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Code examples · runnable

100 runnable code examples.

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100 of 100 examples

Python

How to reverse a string in PythonThree ways to reverse a string — slicing, reversed() + join(), and a loop — plus how to reverse the words in a sentence.How to remove duplicates from a list in PythonRemove duplicate items from a list — dict.fromkeys() (keeps order), set() (fastest), and a loop — plus how to dedupe unhashable items like dicts.How to merge two dictionaries in PythonMerge two dictionaries — the | operator (3.9+), {**a, **b} unpacking, and .update() — plus how to deep-merge nested dicts. See which mutates and which builds a new dict.How to flatten a list of lists in PythonMake a flat list out of a list of lists — a nested comprehension, itertools.chain.from_iterable(), and sum(nested, []) — plus how to flatten arbitrarily nested lists.How to find the index of a list item in PythonFind the index of an item in a list — list.index() for the first match, an in-guard or try/except for missing items, and an enumerate() comprehension for every index — plus finding the first item that matches a condition.How to get the current date and time in PythonGet the current date and time with datetime.now(), format it with strftime(), make it timezone-aware with timezone.utc or ZoneInfo, and read the clock as a Unix timestamp.How to convert bytes to a string in PythonDecode bytes to str with .decode(), why str(b) gives you b'...' instead, how to survive a UnicodeDecodeError, and where bytes come from in the first place.How to list all files in a directory in PythonList a folder with os.listdir() and Path.iterdir(), keep only the files, match patterns with glob, walk subfolders recursively, and read sizes with os.scandir().How to concatenate two lists in PythonFour ways to concatenate lists — the + operator, extend() / += in place, unpacking with [*a, *b], and itertools.chain — plus the aliasing gotcha += hides.How to check if a list is empty in PythonCheck whether a list is empty — the truthiness test if not items:, len(items) == 0, and why items == [] is discouraged — plus how to tell an empty list from None.How to slice a list in PythonSlice a list with items[start:stop] — the half-open rule, omitted ends, negative indices, the step argument, and slice assignment to edit a list in place.How to count occurrences in a list in PythonCount how often items appear in a list — list.count() for a single value, collections.Counter for all of them in one pass, sum() with a condition, and a plain dict tally.How to remove a key from a dictionary in PythonRemove a key from a dict — del for keys that must exist, pop() when you want the removed value, pop(key, None) to never raise — plus a comprehension that drops several keys at once.How to check if a key exists in a dictionary in PythonFour ways to check for a dictionary key — the in operator, dict.get() with a default, try/except KeyError, and setdefault() — plus the falsy-value trap that breaks if d.get(key).How to sort a dictionary by value in PythonSort a dictionary by value — sorted() on .items() with a lambda, operator.itemgetter(1), reverse=True for descending, a tuple key for ties, and Counter.most_common() for counts.How to convert two lists into a dictionary in PythonTurn a list of keys and a list of values into a dict — dict(zip(...)), zip(strict=True), a dict comprehension, and itertools.zip_longest — plus what happens on uneven lengths and duplicate keys.How to check if a string contains a substring in PythonCheck whether a string contains a substring with the in operator, find() and index() when you need the position, casefold() for case-insensitive matching, and re.search() for patterns.How to join a list into a string in PythonJoin a list into a string with ", ".join(items) — plus map(str, ...) for lists of numbers, "\n".join() for line-by-line output, and why += in a loop is the wrong tool.How to convert a string to an int in PythonTurn a string into an int with int() — plus try/except ValueError for untrusted input, int(float()) for decimal strings, int(s, base) for binary and hex, and how to convert a whole list.How to loop with an index in PythonLoop with an index in Python — enumerate(), enumerate(items, start=1) for human-facing numbering, the range(len()) habit to avoid, and zip() for walking two lists together.

