Python: For loop Concatenation

Question

cat = "["
for row in res:
    cat = cat  + (str((row['weeks'])) + ',')

cat = (cat + "]").replace(',]', ']')

The above bit of code gives a result string as:

[30,31,32,33,34,35,36,37,38,39,40]

However what I want is:

[W30,W31,W32,W33,W34,W35,W36,W37,W38,W39,W40]

I have been unsuccessful in concatenating the W before each number. How could I do this? This is the unsuccessful code that I tried:

cat = cat  + (str('W'+ (row['weeks'])) + ',')

Are you trying to make an actual list when this is done, or do you actually want a string? — Rob Foley
– Rob Foley, Commented Jul 10, 2015 at 11:50
I want the result to be a string but in the above mentioned format. — Tauseef Hussain
– Tauseef Hussain, Commented Jul 10, 2015 at 11:52
Ah! Please add an example of the contents of row['weeks']. This will probably stop other guesses. — dhke
– dhke, Commented Jul 10, 2015 at 12:01

mkrieger1 · Accepted Answer · 2015-07-10 12:07:07Z

2

This gives the desired string:

cat = '[{}]'.format(','.join('W{}'.format(i) for i in row['weeks']))
#                            ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
#                            this gives 'W30', 'W31', etc.
#                   ^^^^^^^^^
#                   this joins them with commas: 'W30,W31,...,W40'
#     ^^^^^^^^^^^^^^
#     this puts [ ] around

edited Jul 10, 2015 at 12:07

answered Jul 10, 2015 at 12:01

mkrieger1

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Comments

The6thSense · Accepted Answer · 2015-07-10 11:57:14Z

1

You have done this

cat = cat  + (str('W'+ (row['weeks'])) + ',')

It should be

cat = cat  + ('W'+ str(row['weeks']) + ',')

if you want it to be a list you could do this

cat = ["W"+str(row['weeks']) for row in res]

edited Jul 10, 2015 at 11:57

answered Jul 10, 2015 at 11:51

The6thSense

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3 Comments

Tauseef Hussain Over a year ago

Your initial answer was great, thank you so much. I later realised that I need the final answer in the form of a JSON Object hence the final edit was: cat = cat + ('"W'+ str((row['weeks'])) + '",') cat = (cat + "]").replace(',]', ']')

The6thSense Over a year ago

you are welcome but do see others way it might be efficient and help full for you :)

Tauseef Hussain Over a year ago

So my final list was ["W30","W31","W32","W33","W34","W35","W36","W37","W38","W39","W40"]

The6thSense · Accepted Answer · 2015-07-10 11:55:51Z

1

code:

cat = '[W' + ',W'.join(str(element) for element in row['weeks']) + ']'

edited Jul 10, 2015 at 11:55

The6thSense

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answered Jul 10, 2015 at 11:54

Rob Foley

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Comments

Karl Knechtel · Accepted Answer · 2015-07-10 12:03:52Z

1

Instead of getting an answer from here (since there are many ways to do it), you should understand what went wrong with your own code.

(str('W'+ (row['weeks'])) + ',')

What is the order of operations here?

We find row['weeks'].
We attempt 'W' + that.
We attempt str on that.
We attempt that + ','.

Presumably, row['weeks'] is an integer, so the addition with 'W' will fail. You must do this after applying str to the integer.

answered Jul 10, 2015 at 12:03

Karl Knechtel

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Comments

Pynchia · Accepted Answer · 2015-07-10 11:55:17Z

0

If you want a list made of strings do

cat = [ 'W'+str(row['weeks']) for row in res]

and, if you want a single string, turn the list into a string

string_cat = str(cat)

answered Jul 10, 2015 at 11:55

Pynchia

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Comments

dlask · Accepted Answer · 2015-07-10 11:55:47Z

0

cat = "[" + ",".join('W' + str(row['weeks']) for row in res) + "]"

For each row in res convert row['weeks'] to string, prefix it with 'W', and join all such strings with the ',' character. Then just add opening [ and closing ].

answered Jul 10, 2015 at 11:55

dlask

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