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Zebra: Common Patterns Reference

Copy-paste patterns for typical programming tasks.

File Operations

Read Entire File Safely

File.read/File.write are not throws — a missing file panics rather than raising, so guard with File.exists() first:

if not File.exists("data.txt")
    print("Cannot read file")
else
    var content = File.read("data.txt")
    print(content)

Write File Safely

def write_output()
    File.write("output.txt", "File contents here")
    print("File written successfully")

Process File Line by Line

var content = File.read("file.txt")
var lines = content.split("\n")

for line in lines
    if line.trim().len == 0
        continue  # Skip empty lines

    # Process line...
    print(line)

Read CSV File

var content = File.read("data.csv")
var lines = content.split("\n")
var records = List(List(str))()

for line in lines
    if line.trim().len == 0
        continue

    var fields = line.split(",")
    records.add(fields)

for record in records
    print(record)  # Each field: record.at(0), record.at(1), etc.

String Processing

Split and Process Words

var text = "one two three four five"
var words = text.split(" ")

for word in words
    print("Word: ${word}")

Build String from Parts

var parts = List(str)()
parts.add("hello")
parts.add("world")
parts.add("from")
parts.add("zebra")

var message = parts.join(" ")      # "hello world from zebra"

Find and Replace

var text = "Hello World Hello"

# Replace every occurrence (replaceAll is the same call, spelled out)
var result1 = text.replace("Hello", "Hi")     # "Hi World Hi"
var result2 = text.replaceAll("Hello", "Hi")  # "Hi World Hi"

# Find position
var pos = text.indexOf("World")
if pos >= 0
    print("Found at position ${pos}")

Extract Substring

var text = "Hello, World!"

var greeting = text.substring(0, 5)      # "Hello"
var after_comma = text.substring(7, 12)  # "World"

Clean Input

var user_input = "  hello world  \n"
var clean = user_input.trim()            # "hello world"

Case Conversion

var text = "Hello World"

var upper = text.upper()                 # "HELLO WORLD"
var lower = text.lower()                 # "hello world"

Collections & Lists

Create and Populate List

var numbers = List(int)()
numbers.add(1)
numbers.add(2)
numbers.add(3)

Loop Over List

for item in items
    print(item)

# With index
for i in 0.to(items.count())
    print("${i}: ${items.at(i)}")

Find Item in List

var target = "apple"

for item in items
    if item == target
        print("Found it!")
        break

Filter List

var numbers = List(int)()
numbers.add(1)
numbers.add(2)
numbers.add(3)
numbers.add(4)
numbers.add(5)

var evens = List(int)()
for num in numbers
    if num % 2 == 0
        evens.add(num)

# evens = [2, 4]

Transform List

var numbers = List(int)()
numbers.add(1)
numbers.add(2)
numbers.add(3)

var doubled = List(int)()
for num in numbers
    doubled.add(num * 2)

# doubled = [2, 4, 6]

Remove Duplicates

var items = List(str)()
items.add("apple")
items.add("banana")
items.add("apple")
items.add("cherry")

var seen = HashMap(str, bool)()
var unique = List(str)()
for item in items
    if not seen.contains(item)
        seen.put(item, true)
        unique.add(item)

# unique has: apple, banana, cherry (duplicates removed)

Count Occurrences

var items = List(str)()
items.add("apple")
items.add("apple")
items.add("banana")

var count = 0
for item in items
    if item == "apple"
        count = count + 1

print(count)  # 2

Dictionaries & Maps

Create and Use HashMap

var scores = HashMap(str, int)()

scores.put("Alice", 95)
scores.put("Bob", 87)
scores.put("Charlie", 92)

var alice_score = scores.get("Alice")
if alice_score != nil
    print("Alice: ${alice_score}")

Loop Over HashMap

for key, value in scores
    print("${key}: ${value}")

Check if Key Exists

if scores.contains("Alice")
    print("Alice found")
else
    print("Alice not found")

