1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
|
[
{
"category": "programming",
"language": "javascript",
"attribution": "JavaScript",
"content": "const total = items.reduce((acc, item) => acc + item.price, 0);",
"explanation": "reduce walks the array carrying a running value. The 0 is the starting total; drop it and an empty array throws."
},
{
"category": "programming",
"language": "javascript",
"attribution": "JavaScript",
"content": "const unique = [...new Set(values)].sort((a, b) => a - b);",
"explanation": "A Set discards duplicates. Spreading it back to an array allows sorting; the comparator is required or numbers sort as text."
},
{
"category": "programming",
"language": "javascript",
"attribution": "JavaScript",
"content": "export async function load(url) { const res = await fetch(url); return res.json(); }",
"explanation": "fetch resolves as soon as headers arrive, so .json() is a second await for the body. It does not throw on a 404 - check res.ok."
},
{
"category": "programming",
"language": "javascript",
"attribution": "JavaScript",
"content": "document.querySelectorAll('.row').forEach(el => el.classList.toggle('active'));",
"explanation": "querySelectorAll returns a static NodeList, so it will not change as you mutate the elements in it."
},
{
"category": "programming",
"language": "javascript",
"attribution": "JavaScript",
"content": "if (typeof value !== 'string' || value.length === 0) { return null; }",
"explanation": "A guard clause: reject the bad input first and return, instead of nesting the real work inside an if."
},
{
"category": "programming",
"language": "javascript",
"attribution": "JavaScript",
"content": "const { id, name = 'anonymous', ...rest } = await request.json();",
"explanation": "Destructuring with a default and a rest property. The default applies only when the value is undefined, not when it is null."
},
{
"category": "programming",
"language": "python",
"attribution": "Python",
"content": "squares = [n * n for n in range(1, 21) if n % 3 != 0]",
"explanation": "A list comprehension: the expression, the loop, then the filter. Faster and clearer than appending in a for loop."
},
{
"category": "programming",
"language": "python",
"attribution": "Python",
"content": "def fib(n): return n if n < 2 else fib(n - 1) + fib(n - 2)",
"explanation": "Recursion with a base case. Correct but exponential - each call spawns two more, so it recomputes the same values repeatedly."
},
{
"category": "programming",
"language": "python",
"attribution": "Python",
"content": "with open('data.csv', encoding='utf-8') as f: rows = [line.split(',') for line in f]",
"explanation": "with closes the file even if the body raises. Iterating a file yields lines without loading it all into memory."
},
{
"category": "programming",
"language": "python",
"attribution": "Python",
"content": "counts = {word: text.count(word) for word in set(text.split())}",
"explanation": "A dict comprehension. Note it is quadratic: count rescans the whole string per word. collections.Counter is the right tool."
},
{
"category": "programming",
"language": "python",
"attribution": "Python",
"content": "try: value = int(raw) except ValueError: value = 0",
"explanation": "Catching the specific exception, not a bare except - a bare one would also swallow KeyboardInterrupt."
},
{
"category": "programming",
"language": "rust",
"attribution": "Rust",
"content": "let mut counts: HashMap<String, usize> = HashMap::new();",
"explanation": "mut because inserting mutates it. The type annotation is needed here since nothing yet tells the compiler what it holds."
},
{
"category": "programming",
"language": "rust",
"attribution": "Rust",
"content": "fn main() { println!(\"{}\", args().skip(1).collect::<Vec<_>>().join(\" \")); }",
"explanation": "skip(1) drops the program name, which is always the first argument. Vec<_> lets the compiler infer the element type."
},
{
"category": "programming",
"language": "rust",
"attribution": "Rust",
"content": "let total: i64 = values.iter().filter(|v| **v > 0).sum();",
"explanation": "iter() borrows rather than consuming. The double deref is because filter hands you a reference to the iterator's own reference."
},
{
"category": "programming",
"language": "rust",
"attribution": "Rust",
"content": "match result { Ok(v) => println!(\"{v}\"), Err(e) => eprintln!(\"error: {e}\") }",
"explanation": "match on a Result must cover both arms - the compiler rejects it otherwise, which is how Rust stops an error going unhandled."
},
{
"category": "programming",
"language": "rust",
"attribution": "Rust",
"content": "pub struct Point { pub x: f64, pub y: f64 }",
"explanation": "Fields are private by default in Rust, so each one needs its own pub - making the struct public does not publish its contents."
},
{
"category": "programming",
"language": "rust",
"attribution": "Rust",
"content": "impl Display for Token { fn fmt(&self, f: &mut Formatter) -> Result { write!(f, \"{}\", self.0) } }",
"explanation": "Implementing Display is what makes a type usable in {}. self.0 is the first field of a tuple struct."
},
{
"category": "programming",
"language": "clike",
"attribution": "C",
"content": "int *buf = malloc(len * sizeof(int)); if (!buf) return -1;",
"explanation": "malloc returns NULL when it fails, and checking is not optional. sizeof(int) rather than 4 keeps it correct across platforms."
},
{
"category": "programming",
"language": "clike",
"attribution": "C",
"content": "for (int i = 0; i < n; i++) { sum += a[i] * b[i]; }",
"explanation": "A dot product. The bound is < n, not <= n; off-by-one here reads past the end of the array."
},
{
"category": "programming",
"language": "clike",
"attribution": "C",
"content": "struct node { int value; struct node *next; };",
"explanation": "A linked-list node. It refers to its own type through a pointer, which is legal because a pointer has a known size."
},
{
"category": "programming",
"language": "clike",
"attribution": "Go",
"content": "for i, v := range items { fmt.Printf(\"%d: %v\\n\", i, v) }",
"explanation": "range yields index and value. In Go before 1.22 the loop variable was reused each iteration, which surprised anyone capturing it in a closure."
},
{
"category": "programming",
"language": "clike",
"attribution": "Go",
"content": "if err != nil { return fmt.Errorf(\"read config: %w\", err) }",
"explanation": "Go's error convention. %w wraps the original so errors.Is and errors.As can still see it further up."
},
{
"category": "programming",
"language": "clike",
"attribution": "Java",
"content": "public static void main(String[] args) { System.out.println(\"ready\"); }",
"explanation": "Java's entry point. The signature is fixed: static so it runs without an instance, and taking the command-line arguments."
},
{
"category": "programming",
"language": "shell",
"attribution": "Shell",
"content": "grep -rn \"TODO\" src/ | awk -F: '{print $1}' | sort -u",
"explanation": "grep -n prints file:line:text, awk splits on the colon to take the filename, sort -u removes the duplicates."
},
{
"category": "programming",
"language": "shell",
"attribution": "Shell",
"content": "for f in *.log; do gzip -9 \"$f\" && echo \"packed $f\"; done",
"explanation": "The quotes around $f matter: without them a filename containing a space becomes two arguments."
},
{
"category": "programming",
"language": "shell",
"attribution": "Shell",
"content": "export PATH=\"$HOME/.local/bin:$PATH\" && command -v rg >/dev/null || echo missing",
"explanation": "Prepending puts your own binaries ahead of the system ones. command -v is the portable test for whether something is installed."
}
]
|