Avoid nested loops in PHP and Python

Avoid nested loop notation by using generators in PHP and Python

When decoding and using JSON as shown below, loop processing may be described by nesting.

nest.json

{
  "a": {"A": {"A0": [1, 2], "A1": [3, 4]}, "B": {"A0": [5, 6], "A1": [7, 8]}},
  "b": {"A": {"A0": [10, 20], "A1": [30, 40]}, "B": {"A0": [50, 60], "A1": [70, 80]}}
}

At this time, the value extraction of the index 1 of the key ʻA0` has the following result, but when realizing the processing with PHP or Python, there are cases where it becomes a double loop or a triple loop.

[2, 6, 20, 60]

Since a generator using yield from is available in PHP7 and Python3.3 or later, use this function to verify whether you can break out of the nested loop. For verification, PHP 7.1.5 and Python 3.5.0 are used.

When realized by a nested loop

The PHP and Python code is described as an example when it is realized by triple loop and double loop.

For PHP7

ʻAssuming that array_walketc. is not used, it is implemented only withforeach`.

<?php

$nest = json_decode(file_get_contents("nest.json"), true);

/*
$nest = [
  "a" => ["A" => ["A0" => [1,2], "A1" => [3,4]], "B" => ["A0" => [5,6], "A1" => [7,8]]],
  "b" => ["A" => ["A0" => [10,20], "A1" => [30,40]], "B" => ["A0" => [50,60], "A1" => [70,80]]]
];
*/

$r = [];
foreach ($nest as $v1) {
    foreach ($v1 as $v2) {
        foreach ($v2 as $k => $v3)  {
            if ($k === "A0") {
                $r[] = $v3[1];
            }
        }
    }
}
print_r($r);

$r = [];
foreach ($nest as $v1) {
    foreach ($v1 as $v2) {
        if (isset($v2["A0"][1]))  {
            $r[] = $v2["A0"][1];
        }
    }
}
print_r($r);

For Python 3

Writing almost the same as PHP.

import json
from collections import OrderedDict

nest = json.load(open("nest.json"), object_pairs_hook=OrderedDict)

"""
nest = {
  "a": {"A": {"A0": [1,2], "A1": [3,4]}, "B": {"A0": [5,6], "A1": [7,8]}},
  "b": {"A": {"A0": [10,20], "A1": [30,40]}, "B": {"A0": [50,60], "A1": [70,80]}}
}
"""

r = []
for v1 in nest.values():
    for v2 in v1.values():
        for k, v3 in v2.items():
            if k == "A0" and len(v3) > 1:
                r.append(v3[1])
print(r)

r = []
for v1 in nest.values():
    for v2 in v1.values():
        if "A0" in v2 and len(v2["A0"]) > 1:
            r.append(v2["A0"][1])
print(r)

When realized with yield from

Both PHP and Python can be written in the same way.

For PHP7

It will be relatively simple. There is no need to add loop processing even if the array hierarchy becomes deeper.

<?php

$nest = json_decode(file_get_contents("nest.json"), true);

function zslice ($n)
{
     foreach($n as $k => $v) {
         yield from $v;
     }
}

$r = [];
foreach(zslice(zslice($nest)) as $k => $v) {
    if ($k == "A0") {
        $r[] = $v[1];
    }
}
print_r($r);

$r = [];
foreach(zslice($nest) as $k => $v) {
    if (isset($v["A0"][1])) {
        $r[] = $v["A0"][1];
    }
}
print_r($r);

For Python 3

Since it is handled by dict, a little complicated processing is required, but basically it is the same implementation method as PHP7.

import json
from collections import OrderedDict

nest = json.load(open("nest.json"), object_pairs_hook=OrderedDict)


def zslice(n):
    r = n.values() if isinstance(n, dict) else n
    for v in r:
        if isinstance(v, dict):
            d = v.items()
        elif isinstance(v, tuple) and len(v) > 1:
            d = v[1].items()
        else:
            raise ValueError
        yield from d

r = []
for k, v in zslice(zslice(nest)):
    if k == "A0" and len(v) > 1:
        r.append(v[1])
print(r)

r = []
for k, v in zslice(nest):
    if "A0" in v and len(v["A0"]) > 1:
       r.append(v["A0"][1])
print(r)

Other

It may be simpler and easier to read by separating the zslice as shown below.

import json
from collections import OrderedDict

nest = json.load(open("nest.json"), object_pairs_hook=OrderedDict)


def zslice1(n):
    for v in n.values():
        yield from v.items()


def zslice2(n):
    for k, v in n:
        yield from v.items()

r = []
for k, v in zslice2(zslice1(nest)):
    if k == "A0" and len(v) > 1:
        r.append(v[1])
print(r)

r = []
for k, v in zslice1(nest):
    if "A0" in v and len(v["A0"]) > 1:
       r.append(v["A0"][1])
print(r)

Output result

The output result of PHP and Python sample code described here is described.

For PHP

python

Array
(
    [0] => 2
    [1] => 6
    [2] => 20
    [3] => 60
)
Array
(
    [0] => 2
    [1] => 6
    [2] => 20
    [3] => 60
)

For Python

Output result

[2, 6, 20, 60]
[2, 6, 20, 60]

Supplement: About the order of Python dictionaries

If ʻobject_pairs_hook = OrderedDict is not specified in the second argument of json.load`, the order of the elements may be different if the sample is executed several times.

pattern 1

[60, 20, 6, 2]
[60, 20, 6, 2]

Pattern 2

[6, 2, 60, 20]
[6, 2, 60, 20]

Since the order of elements is not guaranteed when creating dict, use ʻOrderedDict` to maintain consistency.