Vehicle Purchase

Compare

Comparing Numbers

In jq numbers can be compared using the following relational and equality operators.

The result of the comparison is always a boolean value, so either true or false.

1 < 3,      # => true
2 != 2,     # => false
1 == 1.0    # => true
            # All numbers are floating-points, so this is different syntax
            # for the exact same value.

Comparing Strings

The comparison operators above can also be used to compare strings. In that case, a dictionary (lexicographical) order is applied. The ordering is by unicode codepoint value.

"Apple" > "Pear",   # => false
"a" < "above",      # => true
"a" == "A"          # => false

You need to be careful when you compare two variables that appear to contain numeric values but are of type string. Due to the dictionary order, the result will not be the same as comparing values of type number.

10 < 2,     # => false
"10" < "2"  # => true (because "1" comes before "2")

"Strict" Equality

The jq == operator is like Javascript's === in the sense that things that "look" the same, but are of different types, are not equal.

"3" == 3    # => false
            # the value on the left has type string,
            # the value on the right has type number.

Comparing Arrays

Two arrays are equal if all the corresponding elements are equal.

[1, 2, 3] == [1, 2, 3]      # => true
[1, 2, 3] == [1, 3, 2]      # => false, different order
[1, 2, 3] == [1, 2, "3"]    # => false, different types

Comparing Objects

Two objects are equal if they have the same key-value pairs.

{name: "Joe", age: 42} == {age: 42, name: "Joe"}                # => true
{name: "Joe", age: 42} == {age: 42, name: "Jane"}               # => false
{name: "Joe", age: 42} == {age: "42", name: "Joe"}              # => false
{name: "Joe", age: 42} == {age: 42, name: "Joe", height: 175}   # => false

# comparisons will drill down as deeply as required
{a: {b: {c: [1, 2]}}} == {a: {b: {c: [1, 2]}}}                  # => true
{a: {b: {c: [1, 2]}}} == {a: {b: {c: [1, 2, 3]}}}               # => false

Conditionals

If Expression

jq's conditional expression is if A then B else C end.

if-then-else is a filter like all jq builtins: it takes an input and produces an output.

If the expression A produces a "truthy" value, then the if filter evaluates B. Otherwise it evaluates C. The input to the if filter will be passed to B or C.

42 | if . < 50 then "small" else "big" end      # => "small"

5 | if . % 2 == 0 then . / 2 else . * 4 end     # => 20

if A then B else . end
if A then B end

Nested If-Statements

Further conditions can be added with elif.

42 | if . < 33 then "small"
     elif . < 66 then "medium"
     else "big"
     end
# => "medium"

Use as many elif clauses as you need.

Truthiness

The only "false" values in jq are: false and null. Everything else is "true", even the number zero and the empty string, array and object.

Boolean Operators

The boolean operators and and or can be used to build complex queries.

42 | if . < 33 or . > 66 then "big or small"
     else "medium"
     end

To negate, use not. This is a filter not an operator.

42 | if (. < 33 or . > 66 | not) then "medium"
     else "big or small"
     end

Alternative Operator

The alternative operator allows you to specify a "default" value if an expression is false or null.

A // B

# This is identical to
if A then A else B end

To demonstrate

[3, 5, 18] | add / 2       # => 13
[]         | add / 2       # => error: null (null) and number (2) cannot be divided
[]         | add // 0 / 2  # => 0