="01"
j[0, 1]="0 1"

for i in "${!j[@]}"
do
   echo "Index: $i - Value: ${j[$i]}"
done
echo "Length of this array: ${#j[@]}"

Output:

Index: 0, 1 - Value: 0 1
Index: 0,1 - Value: 01
Index: 0,0 - Value: 00
Length of this array: 3

What this shows:

• The entry with index [0, 1], is not the same as the entry with index [0,1]. This shows that everything between [] is regarded as just one index and not as something multidimensional
• When retrieving the dimension of the array, it returns only one number. Because it's still just a vector.

But does this actually matter? Sometimes it probably does, but so far, I seem to be able to live with it:

Retrieve number of rows or columns?

A problem with this 'emulated' multi-dimensional arrays: You can't read-out the number of rows or columns, for there are no real rows and columns. Illustration:

# The array below is 3x2
#
unset j
declare -A j

j[1,1]="11"
j[1,2]="12"
j[2,1]="21"
j[2,2]="22"
j[3,1]="31"
j[3,2]="32"

echo "Length: ${#j[@]}"
echo "Complete array: ${j[@]}"

Output:

Length: 6
Complete array: 21 22 31 31 12 11

This can be a problem, when you want to loop over the rows - Can't do that. But see below for a solution.

Iterate over rows?

If there is not really such a thing as rows and columns, than how to iterate over them?

That's probably not so difficult, Just remember that the 'multidimensional index' is just one index, and that you can iterate over it. E.g.:

for i in "${!imwiz[@]}"
do
   echo "Index: $i"
   echo "Value: ${imwiz[$i]}"
done

Retrieve a specific value?

How to retrieve a specific value? I suspect that isn't that hard either, and goes similarly as iterating over the array

Solutions

• When arrays have a numerical index, always start with the same number ⇒ I prefer base 1, just as in matrix algebra
• For two-dimensional arrays, the first dimensions is always rows (x) and the second is always columns (y) - Just as in matrix algebra
• Don't use spaces around the , that separates indices: You need a uniform syntax, and using spaces actually messes up language highlighting in Sublime Text
• Store the dimensions in associated variables (when needed). E.g., in the example above: j_rows=3 and j_columns=2

Examples

As stated above, these are not really multidimensional arrays, just arrays with fancy indices. It doesn't matter if these indices are numerical or symbolic:

declare -A j
j[0,0,0]="000"
j[0,0,1]="001"
j[0,1,0]="010"
j[0,1,1]="011"
j[1,0,0]="100"
j[1,0,1]="101"
j[1,1,0]="110"
j[1,1,1]="111"

echo "${j[0,0,0]} ${j[0,0,1]} ${j[0,1,0]} ${j[0,1,1]}"
echo "${j[1,0,0]} ${j[1,0,1]} ${j[1,1,0]} ${j[1,1,1]}"

unset j

declare -A j

j[fruit,one]=Mango
j[fruit,two]=Apple
j[bird,1]=Cockatail
j[bird,2]=Spottingbird
j[flower,1]=Rose
j[flower,2]=Sunflower
j[animal]=Tiger

for i in "${j[@]}"
do
   echo "Entry: $i"
done

Output:

Entry: Cockatail
Entry: Spottingbird
Entry: Rose
Entry: Sunflower
Entry: Tiger
Entry: Apple
Entry: Mango

Note that the entries seem to appear in arbitrary order

Loop through an array

Again: These are not multidimensional arrays. They only seem that way.....

This works [1]:

declare -A j
j[fruit]=Mango
j[bird]=Cockatail
j[flower]=Rose
j[animal]=Tiger

for i in "${j[@]}"
do
   echo "Entry: $i"
done	

Output:

Entry: Mango
Entry: Rose
Entry: Tiger
Entry: Cockatail

This works:

declare -A j

j[fruit,1]=Mango
j[fruit,2]=Apple
j[bird,1]=Cockatail
j[bird,2]=Spottingbird
j[flower,1]=Rose
j[flower,2]=Sunflower
j[animal,1]=Tiger
j[animal,1]=Mouse

for i in "${j[@]}"
do
   echo "Entry: $i"
done

Output:

Entry: Cockatail
Entry: Spottingbird
Entry: Rose
Entry: Sunflower
Entry: Mouse
Entry: Mango
Entry: Apple

The only problem: The entries seem to be quite random. This is also the case if I insert statement unset j at the beginning of the script.

Looping through the rows of an associate array: This one isn't as cool as the code before, because the index is given explicitly:

i=1
for ((i; i<=$number_of_sites; i++))
do
   echo "Row ${i}: ${site[$i,1]} &  ${site[$i,2]}"
done

Loop through the index of an array

Use the symbol ! to refer to an array's index, rather than its content. Remember that with associative arrays, you define the index yourself. There is no numerical index:

declare -A j

j[fruit,1]=Mango
j[fruit,2]=Apple
j[bird,1]=Cockatail
j[bird,2]=Spottingbird
j[flower,1]=Rose
j[flower,2]=Sunflower
j[animal,1]=Tiger
j[animal,1]=Mouse

for i in "${!j[@]}"
do
   echo "Index: $i"
done

With output:

Index: bird,1
Index: bird,2
Index: flower,1
Index: flower,2
Index: animal,1
Index: fruit,1
Index: fruit,2

Loop over index + value

Again, not very exciting, but maybe instructive at times:

unset j
declare -A j

j[fruit,one]=Mango
j[fruit,two]=Apple
j[bird,1]=Cockatail
j[bird,2]=Spottingbird
j[flower,1]=Rose
j[flower,2]=Sunflower
j[animal]=Tiger

for i in "${!j[@]}"
do
   echo "Index: $i - Value: ${j[$i]}"
done

Output:

Index: bird,1 - Value: Cockatail
Index: bird,2 - Value: Spottingbird
Index: flower,1 - Value: Rose
Index: flower,2 - Value: Sunflower
Index: animal - Value: Tiger
Index: fruit,two - Value: Apple
Index: fruit,one - Value: Mango

Length of an array

Use the symbol # to retrieve the length of an array. Since associative arrays are just vectors with fancy indices, there is only one dimension to retrieve: Its length:

unset j
declare -A j

j[0,0]="00"
j[0,1]="01"
j[1,0]="10"
j[1,1]="11"
j[2,0]="20"
j[2,1]="21"

echo "Length: ${#j[@]}"

Output:

Length: 6

Awk?

And for something entirely different: I kinda move from spreadsheets to associative arrays and back. A while ago I saw on YouTube Gary Explains: EVERYONE Needs to Learn a Little Bit of AWK! - Maybe awk is what I have been looking for whole my life?

See also

• Awk
• Declare (Bash)
• Unset (Bash)

Sources

• https://linuxhint.com/associative_array_bash - Only vectors. No multidimensional arrays
• https://stackoverflow.com/questions/11233825/multi-dimensional-arrays-in-bash
• https://stackoverflow.com/questions/3112687/how-to-iterate-over-associative-arrays-in-bash