Control flow
Mojo includes several traditional control flow structures for conditional and repeated execution of code blocks.
The if statement
Mojo supports the if statement for conditional code execution. With it you can
conditionally execute an indented code block if a given
boolean expression evaluates to True.
temp_celsius = 25
if temp_celsius > 20:
print("It is warm.")
print("The temperature is", temp_celsius * 9 / 5 + 32, "Fahrenheit." )
temp_celsius = 25
if temp_celsius > 20:
print("It is warm.")
print("The temperature is", temp_celsius * 9 / 5 + 32, "Fahrenheit." )
It is warm.
The temperature is 77.0 Fahrenheit.
It is warm.
The temperature is 77.0 Fahrenheit.
You can write the entire if statement as a single line if all you need to
execute conditionally is a single, short statement.
temp_celsius = 22
if temp_celsius < 15: print("It is cool.") # Skipped because condition is False
if temp_celsius > 20: print("It is warm.")
temp_celsius = 22
if temp_celsius < 15: print("It is cool.") # Skipped because condition is False
if temp_celsius > 20: print("It is warm.")
It is warm.
It is warm.
Optionally, an if statement can include any number of additional elif
clauses, each specifying a boolean condition and associated code block to
execute if True. The conditions are tested in the order given. When a
condition evaluates to True, the associated code block is executed and no
further conditions are tested.
Additionally, an if statement can include an optional else clause providing
a code block to execute if all conditions evaluate to False.
temp_celsius = 25
if temp_celsius <= 0:
print("It is freezing.")
elif temp_celsius < 20:
print("It is cool.")
elif temp_celsius < 30:
print("It is warm.")
else:
print("It is hot.")
temp_celsius = 25
if temp_celsius <= 0:
print("It is freezing.")
elif temp_celsius < 20:
print("It is cool.")
elif temp_celsius < 30:
print("It is warm.")
else:
print("It is hot.")
It is warm.
It is warm.
Mojo currently does not support the equivalent of a Python match or C switch
statement for pattern matching and conditional execution.
Short-circuit evaluation
Mojo follows short-circuit evaluation
semantics for boolean operators. If the first argument to an or operator
evaluates to True, the second argument is not evaluated.
def true_func() -> Bool:
print("Executing true_func")
return True
def false_func() -> Bool:
print("Executing false_func")
return False
print('Short-circuit "or" evaluation')
if true_func() or false_func():
print("True result")
def true_func() -> Bool:
print("Executing true_func")
return True
def false_func() -> Bool:
print("Executing false_func")
return False
print('Short-circuit "or" evaluation')
if true_func() or false_func():
print("True result")
Short-circuit "or" evaluation
Executing true_func
True result
Short-circuit "or" evaluation
Executing true_func
True result
If the first argument to an and operator evaluates to False, the second
argument is not evaluated.
print('Short-circuit "and" evaluation')
if false_func() and true_func():
print("True result")
print('Short-circuit "and" evaluation')
if false_func() and true_func():
print("True result")
Short-circuit "and" evaluation
Executing false_func
Short-circuit "and" evaluation
Executing false_func
Conditional expressions
Mojo also supports conditional expressions (or what is sometimes called a
ternary conditional operator)
using the syntaxtrue_result if boolean_expression else false_result, just as
in Python. This is most often used as a concise way to assign one of two
different values to a variable, based on a boolean condition.
temp_celsius = 15
forecast = "warm" if temp_celsius > 20 else "cool"
print("The forecast for today is", forecast)
temp_celsius = 15
forecast = "warm" if temp_celsius > 20 else "cool"
print("The forecast for today is", forecast)
The forecast for today is cool
The forecast for today is cool
The alternative, written as a multi-line if statement, is more verbose.
if temp_celsius > 20:
forecast = "warm"
else:
forecast = "cool"
print("The forecast for today is", forecast)
if temp_celsius > 20:
forecast = "warm"
else:
forecast = "cool"
print("The forecast for today is", forecast)
The forecast for today is cool
The forecast for today is cool
The while statement
The while loop repeatedly executes a code block while a given boolean
expression evaluates to True. For example, the following loop prints values
from the Fibonacci series that are less than 50.