Count by Category

var items = List(str)()
items.add("red")
items.add("blue")
items.add("red")
items.add("blue")
items.add("red")

var counts = HashMap(str, int)()

for item in items
    var current = counts.get(item)
    if current != nil
        counts.put(item, current + 1)
    else
        counts.put(item, 1)

# counts: red->3, blue->2

Error Handling

Safe Function that Can Fail

def divide(a: int, b: int): int throws
    if b == 0
        raise "Cannot divide by zero"
    return a / b

def attempt_divide()
    var v = divide(10, 0)
    print(v)
catch |err|
    print("Error: ${err.message}")

class Main
    static
        def main
            var value = divide(10, 2) catch 0
            print("Result: ${value}")

            attempt_divide()

Propagate Errors Up

def process_file(filename: str): int throws
    var content = File.read(filename)
    var line_count = content.split("\n").count()
    return line_count

Handle Multiple Errors

def calculate(a_str: str, b_str: str): int throws
    var a = a_str.tryInt()
    if a == nil
        raise "Invalid first number"

    var b = b_str.tryInt()
    if b == nil
        raise "Invalid second number"

    return a + b

Try Multiple Operations

def process_data() throws
    # Step 1
    var input = File.read("input.txt")

    # Step 2: Process
    var processed = input.upper()

    # Step 3
    File.write("output.txt", processed)

Null Safety

Safe Null Check

var x: int? = get_value()

if x != nil
    print("Value: ${x}")
else
    print("No value")

Use Default Value

var x: int? = get_value()
var value = x orelse 0  # Use 0 if nil
print(value)

Optional Chain

var user: User? = get_user()

if user != nil
    if user.address != nil
        print(user.address)

Nested Null Checks

var data: Data? = fetch_data()

if data != nil
    var items = data.items
    if items != nil
        for item in items
            print(item)

Type Conversions

String to Number

var num_str = "42"
var num = num_str.tryInt()

if num != nil
    print(num + 1)
else
    print("Invalid number")

Number to String

var num = 42
var text = num.toString()
var message = "The answer is ${text}"

String Interpolation (Preferred)

var num = 42
print("The answer is ${num}")

Type Checking

var value = 42

# Check if it's an int
var as_str = value.toString()  # Force conversion

# Check compatibility
if value > 0
    print("Positive")

Classes & Objects

Create Class with Constructor

class Person
    var name: str = ""
    var age: int = 0
    
    cue init(name: str, age: int)
        this.name = name
        this.age = age
    
    def describe(): str
        return "${name} is ${age} years old"

var person = Person("Alice", 30)
print(person.describe())

Use Interfaces

interface Animal
    def speak(): str

class Dog implements Animal
    var name: str = ""
    
    def speak(): str
        return "Woof!"

var dog = Dog()
dog.name = "Buddy"
print(dog.speak())

Implement Interface

interface Shape
    def area(): float

class Circle implements Shape
    var radius: float = 0.0
    
    def area(): float
        return 3.14159 * radius * radius

var circle = Circle()
circle.radius = 5.0
print(circle.area())

Static Methods

class Math
    static
        def add(a: int, b: int): int
            return a + b

var result = Math.add(2, 3)  # 5

Make a Class a Value (print, ==, keys, sort)

Cues are the methods the compiler calls (Chapter 07, "Cues"). Without them, == on a class is identity, a Set or HashMap keys it by identity, and sort() does not build.

class Point
    var x: int
    var y: int

    cue init(x: int, y: int)
        .x = x
        .y = y

    cue toString(): str
        return "(${.x}, ${.y})"

    cue equals(other: Point): bool
        return .x == other.x and .y == other.y

    cue hash(): int                    # hash the fields equals compares
        return .x * 31 + .y

    cue compare(other: Point): int     # by x, then by y
        if .x != other.x
            return .x - other.x
        return .y - other.y

def main()
    var visited: Set(Point) = Set()
    visited.add(Point(1, 2))
    visited.add(Point(1, 2))
    print(visited.count())             # 1
    var path = [Point(3, 0), Point(1, 2), Point(1, 1)]
    path.sort()
    print(path.at(0))                  # (1, 1)

