Mojo String Basics

A string is a sequence of characters stored as a single value. Names, sentences, file paths, and messages are all strings. Mojo's String type provides a rich set of operations for creating, inspecting, and transforming text.

Creating Strings

fn main():
    var greeting: String = "Hello, Mojo!"
    var empty: String = ""
    var number_as_text: String = "42"

    print(greeting)
    print(len(empty))    # 0
    print(number_as_text)

String literals always use double quotes in Mojo. The characters inside are stored in memory as a contiguous sequence of bytes.

Multi-line Strings

fn main():
    var poem = """
    Roses are red,
    Mojo is fast,
    Python is friendly,
    Both are a blast.
    """
    print(poem)

Triple double-quotes let you span a string across multiple lines without inserting special characters manually.

String Length

fn main():
    var word = "Compiler"
    print(len(word))   # 8

  C  o  m  p  i  l  e  r
  0  1  2  3  4  5  6  7   ← index positions
  len("Compiler") = 8

Accessing Characters

Each character in a string sits at a numbered position called an index. Mojo uses zero-based indexing — the first character is at index 0.

fn main():
    var city = "Tokyo"
    print(city[0])   # T
    print(city[1])   # o
    print(city[4])   # o

  T  o  k  y  o
  0  1  2  3  4
  ↑              ← city[0] = 'T'
           ↑    ← city[3] = 'y'

String Concatenation

Concatenation joins two strings end to end using the + operator.

fn main():
    var first = "Mojo"
    var second = " is fast"
    var sentence = first + second
    print(sentence)   # Mojo is fast

"Mojo" + " is fast"
  M o j o   i s   f a s t
  └─────┘   └───────────┘
  first       second
  └──────────────────────┘
         sentence

You cannot concatenate a string with a number directly. Convert the number to a string first using String().

fn main():
    var score = 95
    var message = "Your score: " + String(score)
    print(message)   # Your score: 95

String Repetition

The * operator repeats a string a given number of times.

fn main():
    var line = "-" * 20
    print(line)   # --------------------

    var cheer = "Go! " * 3
    print(cheer)  # Go! Go! Go!

Useful String Methods

upper() and lower()

fn main():
    var text = "Hello Mojo"
    print(text.upper())   # HELLO MOJO
    print(text.lower())   # hello mojo

strip()

Removes leading and trailing whitespace (spaces, tabs, newlines).

fn main():
    var messy = "   lots of space   "
    print(messy.strip())   # "lots of space"

replace()

fn main():
    var sentence = "I love Python"
    var updated = sentence.replace("Python", "Mojo")
    print(updated)   # I love Mojo

find()

Returns the index of the first occurrence of a substring. Returns -1 if not found.

fn main():
    var text = "Mojo is great"
    print(text.find("great"))   # 8
    print(text.find("slow"))    # -1

  M  o  j  o     i  s     g  r  e  a  t
  0  1  2  3  4  5  6  7  8  9  10 11 12
                             ↑
                          "great" starts at index 8

startswith() and endswith()

fn main():
    var filename = "report_2025.pdf"
    print(filename.startswith("report"))   # True
    print(filename.endswith(".pdf"))       # True
    print(filename.endswith(".docx"))      # False

String Slicing

Extract a portion of a string by specifying a start index and an end index. The slice includes the start position but excludes the end position.

fn main():
    var message = "Hello, Mojo!"
    # Extract "Mojo"
    var sub = message[7:11]
    print(sub)   # Mojo

  H  e  l  l  o  ,     M  o  j  o  !
  0  1  2  3  4  5  6  7  8  9  10 11

  message[7:11]
          ↑──────↑
          7      11 (excluded)
  Result: M o j o

Checking String Contents

fn main():
    var code = "mojo123"
    print(code.isdigit())   # False — has letters
    print("999".isdigit())  # True — all digits
    print(code.isalpha())   # False — has digits
    print("Mojo".isalpha()) # True — all letters

Escape Characters

Some characters need a special two-character code called an escape sequence to appear inside a string.

Escape | Meaning
-------|------------------
\n     | New line
\t     | Tab (horizontal)
\\     | Backslash itself
\"     | Double quote

fn main():
    print("Line 1\nLine 2")
    print("Name:\tMojo")
    print("Path: C:\\Users\\Mojo")
    print("She said, \"Mojo rocks!\"")

Output:

Line 1
Line 2
Name:   Mojo
Path: C:\Users\Mojo
She said, "Mojo rocks!"

Strings Are Immutable

You cannot change a single character inside a string after creating it. Every string operation creates a new string object rather than modifying the original.

Immutable string model:

  original = "Mojo"
  modified = original.replace("M", "B")

  original → "Mojo"   (unchanged)
  modified → "Bojo"   (new string)

Key Takeaways

Strings store sequences of characters enclosed in double quotes. Use triple quotes for multi-line strings. Access individual characters by index starting at zero. The + operator concatenates strings and * repeats them. Methods like upper(), strip(), replace(), and find() transform and search strings. Slicing extracts substrings with start and end indices. Strings are immutable — every change produces a new string object.