Go

How to reverse a string in GoReverse a string in Go the Unicode-safe way — []rune plus a two-pointer swap, slices.Reverse, and strings.Builder — plus why reversing []byte corrupts non-ASCII text.How to convert a string to an int in GoConvert a string to an int in Go with strconv.Atoi — plus ParseInt for hex, binary, and bit sizes, ParseFloat for decimal strings, and how to handle bad input and overflow the idiomatic way.How to check if a key exists in a map in GoCheck if a key exists in a Go map with the comma-ok idiom — v, ok := m[key] — plus the zero-value trap, existence-only sets with map[string]struct{}, value lookups, and how nil maps behave.How to check if a slice contains an element in GoCheck if a Go slice contains a value with slices.Contains (Go 1.21+), the classic loop for older code, slices.ContainsFunc for custom matches, slices.Index when you need the position, and a map set for repeated lookups.How to sort a slice in GoSort a Go slice with slices.Sort (Go 1.21+), order structs and custom keys with slices.SortFunc and cmp.Compare, sort.Slice for older code, stable multi-key sorts, and how to sort without mutating the original.How to remove an element from a slice in GoRemove an element from a Go slice with slices.Delete (Go 1.21+), the classic append(s[:i], s[i+1:]...) idiom, O(1) swap-with-last removal when order does not matter, slices.DeleteFunc for bulk filtering, and the aliasing trap that leaks memory.How to check if a string contains a substring in GoCheck whether a Go string contains a substring with strings.Contains — plus HasPrefix/HasSuffix for anchored checks, ContainsAny and ContainsRune for character sets, Index when you need the position, and how to search case-insensitively when there is no ContainsFold.How to split a string in GoSplit a string in Go with strings.Split — plus strings.Fields for whitespace, SplitN to cap the pieces, strings.Cut for key=value, bufio.Scanner for lines, and FieldsFunc/regexp for several delimiters at once.How to concatenate strings in GoConcatenate strings in Go with + for a couple of pieces, strings.Builder inside a loop, and strings.Join for a []string — plus fmt.Sprintf for formatting, bytes.Buffer for byte output, and why += in a loop is O(n²).How to trim whitespace from a string in GoTrim whitespace from a string in Go with strings.TrimSpace — plus custom cutsets with Trim, the TrimLeft-vs-TrimPrefix trap, TrimFunc for zero-width characters, and collapsing inner whitespace with Fields + Join.How to convert an int to a string in GoConvert an int to a string in Go with strconv.Itoa — plus FormatInt for other bases, fmt.Sprintf for padded output, why string(65) gives "A" not "65", and how to build strings from many ints without O(n²) concatenation.How to iterate over a map in GoIterate over a Go map with for k, v := range m — plus keys-only and values-only loops, why the iteration order is deliberately randomised, how to get a stable order with slices.Sorted(maps.Keys(m)), sorting by value, and what is legal to change while the loop runs.How to remove a key from a map in GoRemove a key from a Go map with the delete builtin — plus why deleting a missing key is a safe no-op, deleting while ranging, clear() vs make() vs nil, and why delete never gives the memory back.How to remove duplicates from a slice in GoRemove duplicates from a Go slice with a map[T]struct{} seen-set that preserves first-seen order, or slices.Sort + slices.Compact when sorted output is fine — plus CompactFunc for case-insensitive dedupe, deduping structs by a key field, and the zero-allocation s[:0] filter.How to loop with an index in GoLoop with an index in Go — for i, v := range s, the index-only and value-only forms, the classic three-clause loop for backwards and custom steps, why ranging a string gives byte offsets, and the Go 1.22 loop-variable change.How to convert a struct to JSON in GoTurn a Go struct into JSON with json.Marshal — plus struct tags for key names, omitempty and hidden fields, MarshalIndent for pretty output, nested structs, maps and time.Time, and streaming with json.Encoder.How to parse JSON into a struct in GoParse JSON into a Go struct with json.Unmarshal — struct tags and field matching, JSON arrays and nested objects, map[string]any for unknown payloads, pointer fields to tell missing from zero, and json.Decoder for strict parsing.How to format a date in GoFormat a date in Go with t.Format and the reference-time layout "2006-01-02 15:04:05" — why it is that date, the predefined layouts like time.RFC3339 and time.DateOnly, parsing back with time.Parse, 12-hour clocks and padding, and time zones.How to wait for goroutines to finish in GoWait for goroutines to finish in Go with sync.WaitGroup — Add, Done and Wait — plus why a bare go f() prints nothing, how to collect results without a data race, mutex and atomic counters, channel fan-in, and errgroup-style error handling with only the standard library.How to handle errors in GoHandle errors in Go the idiomatic way — check err != nil and return early, add context with fmt.Errorf and %w, match sentinels with errors.Is, pull typed errors out with errors.As, join several failures with errors.Join, and know when panic/recover is the wrong tool.