fib_prev = 0
fib_curr = 1
print(fib_prev, end="")
while fib_curr < 50:
print(",", fib_curr, end="")
fib_prev, fib_curr = fib_curr, fib_prev + fib_curr
fib_prev = 0
fib_curr = 1
print(fib_prev, end="")
while fib_curr < 50:
print(",", fib_curr, end="")
fib_prev, fib_curr = fib_curr, fib_prev + fib_curr
0, 1, 1, 2, 3, 5, 8, 13, 21, 34
0, 1, 1, 2, 3, 5, 8, 13, 21, 34
A continue statement skips execution of the rest of the code block and
resumes with the loop test expression.
n = 0
while n < 5:
n += 1
if n == 3:
continue
print(n, end=", ")
n = 0
while n < 5:
n += 1
if n == 3:
continue
print(n, end=", ")
1, 2, 4, 5,
1, 2, 4, 5,
A break statement terminates execution of the loop.
n = 0
while n < 5:
n += 1
if n == 3:
break
print(n, end=", ")
n = 0
while n < 5:
n += 1
if n == 3:
break
print(n, end=", ")
1, 2,
1, 2,
Optionally, a while loop can include an else clause. The body of the else
clause executes when the loop's boolean condition evaluates to False, even if
it occurs the first time tested.
n = 5
while n < 4:
print(n)
n += 1
else:
print("Loop completed")
n = 5
while n < 4:
print(n)
n += 1
else:
print("Loop completed")
Loop completed
Loop completed
The else clause does not execute if a break or return statement
exits the while loop.
n = 0
while n < 5:
n += 1
if n == 3:
break
print(n)
else:
print("Executing else clause")
n = 0
while n < 5:
n += 1
if n == 3:
break
print(n)
else:
print("Executing else clause")
1
2
1
2
The for statement
The for loop iterates over a sequence, executing a code block for each
element in the sequence.
The Mojo for loop can iterate over any type that implements an __iter__()
method that returns a type that defines __next__() and __len__() methods.
Iterating over Mojo collections
All of the collection types in the collections
module support for loop iteration. See the
Collection types documentation for more
information on Mojo collection types.
Iterating over Mojo native collections currently assigns the loop index variable
a Reference to each item, not the
item itself. You can access the item using the dereference operator, [], as
shown in the examples below. This may change in a future version of Mojo.
The following shows an example of iterating over a Mojo
List.
from collections import List
states = List[String]("California", "Hawaii", "Oregon")
for state in states:
print(state[])
from collections import List
states = List[String]("California", "Hawaii", "Oregon")
for state in states:
print(state[])
California
Hawaii
Oregon
California
Hawaii
Oregon
The same technique works for iterating over a Mojo
Set.
from collections import Set
values = Set[Int](42, 0)
for item in values:
print(item[])
from collections import Set
values = Set[Int](42, 0)
for item in values:
print(item[])
42
0
42
0
There are two techniques for iterating over a Mojo
Dict. The first is to iterate directly
using the Dict, which produces a sequence of the dictionary's keys.
from collections import Dict
capitals = Dict[String, String]()
capitals["California"] = "Sacramento"
capitals["Hawaii"] = "Honolulu"
capitals["Oregon"] = "Salem"
for state in capitals:
print(capitals[state[]] + ", " + state[])
from collections import Dict
capitals = Dict[String, String]()
capitals["California"] = "Sacramento"
capitals["Hawaii"] = "Honolulu"
capitals["Oregon"] = "Salem"
for state in capitals:
print(capitals[state[]] + ", " + state[])
Sacramento, California
Honolulu, Hawaii
Salem, Oregon
Sacramento, California
Honolulu, Hawaii
Salem, Oregon
The second approach to iterating over a Mojo Dict is to invoke its
items() method, which produces a
sequence of DictEntry objects.
