CODEWITHSUNDEEP
pythonfor-loop
Tung Son Nguyen
Tung Son Nguyen

Reputation: 3

Increment happens before it should

a_string = 'abc'

destination = [2, 3]    

edges = { (1, 'a') : [2, 3],
          (2, 'a') : [2],
          (3, 'b') : [4, 3],
          (4, 'c') : [5] }

def make(a_string, destination, edges):
    n = 0
    while n + 1 < len(a_string):
        letter = a_string[n]
        letter2 = a_string[n + 1]
        for d in destination:                              # (1)
            if (d, letter2) in edges:
                for state in edges[(d, letter2)]:
                    destionation.append(state)
            destination.remove(d)
        n += 1                                             # (2)
    return destination

The code return [], but I expect to see [5], so I think the problem is that it increment n unexpectedly then make letter2 change. Why does this code increment n (at position 2) before completing the for loop (at position 1)?

Upvotes: 0

Views: 125

Answers (3)

Minion91
Minion91

Reputation: 1929

n will not be incremented before the loop is done. The thing you are probably missing is that the while loop checks for n+1 instead of n.

edit now we have more information:

The problem is that you are removing items from an iterator which has undefined behavior.

try

for d in destination[:]:

This is the slice operator on the entire array, so it acts as a copy constructor. You are now looping over a different object and the remove should be safe.

Upvotes: 1

Burhan Khalid
Burhan Khalid

Reputation: 174614

You can iterate over strings as well, and using the index method of strings you can grab the next position of the character.

Combining those two, your initial outer loop can be simplified:

def make(a_string, destination, edges):

    for letter in a_string:
        while a_string.index(letter)+1 < len(a_string):
            next_letter = a_string[a_string.index(letter)+1]

In addition, you shouldn't name your variable string as it is the name of a module.

Upvotes: 0

Asciiom
Asciiom

Reputation: 9975

If you don't add 1 to n at the end of the loop, the loop condition stays the same and the loop will be executed forever. It's not executed in the for loop but in the while loop body. (Indentation determines to which block of code a line belongs!)

Upvotes: 0

Related Questions