Python For Loops
Python For Loops
A for loop is used for iterating over a sequence (that is either a list, a tuple or a string).
This is less like the for keyword in other programming language, and works more like an iterator method as found in other object-orientated programming languages.
With the for loop we can execute a set of statements, once for each item in a list, tuple, set etc.
Example
Print each fruit in a fruit list:
fruits = ["apple", "banana", "cherry"]
for
x in fruits:
print(x)
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The for loop does not require an indexing variable to set beforehand, as the for command itself allows for this.
The break Statement
With the break statement we can stop the loop before it has looped through all the items:
Example
Exit the loop when x is "banana":
fruits = ["apple", "banana", "cherry"]
for x in fruits:
if x ==
"banana":
break
print(x)
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The continue Statement
With the continue statement we can stop the current iteration of the loop, and continue wit the next:
Example
Do not print banana:
fruits = ["apple", "banana", "cherry"]
for x in fruits:
if x ==
"banana":
continue
print(x)
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The range() Function
To loop through a set of code a specified number of times, we can use the range() function,The range() function returns a sequence of numbers, starting from 0 by default, and increments by 1 (by default), and ends at a specified number.
Note that range(6) is not the values of 0 to 6, but the values 0 to 5.
The range() function defaults to 0 as a starting value, however it is possible to specify the starting value by adding a parameter: range(2, 6), which means values from 2 to 6 (but not including 6):
The range() function defaults to increment the sequence by 1, however it is possible to specify the increment value by adding a third parameter: range(2, 30, 3):
Example
Increment the sequence with 3 (default is 1):
for x in range(2, 30, 3):
print(x)
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Recursion
Python also accepts function recursion, which means a defined function can call itself.
Recursion is a common mathematical and programming concept. It means that a function calls itself. This has the benefit of meaning that you can loop through data to reach a result.
The developer should be very careful with recursion as it can be quite easy to slip into writing a function which never terminates, or one that uses excess amounts of memory or processor power. However, when written correctly recursion can be a very efficient and mathematically-elegant approach to programming.
In this example, tri_recursion() is a function that we have defined to call itself ("recurse"). We use the k variable as the data, which decrements (-1) every time we recurse. The recursion ends when the condition is not greater than 0 (i.e. when it is 0).
To a new developer it can take some time to work out how exactly this works, best way to find out is by testing and modifying it.
Example
Recursion Example
def tri_recursion(k):
if(k>0):
result = k+tri_recursion(k-1)
print(result)
else:
result = 0
return result
print("\n\nRecursion Example Results")
tri_recursion(6)
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