q vs python: If/else

Sep 2020

(Series) A side-by-side comparison of q/kdb+ and python for qbies (q newbies).

This cheat sheet is a list of code snippets that do the same thing in q and python -- something I wished existed when I first came to q/KDB+ from python. Note that in both languages, the code shown is sometimes not the most elegant or succinct; that's on purpose because I wanted to show the same constructs in both languages. For a comprehensive intro to q, check out Q for Mortals.

Note q code snippets are edited to have line breaks for readability (my opinion; ymmv). Multiline statements do not work at the prompt, but they do work in scripts. See docs.

Versions: q (3.6), python (3.7)

If*

q)name:`bob
q)x:0

q)if[name=`bob;
    x:x+1;  / run this
    x+:1;   / run this too
    x:x*10  / also this
    ];

q)x
20

>>> name = 'bob'
>>> x = 0

>>> if name == 'bob':
...     x = x + 1
...     x += 1
...     x = x * 10


>>> x
20

If/else*

q)func:{[name]
    y:0;
    x:$[name=`foo;
        0;
      name=`bar;
        [
          y:9;
          1
          ]
      name=`baz;
        2
      /else
        3
      ];
    :x+y;
    };

q)func `foo
0

q)func `bar
10

q)func[`baz]  / square brackets optional
2

q)func[`qux]
3

>>> def func(name):
...     y = 0
...     if name == 'foo':
...         x = 0
...     elif name == 'bar':
...         y = 9
...         x = 1
...     elif name == 'baz':
...         x = 2
...     else:
...         x = 3
...     return x + y





>>> func('foo')
0

>>> func('bar')
10

>>> func('baz')
2

>>> func('qux')
3