JavaScript

How to remove an item from an array in JavaScriptRemove a specific item from a JavaScript array: indexOf + splice in place, toSpliced() for a new array, filter() for every match or a condition, why delete leaves a hole, and how to remove items inside a loop without skipping any.How to remove a property from an object in JavaScriptRemove a property from a JavaScript object with delete, or build a copy without it using rest destructuring — plus dropping several keys at once, why setting a key to undefined is not the same, and the frozen-object and Map edge cases.How to check if a string contains a substring in JavaScriptCheck whether a JavaScript string contains a substring with includes(), test the start or end with startsWith() and endsWith(), find the position with indexOf(), ignore case, and match patterns with a regex.How to loop through an array in JavaScriptLoop through an array in JavaScript with for...of, get the index with entries() or forEach, stop early with break, some() or find(), use the classic for loop for backwards and stepped walks, and see why for...in is the wrong tool for arrays.How to replace all occurrences of a string in JavaScriptReplace every occurrence of a substring in JavaScript with replaceAll() — plus the global regex, how to put a variable in a RegExp safely with RegExp.escape, and the $& / $1 replacement-pattern gotcha.How to capitalize the first letter of a string in JavaScriptCapitalize the first letter of a string with charAt(0).toUpperCase() + slice(1), plus why str[0] throws on an empty string, how to capitalize every word, emoji and surrogate pairs, and locale-aware toLocaleUpperCase().How to deep clone an object in JavaScriptDeep clone an object or array with structuredClone, see why spread and Object.assign only copy one level, what JSON.parse(JSON.stringify()) silently breaks, and what structuredClone cannot copy.How to check if an array contains a value in JavaScriptCheck if a JavaScript array contains a value with includes(), the older indexOf() !== -1 test and its NaN blind spot, some() for objects and custom tests, and a Set when you look up many values.How to round to 2 decimal places in JavaScriptRound a number to 2 decimal places in JavaScript with Math.round(x * 100) / 100, fix the 1.005 floating-point trap, show exactly two decimals with toFixed or Intl.NumberFormat, and turn a float into a whole number with Math.trunc, floor, ceil or round.How to sort an array of objects by property in JavaScriptSort an array of objects by a property: subtract for numbers, localeCompare for strings, Date.parse for dates, || for tie-breaks, and toSorted() when the original must not change.How to check if a key exists in an object in JavaScriptCheck if a key exists in a JavaScript object with Object.hasOwn or the in operator, why obj.key !== undefined gives the wrong answer, hasOwnProperty in older code, Map.has, and optional chaining for nested keys.How to loop through an object in JavaScriptLoop through a JavaScript object with for...of over Object.entries(obj), plus Object.keys and Object.values, why for...in also visits inherited keys, the order keys come back in, and mapping an object with Object.fromEntries.How to merge two objects in JavaScriptMerge two JavaScript objects with spread syntax or Object.assign, see which one mutates and which key wins, why nested objects get replaced instead of merged, a small recursive deep merge, and how to add a property only when a condition holds.How to use async/await in a loop in JavaScriptUse async/await in a JavaScript loop: why forEach does not wait, for...of with await for one-at-a-time work, Promise.all with map for parallel work, Promise.allSettled when some fail, for await...of, and a sleep() helper built from setTimeout.How to remove duplicates from an array in JavaScriptRemove duplicates from a JavaScript array with [...new Set(arr)], the older filter + indexOf, a Map keyed by id for arrays of objects, a case-insensitive seen-set, and merge-then-dedupe with spread or the ES2025 Set.prototype.union().How to format a number with commas in JavaScriptFormat a number with commas as thousands separators in JavaScript: toLocaleString('en-US'), a reusable Intl.NumberFormat with fixed decimals, currency (USD, EUR, JPY), other locales' separators, compact 1.2K notation, BigInt, and the regex fallback with its limits.How to convert a string to a number in JavaScriptConvert a string to a number with Number() or unary +, read a number off the front of text with parseInt and parseFloat (always with a radix), avoid the Number("") === 0 trap, validate with Number.isFinite, and use BigInt past 2^53.How to flatten an array in JavaScriptFlatten an array of arrays in JavaScript: flat() for one level, flat(Infinity) for any depth, flatMap() to map and flatten in one pass, the older concat and reduce answers, and a recursive flatten for custom rules.How to find an object in an array by property in JavaScriptFind an object in a JavaScript array by a property value with find(), get its position with findIndex() or search from the end with findLast(), collect every match with filter(), pluck one property with map(), and build a Map or Object.groupBy index for repeated lookups.How to check for null or undefined in JavaScriptCheck for null or undefined in JavaScript with value == null, the one good use of loose equality: plus null vs undefined, typeof for undeclared variables, the truthiness traps (0, "", false, NaN), ?? vs ||, ??=, optional chaining and a blank-string check.

