How can I keep track of a variable during a For loop?

Question

Basically, this is what I am trying to do. I have an CSV file that I read in using Python 3. So, here is the basic layout of the rows:

Row1: On
Row2: <empty>
Row3: <empty>
Row4: On
Row5: On
Row6: <empty>
Row7: Off
Row8: <empty>

The code to access this would be:

for row in file:
     var = row[0]
     print(var)

The output I would like to see for each line after running the script would be:

for row in file:
     print(var)

On
On
On
On
On
On
Off
Off

I am not sure how to do it, but I am trying to keep track of the variable as the program moves through the for loop. Here is the logic:

for loop:
  1.  If row[0] has the string 'On', then assign 'On' to var
  2.  If the next row[0] is empty, then I want the var to retain the previous value of 'On'.
  3.  Var will not change from 'On' until row[0] has a different value such as 'Off'.  Then, Var will be assigned 'Off'.

Hopefully this question makes sense. I am not sure how to do this in Python 3.

What do you mean when you say a row is empty? A blank line? Commas with no values between them? Empty strings? — Mark Reed, Nov 01 '15 at 00:33
So just declare `var = None` outside the loop? Then, `if row != "": var = row`? Or am I missing something? — Teepeemm, Nov 01 '15 at 00:33

score 1 · Answer 1 · edited May 23 '17 at 10:27

# set an initial value for `var`
var = None
for row in file:
    # `row` should now contain the text content of a line
    if row:
        # if the string is not empty
        # set var to the value of the string
        var = row
    # print the value of var
    print(var)

In Python, empty strings are "falsey" while non-empty strings are "truthy". By using the statement if row: we would only proceed into the if statement when row contains non-empty strings, such as "On", or "Off".

score 0 · Answer 2 · answered Nov 01 '15 at 00:36

0

A simple if/else will do

var = some_initial_value
for row in file:
     var = row[0] if row[0] else var
     print(var)

answered Nov 01 '15 at 00:36

tdelaney

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How can I keep track of a variable during a For loop?

2 Answers2