Within the loop body, you can then access the key and value fields of the
entry.
for item in capitals.items():
print(item[].value + ", " + item[].key)
for item in capitals.items():
print(item[].value + ", " + item[].key)
Sacramento, California
Honolulu, Hawaii
Salem, Oregon
Sacramento, California
Honolulu, Hawaii
Salem, Oregon
Another type of iterable provided by the Mojo standard library is a range,
which is a sequence of integers generated by the
range() function. It differs from the
collection types shown above in that it's implemented as a
generator,
producing each value as needed rather than materializing the entire sequence
in memory. Additionally, each value assigned to the loop index variable is
simply the Int value rather than a Reference to the value, so you should
not use the dereference operator on it within the loop. For example:
for i in range(5):
print(i, end=", ")
for i in range(5):
print(i, end=", ")
0, 1, 2, 3, 4,
0, 1, 2, 3, 4,
for loop control statements
A continue statement skips execution of the rest of the code block and
resumes the loop with the next element of the collection.
for i in range(5):
if i == 3:
continue
print(i, end=", ")
for i in range(5):
if i == 3:
continue
print(i, end=", ")
0, 1, 2, 4,
0, 1, 2, 4,
A break statement terminates execution of the loop.
for i in range(5):
if i == 3:
break
print(i, end=", ")
for i in range(5):
if i == 3:
break
print(i, end=", ")
0, 1, 2,
0, 1, 2,
Optionally, a for loop can include an else clause. The body of the else
clause executes after iterating over all of the elements in a collection.
for i in range(5):
print(i, end=", ")
else:
print("\nFinished executing 'for' loop")
for i in range(5):
print(i, end=", ")
else:
print("\nFinished executing 'for' loop")
0, 1, 2, 3, 4,
Finished executing 'for' loop
0, 1, 2, 3, 4,
Finished executing 'for' loop
The else clause executes even if the collection is empty.
from collections import List
empty = List[Int]()
for i in empty:
print(i[])
else:
print("Finished executing 'for' loop")
from collections import List
empty = List[Int]()
for i in empty:
print(i[])
else:
print("Finished executing 'for' loop")
Finished executing 'for' loop
Finished executing 'for' loop
The else clause does not execute if a break or return statement
terminates the for loop.
from collections import List
animals = List[String]("cat", "aardvark", "hippopotamus", "dog")
for animal in animals:
if animal[] == "dog":
print("Found a dog")
break
else:
print("No dog found")
from collections import List
animals = List[String]("cat", "aardvark", "hippopotamus", "dog")
for animal in animals:
if animal[] == "dog":
print("Found a dog")
break
else:
print("No dog found")
Found a dog
Found a dog
Iterating over Python collections
The Mojo for loop supports iterating over Python collection types. Each item
retrieved by the loop is a
PythonObject wrapper around
the Python object. Refer to the Python types
documentation for more information on manipulating Python objects from Mojo.
The following is a simple example of iterating over a mixed-type Python list.
from python import Python
# Create a mixed-type Python list
py_list = Python.evaluate("[42, 'cat', 3.14159]")
for py_obj in py_list: # Each element is of type "PythonObject"
print(py_obj)
from python import Python
# Create a mixed-type Python list
py_list = Python.evaluate("[42, 'cat', 3.14159]")
for py_obj in py_list: # Each element is of type "PythonObject"
print(py_obj)
42
cat
3.14159
42
cat
3.14159
Iterating over a Mojo collection currently assigns the loop index variable a
Reference to each element, which then requires you to use the dereference
operator within the loop body. In contrast, iterating over a Python collection
assigns a PythonObject wrapper for the element, which does not require you
to use the dereference operator.
There are two techniques for iterating over a Python dictionary. The first is to iterate directly using the dictionary, which produces a sequence of its keys.
from python import Python
# Create a mixed-type Python dictionary
py_dict = Python.evaluate("{'a': 1, 'b': 2.71828, 'c': 'sushi'}")
for py_key in py_dict: # Each element is of type "PythonObject"
print(py_key, py_dict[py_key])
from python import Python
# Create a mixed-type Python dictionary
py_dict = Python.evaluate("{'a': 1, 'b': 2.71828, 'c': 'sushi'}")
for py_key in py_dict: # Each element is of type "PythonObject"
print(py_key, py_dict[py_key])
a 1
b 2.71828
c sushi
a 1
b 2.71828
c sushi
The second approach to iterating over a Python dictionary is to invoke its
items() method, which produces a sequence of 2-tuple objects.
Within the loop body, you can then access the key and value by index.
for py_tuple in py_dict.items(): # Each element is of type "PythonObject"
print(py_tuple[0], py_tuple[1])
for py_tuple in py_dict.items(): # Each element is of type "PythonObject"
print(py_tuple[0], py_tuple[1])
a 1
b 2.71828
c sushi
a 1
b 2.71828
c sushi
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