Java

How to convert a string to an int in JavaConvert a String to an int in Java with Integer.parseInt, know when Integer.valueOf and its cache matter, catch NumberFormatException cleanly, check whether a string is numeric first, and reach for Long.parseLong or BigInteger when the number is too big.How to iterate over a map in JavaIterate over a Java Map with a for-each loop over entrySet(), plus Map.forEach((k, v) -> …), keySet() and values(), why keySet() + get() does extra lookups, the iteration order of HashMap, LinkedHashMap and TreeMap, removing entries safely, and streams over entries.How to convert an array to a list in JavaConvert a Java array to a List with new ArrayList<>(Arrays.asList(arr)), the fixed-size Arrays.asList view, List.of (Java 9+) and a stream for int[], then turn it into a Set or back into an array with toArray(new String[0]) and mapToInt.How to generate a random number in a range in JavaGenerate a random int between min and max in Java with ThreadLocalRandom.current().nextInt(min, max + 1), avoid the exclusive-bound off-by-one, seed Random for repeatable numbers, get doubles and streams of random numbers, and build secure tokens and random alphanumeric strings with SecureRandom.How to check if an array contains a value in JavaCheck if a Java array contains a value with Arrays.asList(array).contains() for object arrays, Arrays.stream().anyMatch() for int[] and other primitives, a plain loop with equals() or Objects.equals(), a field test for arrays of objects, and a HashSet or Arrays.binarySearch() for many lookups.How to initialize a list in one line in JavaInitialize a Java List, Set or Map in one line with List.of, Set.of and Map.of (Java 9+), wrap them in new ArrayList<>(…) or new HashMap<>(…) when you need to add, and know where Arrays.asList, Stream.toList(), Map.ofEntries and a static final map fit. Plus why double-brace initialization is a trap.How to declare and initialize an array in JavaDeclare and initialize a Java array with the {…} initializer, new int[] {…} in-line, or new int[n] with default values, then fill it with Arrays.fill, Arrays.setAll or IntStream. Also covers 2-D and jagged arrays, length, and index bounds.How to print an array in JavaPrint a Java array with Arrays.toString, see why println(array) shows [I@ and a hash, print 2-D arrays with Arrays.deepToString, choose your own separator with String.join or Collectors.joining, and learn why Lists and char[] print fine on their own.How to convert a string to an enum in JavaConvert a string to an enum in Java with valueOf(), and handle what it throws: a case-insensitive lookup that returns Optional, a static map for custom codes, values() to loop, name() vs toString(), == vs equals() and switch on an enum.How to break out of nested loops in JavaBreak out of nested loops in Java with a labeled break outer;, skip to the next outer iteration with continue outer;, move the loops into a method and return, use a boolean flag, or replace a search with anyMatch and findFirst. Plus why a plain break only leaves the innermost loop.How to split a string in JavaSplit a string in Java with split(), which takes a regex: escape . | and $ or use Pattern.quote, keep trailing empty strings with a limit of -1, split once with a limit of 2, split on whitespace, and turn the pieces into a List.How to sort a Map by value in JavaSort a Java Map by value: stream entrySet() with Map.Entry.comparingByValue() into a LinkedHashMap, descending order and tie-breaks, a sorted List of entries and top N, why a TreeMap sorts by key, and HashMap vs LinkedHashMap vs TreeMap.How to read a file into a String in JavaRead a whole file into a String with Files.readString (Java 11+), get a List of lines with readAllLines, stream a large file line by line with Files.lines or a BufferedReader, turn an InputStream into a String with readAllBytes, and handle missing files and the wrong charset.How to round to 2 decimal places in JavaRound a number to 2 decimal places in Java with BigDecimal.setScale(2, RoundingMode.HALF_UP), see why Math.round(x * 100.0) / 100.0 turns 1.005 into 1.0, format with String.format("%.2f") or DecimalFormat, pick a RoundingMode, drop trailing zeros and fix the BigDecimal.divide ArithmeticException.How to concatenate two lists in JavaJoin two lists in Java by copying the first into a new ArrayList and calling addAll, or with Stream.concat(...).toList() on Java 16+. Also covers many lists with flatMap, the UnsupportedOperationException from List.of and Arrays.asList, merging without duplicates, and concatenating arrays.How to sort a list of objects by property in JavaSort a Java list of objects by a property with list.sort(Comparator.comparing(Person::name)): comparingInt to avoid boxing, reversed() for descending, thenComparing for ties, case-insensitive and null-safe keys, Comparable, and a sorted copy with stream().sorted().toList().How to compare strings in JavaCompare strings in Java with equals(), and see why == only seems to work on literals. Plus null-safe checks with "literal".equals(x) and Objects.equals, equalsIgnoreCase, compareTo and Collator for ordering, contentEquals, isEmpty vs isBlank, and switch on strings.How to join a list into a string in JavaJoin a Java List<String> into one string with String.join(", ", list), use Collectors.joining for numbers, objects, prefixes and suffixes, handle null elements, build strings with StringJoiner or a StringBuilder loop without a trailing comma, and repeat a string with String.repeat.How to remove items from a list while iterating in JavaRemove items from a Java list while looping without a ConcurrentModificationException: removeIf (Java 8+), Iterator.remove(), a backwards index loop and a filtered stream copy, plus the remove(int) vs remove(Object) trap, maps, and why Arrays.asList and List.of throw UnsupportedOperationException.How to get the current date and time in JavaGet the current date and time in Java with LocalDateTime.now(), format it as yyyy-MM-dd HH:mm:ss.SSS with DateTimeFormatter, avoid the YYYY, hh and mm pattern traps, use ZonedDateTime and Instant for time zones and epoch millis, and convert to and from java.util.Date.

