Incrementing counter within for loop

Question

I am having trouble properly incrementing a counter variable within a for loop.

I have a variable called flag, and I want to create a new variable called num based on the values in flag.

Input:

'flag'
FALSE
TRUE
TRUE
TRUE
FALSE
TRUE
FALSE
TRUE
TRUE

Desired output:

'flag' 'num'
FALSE   1
TRUE    1
TRUE    1
TRUE    1
FALSE   2
TRUE    2
FALSE   3
TRUE    3
TRUE    3

The goal is to start the num counter at 1 and keep that counter at 1 until another False instance occurs in the flag column. This would be continued until the end of the df.

My code:

def num(z):  
    i = 0
    for x in range (0,len(z)):
        if z['flg'] == False:
            return i + 1
        else:
            return i

df['num']=df.apply(num, axis=1)

I have tried incrementing the I counter in numerous places, but no luck...

`z['flg']` always returns the same thing. Your loop does not affect it. — Ma0, Aug 28 '17 at 14:07
Also, if you meant to index `z` using `x`, iterate over `z.keys()`, not its length. And you shouldn't be returning in the loop. you should be doing `i = i+1` and returning at the end of the loop — user1999728, Aug 28 '17 at 14:09

Maarten Fabré · Accepted Answer · 2017-08-28T14:14:18.417

8

df['num'] = (~df['flag']).cumsum()

uses the fact that False = 0, True == 1

if df['num'] is not a column of True and False values, but a column of strings, I suggest you change that, or change the test to df['num'] = (df['flag'] == 'FALSE').cumsum()

edited Aug 28 '17 at 14:14

answered Aug 28 '17 at 14:09

Maarten Fabré

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very nice solution ;) – jezrael Aug 28 '17 at 14:11
Very simple solution too. – Bharath M Shetty Aug 28 '17 at 14:13

score 0 · Answer 2 · answered Aug 28 '17 at 15:09

Here is your code in a corrected matter, but the solution by Maarten Fabré is nicer:

def num(z):
    i = 0
    numcol = []
    for flag in z:
        if not flag:
            i += 1
        numcol.append(i)
    return numcol

df['num'] = num(df['flag'])

At first I wanted to do it with a generator, but I did not get it to work with the pandas dataframe.

Incrementing counter within for loop

2 Answers2