Question

The snippet of code that’s messing up:

equ2 = ['+', '10', '+', '2', '-', '2', '+', '4']
flag4 = [ ]

for k in equ2[:]:
    if equ2[equ2.index(k)-1] == '+':
        del(equ2[equ2.index(k)])
        flag4.append('-' + k)
    elif equ2[equ2.index(k)-1] == '-':
        del(equ2[equ2.index(k)])
        flag4.append('+' + k)
print flag4

*Edit: I messed up the variables, sorry about that 😡

*Edit2: Sorry again D: jedwards, that is the output I’m getting, I got the other output by deleting from the copied list

Output I’m getting:

['-10', '-2', '--', '-2', '-4']

Output I want:

['-10', '-2', '+'2, '-4']

I figured out that when it got to the ‘-‘, it checks the original list (now modified to + – 2 + 4), and sees that the item before the ‘-‘ is a ‘+’, so it appends ‘–‘, then checks the original list again (now + 2 + 4) and sees the ‘+’ before the 2, so it appends ‘-2’ (fixes itself sorta after that). Is it checking the original list the whole time?

Answer 1

I’m not really conviced this is the best way to do this, the following works

equ2 = ['+', '10', '+', '2', '-', '2', '+', '4']
flag4 = list()

chgsgn = dict([('+','-'), ('-','+')])

while len(equ2):
    sgn = equ2.pop(0)
    num = equ2.pop(0)

    flag4.append(chgsgn[sgn] + num)


print flag4

Which outputs

['-10', '-2', '+2', '-4']

Edit: Alternatively, using list comprehensions and not modifying equ2:

chgsgn = dict([('+','-'), ('-','+')])
flag4 = [chgsgn[sgn]+num for (sgn,num) in zip(*[iter(equ2)]*2)]
print flag4

Which also outputs

['-10', '-2', '+2', '-4']