C#

How to loop through an enum in C#Loop through every value of an enum in C# with Enum.GetValues<T>(), get the names with GetNames, turn an enum into a List, and handle the surprises: sort order, duplicate values, [Flags] members, generic helpers and display text.How to convert an int to an enum in C#Convert an int to an enum in C# with a cast, check the value with Enum.IsDefined first, get the int back with (int), get a member name from a value with ToString or Enum.GetName, and combine values with a [Flags] enum.How to check if a string contains a substring in C#Check if a C# string contains a substring with Contains, and do it case-insensitively with StringComparison.OrdinalIgnoreCase. Plus Ordinal vs InvariantCulture vs CurrentCulture, the Turkish-I trap, IndexOf, StartsWith and EndsWith, matching any of several words with SearchValues, and whole-word checks with Regex.How to iterate over a dictionary in C#Iterate over a C# Dictionary with foreach over KeyValuePair (or deconstructed into key and value), Keys and Values, why Keys + dict[key] does extra lookups, updating values, removing entries during the loop, iteration order, and merging two dictionaries.How to deep clone an object in C#Deep clone an object in C# with a hand-written DeepClone method or a System.Text.Json round trip, see why MemberwiseClone, record with and new List<T>(list) only copy one level, clone a generic list, and keep shared references and cycles intact.How to catch multiple exceptions in C#Catch multiple exceptions at once in C# with an exception filter, catch (Exception e) when (e is FormatException or OverflowException), order separate catch blocks correctly, filter on exception data, and rethrow properly: throw vs throw ex, wrapping with an inner exception, and ExceptionDispatchInfo.How to generate a random number in a range in C#Generate a random int between min and max in C# with Random.Shared.Next(min, max + 1), avoid the exclusive-bound off-by-one, fix the "Random keeps returning the same number" bug, get doubles and longs, shuffle a List<T>, and build random alphanumeric strings and secure tokens with RandomNumberGenerator.How to calculate the difference between two dates in C#Calculate the difference between two dates in C#: subtract DateTimes to get a TimeSpan, read Days vs TotalDays, count calendar days with DateOnly.DayNumber, calculate age from a birthday, split a gap into years, months and days, print relative time like "3 hours ago", and get the real elapsed time across DST with DateTimeOffset.How to check for null in C#Check for null in C# with is null and is not null, supply defaults with ?? and ??=, reach through possibly-null objects with ?. and ?[], work with int? and convert it to int, and track down and prevent NullReferenceException.How to sort a list of objects by property in C#Sort a C# List<T> of objects by a property with LINQ OrderBy(p => p.Name) or in place with list.Sort((a, b) => a.Age.CompareTo(b.Age)): descending order, ThenBy for ties, ordinal and case-insensitive strings, null keys, IComparable<T>, and sorting by a property name given as a string.How to convert a byte array to a hex string in C#Convert a byte array to a hex string in C# with Convert.ToHexString, get lowercase with ToHexStringLower, read hex