String

Initialization

from String import String

var s = String("abcde")
print(s)

abcde

Indexing

Loop through the string and print each item

for i in range(len(s)):
    print(s[i])

a
b
c
d
e

Both slicing and indexing are on bytes, not characters, for example an emoji is 4 bytes so you need to use this slice of 4 bytes to print the character:

var utf8 = String("mojo🔥")
print(utf8[4:8])

🔥

Slicing

Print part of the string sting using a slice

Slice from 2 up to 4 non-inclusive:

print(s[2:4])

cd

Slice all characters starting from 1:

print(s[1:])

bcde

Slice all characters up to the second last:

print(s[:-1])

abcd

Only get every second item after the start position:

print(s[::2])

ace

Appending

Returns a new string by copying memory

let x = String("Left")
let y = String("Right")

var c = x + y

c += "🔥"

print(c)

LeftRight🔥

Join

var j = String("🔥")

The join function has a similar syntax to Python's join. You can join elements using the current string as a delimiter.

print(j.join('a', 'b'))
print(j.join(40, 2))

a🔥b
40🔥2

You can also use it to join elements of a StaticIntTuple.

from Index import StaticIntTuple

var sit = StaticIntTuple[3](1,2,3)
print(j.join(sit))

1🔥2🔥3

atol

The term comes from the C stdlib for ASCII to long-integer, it converts a string to an Int (currently just works with base-10 / decimal):

from String import atol

var n = atol("19")
print(n)

This will throw an error because there's a character that's not a digit:

var e = atol("hi")
print(e)

Error: String is not convertible to integer.

chr

Use chr to convert an integer between 0 and 255 to a string containing the single character

from String import chr

print(chr(97))

ord

Stands for ordinal which means the position of the character in ASCII

from String import ord

print(ord('a'))

Only 1 byte utf8 (ASCII) characters currently work, anything outside will currently wrap:

print(ord('🔥'))

-16

isdigit

Check if the character passed in is a valid decimal between 0 and 9, which in ASCII is 48 to 57

from String import isdigit

print(isdigit(ord('8')))
print(isdigit(ord('a')))

True
False

Conversions

To `DynamicVector[Int8]`

In Mojo a string is backed by a DynamicVector[Int8], which you can access via the buffer member variable:

let buffer = String("mojo").buffer
print(buffer.capacity)

Force an error so Mojo shows us the type:

buffer = ""

error: Expression [19]:25:14: cannot implicitly convert 'StringLiteral' value to 'DynamicVector[SIMD[si8, 1]]' in assignment
    buffer = ""
             ^~

From `DynamicVector[Int8]`

from Vector import DynamicVector

var x = DynamicVector[Int8](4)
x.push_back(109)
x.push_back(111)
x.push_back(106)
x.push_back(111)

let s = String(x.data, 4)
print(s)

mojo