Question about a Python for loop

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In summary, the conversation discusses the use of a "list comprehension" in Python, which is a common construct for creating lists. The code snippet provided by one person has syntax and conceptual errors, and the other person advises testing code in the interactive interpreter before posting it. The conversation also mentions that list comprehension is not present in older languages and provides a link for more information.
  • #1

member 428835

Hi All

I'm learning python and have never seen a for loop positioned like this:
Python:
integers = [4, 5, 7]
strings = [str(integer) for integer in integers]
I've only ever seen for loops positioned at the beginning of lines. Is this usual? How can I know when this is okay? I think I would have tried something like this instead
Python:
integers = [4, 5, 7]
strings = range(len(integers))
for i in integers:
    strings(i) = [str(i)]
 
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  • #2
It's called a "list comprehension" and is a fairly common construct, yes.
 
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  • #3
Ibix said:
It's called a "list comprehension" and is a fairly common construct, yes.
In Python... It's not a construct that's present in other, older languages, AFAIK. Possibly it's present in some of the newer languages.
 
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  • #5
joshmccraney said:
I think I would have tried something like this instead
Your code has a syntax error in its last line. It also has two conceptual errors (three if you are using Python 3).

A really good piece of advice is to test any snippet of code in the interactive interpreter before posting it. Even better, post the actual interpreter session:

Python:
>>> integers = [4, 5, 7]
>>> strings = range(len(integers))
>>> for i in integers:
...     strings(i) = [str(i)]
...
  File "<stdin>", line 2
SyntaxError: cannot assign to function call
>>> for i in integers:
...     strings[i] = [str(i)]
...
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
TypeError: 'range' object does not support item assignment
>>> strings = list(range(len(integers)))
>>> for i in integers:
...     strings[i] = [str(i)]
...
Traceback (most recent call last):
  File "<stdin>", line 2, in <module>
IndexError: list assignment index out of range
>>> for i in integers:
...     print(i)
...     strings[i] = [str(i)]
...
4
Traceback (most recent call last):
  File "<stdin>", line 3, in <module>
IndexError: list assignment index out of range
>>> for idx, i in enumerate(integers):
...     strings[idx] = [str(i)]
...
>>> strings
[['4'], ['5'], ['7']]
>>> for idx, i in enumerate(integers):
...     strings[idx] = str(i)
...
>>> strings
['4', '5', '7']
Only the very last is actually what is intended.
 
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1. What is a for loop in Python?

A for loop in Python is a control flow statement that allows you to execute a block of code repeatedly for a specific number of times. It iterates over a sequence, such as a list or a string, and performs the same action on each element of the sequence.

2. How do you write a for loop in Python?

To write a for loop in Python, you need to use the for keyword, followed by a variable name, the in keyword, and a sequence. Then, you need to indent the code block that you want to execute inside the loop. For example:
for num in range(1, 6):
    print(num)

This loop will print the numbers from 1 to 5.

3. Can you have nested for loops in Python?

Yes, you can have nested for loops in Python. This means that you can have a for loop inside another for loop. Each time the outer loop runs, the inner loop will also run. This is useful when you need to perform a repetitive task on a multi-dimensional data structure, such as a list of lists.

4. What is the difference between a for loop and a while loop in Python?

A for loop in Python is used when you know exactly how many times you want to execute a block of code. It iterates over a sequence, while a while loop is used when you want to execute a block of code until a specific condition is met. It doesn't require a sequence, and the loop will continue to run until the condition is no longer true.

5. How can you exit a for loop in Python?

To exit a for loop in Python, you can use the break keyword. This will immediately terminate the loop, and the code execution will continue after the loop. You can also use the continue keyword to skip the current iteration and move on to the next one.

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