back with Convert.FromHexString, know what BitConverter.ToString and the "x2" loop are still good for, and turn an int into hex and back with ToString("X") and NumberStyles.HexNumber.How to convert a string to a byte array in C#Convert a string to a byte array in C# with Encoding.UTF8.GetBytes and back with GetString, pick the right encoding, get the raw UTF-16 bytes without one, deal with the UTF-8 BOM, and decode invalid or chunked bytes safely.How to convert a string to an enum in C#Convert a string to an enum in C# with Enum.TryParse or Enum.Parse<T>, ignore case, reject numbers and comma lists that parse anyway, map your own codes and labels to enum values, and turn an enum back into a string with ToString(), nameof and format strings.How to remove duplicates from a list in C#Remove duplicates from a List<T> in C# with Distinct().ToList(), dedupe objects by a property with DistinctBy, remove duplicates in place with a HashSet, make Distinct() work on your own classes with records or Equals and GetHashCode, use Distinct with a lambda via EqualityComparer<T>.Create, and find or keep the last duplicate.How to loop with an index in C#Get the index of the current foreach iteration in C#: Index() on .NET 9+, the Select((item, i) => …) overload on older versions, a plain for loop for arrays and lists, a manual counter (and the continue and IndexOf traps), and Zip for walking two collections by position.How to group by with LINQ in C#Group items with LINQ in C#: GroupBy with a key selector, element and result selectors, grouping by multiple columns with anonymous types, tuples or records, query syntax (group … by … into), per-group Count/Sum and a HAVING filter, CountBy and AggregateBy, and ToLookup.How to convert an object to JSON in C#Convert an object to a JSON string in C# with System.Text.Json: JsonSerializer.Serialize, pretty printing and camelCase, renaming and hiding properties, enums as strings, leaving out null properties, fixing the "possible object cycle" error, and the \u escapes it writes for non-ASCII text.How to parse JSON into an object in C#Parse JSON into a C# object with System.Text.Json: JsonSerializer.Deserialize<T> into a record or class, case-insensitive and snake_case name matching, arrays, nested objects and dictionaries, JsonNode when you want a dynamic object, JsonDocument for read-only access, and strict parsing with required members and JsonException.How to convert a string to an int in C#Convert a string to an int in C# with int.TryParse for input you don't trust and int.Parse for text you do, see how Convert.ToInt32 differs, check whether a string is a number, parse hex, thousands separators and other formats, and move to long or BigInteger when the value is too big.How to join a list into a string in C#Join a C# List<string> or any IEnumerable<string> into one string with string.Join(", ", list), join numbers and objects with string.Join<T> and LINQ Select, handle null elements, concatenate with string.Concat instead of Aggregate, build strings with StringBuilder.AppendJoin without a trailing comma, and repeat a string.

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Examples, explained

Yes. Open any example to edit and run its code on the CompileBytes engine without installing anything.