How does the flow of execution go in nested if/else statements?

Question

If I have two if statements and then an else, how do I know which if statement the else applies to? Is it by the indentation? For example this,

if x == 2:
    if y == 3:
        x = y
else:
    y = x

Which if statement does the else refer to?

In python, it will be indents. Other languages use curly braces. What you're looking for is called 'scope' — Will, Aug 10 '15 at 16:13

score 6 · Accepted Answer · answered Aug 10 '15 at 16:13

6

else applies to the first if statement, see the indentation.

answered Aug 10 '15 at 16:13

Avinash Raj

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And if the else were indented to match the second if, it would apply to that if? – Sygnerical Aug 10 '15 at 16:14
1

@Sygnerical yes. You know you can try these things out in the interpreter? – jonrsharpe Aug 10 '15 at 16:15
1

ya, if the else is intended according to the second if then it applies to the second. – Avinash Raj Aug 10 '15 at 16:15
@jonrsharpe I'm on a phone right now, no access to a computer – Sygnerical Aug 10 '15 at 16:16
1

@Sygnerical so, given that you can't act on it, why ask? – jonrsharpe Aug 10 '15 at 16:17
1

@jonrsharpe i was curious and just wanted to make sure. I'm reading a pdf about python and it didn't explain that bit – Sygnerical Aug 10 '15 at 16:19

score 3 · Answer 2 · answered Aug 10 '15 at 16:14

3

You gotta look at the indentation to figure out to what which if does that else belong to.

In this case

if x == 2:
|    if y == 3:
|    |    x = y
|    |
|    else:
|         pass # example
else:
    y = x

it belongs to the if x = 2:

answered Aug 10 '15 at 16:14

jramirez

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score 2 · Answer 3 · edited Sep 29 '19 at 23:33

First, the above code checks:

if x == 2:

If this boolean is False, the code moves to the else:

else:
     y = x

If if x == 2 is True, the code moves to the nested if-statement:

if y == 3:
     x = y

According to this page http://www.python-course.eu/python3_blocks.php, it talks about how instead of braces to group statements into blocks, Python uses indentation: "Python programs get structured through indentation, i.e. code blocks are defined by their indentation."

_{(source: python-course.eu)}

tl;dr: YES, in Python, indentation matters.

How does the flow of execution go in nested if/else statements?

3 Answers3