Regular Expressions

Validate Pattern

var email_pattern = Regex.compile("[a-z0-9]+@[a-z]+\\.[a-z]+")

if email_pattern.match("user@example.com")
    print("Valid email")

Find Matches

var numbers = Regex.compile("\\d+")
var text = "I have 3 apples and 7 oranges"

var matches = numbers.findAll(text)
# matches = ["3", "7"]

for match in matches
    print(match)

Split by Pattern

var delimiter = Regex.compile("\\s+")  # One or more spaces
var text = "one  two   three"
var words = delimiter.split(text)
# words = ["one", "two", "three"]

Replace Pattern

var pattern = Regex.compile("\\d+")
var text = "Version 2.3.1"

var replaced = pattern.replace(text, "X")   # every match
# replaced = "Version X.X.X"

Command-Line Arguments

Read Arguments

class Main
    static
        def main
            var args = sys.args()

            if args.count() > 0
                print("Arguments:")
                for arg in args
                    print("  - ${arg}")
            else
                print("No arguments provided")

Parse Flags

class Main
    static
        def main
            var args = sys.args()
            var verbose = false
            var input_file = ""

            for arg in args
                if arg == "-v" or arg == "--verbose"
                    verbose = true
                else if arg.startsWith("--input=")
                    input_file = arg.substring(8, arg.len)

            print("Verbose: ${verbose}")
            print("Input: ${input_file}")

Environment Variables

Environment and Exit

# Exit with status code
sys.exit(1)

# Use Arg for structured argument parsing
var result = Arg.parse()

Loops & Iteration

Count Loop

for i in 0.to(10)
    print(i)  # 0, 1, 2, ..., 9

While Loop

var i = 0
while i < 10
    print(i)
    i = i + 1

Loop Until Condition

var count = 0
while count < 5
    print(count)
    count = count + 1

Early Exit

for item in items
    if item == target
        print("Found!")
        break

while condition
    if error_detected
        break

Skip Iteration

for item in items
    if item == skip_value
        continue
    
    print(item)

Lazy Sequence (Generator)

A def returning Iter(T) produces values one yield at a time, so the caller can stop early and nothing is collected first (Chapter 05, "Generators"). For your own iterable class, see cue iter / cue next in Chapter 07.

def readTokens(text: str): Iter(str)
    var parts: List(str) = text.split(",")
    for p in parts
        var t = p.trim()
        if t.len > 0
            yield t

def main()
    for tok in readTokens("a, b,, c")
        print(tok)                     # a, b, c (one per line)

Practical Patterns

CSV Reader

def read_csv(filename: str): List(List(str)) throws
    var content = File.read(filename)
    var lines = content.split("\n")
    var records = List(List(str))()

    for line in lines
        if line.trim().len == 0
            continue

        var fields = line.split(",")
        records.add(fields)

    return records

Word Counter

def count_words(filename: str): HashMap(str, int) throws
    var content = File.read(filename)
    var text = content.lower()
    var words = text.split(" ")
    var counts = HashMap(str, int)()

    for word in words
        var clean = word.trim()
        if clean.len > 0
            var current = counts.get(clean)
            if current != nil
                counts.put(clean, current + 1)
            else
                counts.put(clean, 1)

    return counts

Pipeline Processing

The -> pipeline operator's right-hand side must be a call to a free function (x -> f(), meaning f(x)) — it can't take a .method() on its own, and it doesn't span multiple lines. For chaining methods on a value, just chain the method calls directly:

var input = "  hello world  from zebra  "

var result = input.trim().lower().split(" ")

# result is a List(str); note double spaces in the input produce empty
# elements between words, since split(" ") is a raw split

Quick Debugging

var x = 42
print("x = ${x}")

Check Collection Size

var items = List(int)()
items.add(1)
items.add(2)
print("Count: ${items.count()}")

Trace Execution

def function()
    print("Start")
    # ... code ...
    print("After step 1")
    # ... code ...
    print("Done")

Assert Conditions

assert x > 0, "x must be positive"
assert items.count() > 0, "list must not be empty"

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