dotfiles/notes/shell/bash_arithmetic.md

Working with numbers in bash

Bash interprets everything by default as a string, however it can also work with numbers, just be careful with your conditional operators

Storing numbers

declare and local both can make integer variables with the -i flag

declare -i a=2

f () { local -i a=3 ;}

Variables can be manipulated in double parenthesis, very similar to c/python. Use $(()) to return the value instead. Edges and centers seem whitespace insensitive. No need to $ in front of variables inside (())

Work as expected: + - * % Exponent: ** Floored division: /

declare -i a=2 b=3  # Multiple on the same line are fine

(( a++ ))    # a == 3
(( a+=b ))   # a == 6
(( a = 2 ))  # a == 2
((a=4+b))    # a == 7
(( a = 4 + b )) # Spacing is optional, same as above

declare -i c=$((a**b + 4))  # c == 7**3 + 4 == 347

declare -ia arr=(0 1 2 3)
(( arr[a] = 3 ))   # Array indexing also no longer needs $
(( arr[100] = 3 )) # Array is acc an integer hashmap, so over-indexing works

Using numbers

Conditionals are different for numbers in bash. Mapping c to bash:

 C  | Bash
----|-----
 == | -eq
 != | -ne
 >  | -gt
 <  | -lt
 >= | -ge
 <= | -le

declare -i a=3 b=2

if [[ $((a**b)) -ge 1000 ]]; then
  echo "Exponentiation with ${a}^${b} is pretty big"
elif [[ $a -lt $b ]]; then
  echo "$a is smaller than $b"
else
  echo "$a isn't smaller than $b"
fi

echo "You rolled a: $(( RANDOM % 6 + 1 )) on a 6 sided dice"

Floats

Bash has no support for floats, don't even try. If you need floats, use awk, which has full support. To still sort of use rational numbers in bash, store a numerator and denominator, then pass both to awk when you need the float

declare -i num_1=4 denom_1=9
declare -i num_2=2 denom_2=9

(( num_1 += num_2 ))

awk -v n1=$num_1 -v d1=$denom_1 -v n2=$num_2 -v d2=$denom_2 \
  'BEGIN { printf "%.2f\n", n1/d1 + n2/d2 }'

Bash has floored division / and remainders %, though the awk approach is easier. Note awk's syntax is identical to c, so it uses ^ for exponentiation and the standard ==, <=... for comparison