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# Converting List of String to Integer

 P: n/a Hi Everyone, I am relatively new to Python so please forgive me for what seems like a basic question. Assume that I have a list, a, composed of nested lists with string representations of integers, such that a = [['1', '2'], ['3'], ['4', '5', '6'], ['7', '8', '9', '0']] I would like to convert this to a similar list, b, where the values are represented by integers, such as b = [[1, 2], [3], [4, 5, 6], [7, 8, 9, 0]] I have unsuccessfully tried the following code: n = [] for k in a: n.append([int(v) for v in k]) print n Does anyone know what I am doing wrong? Thanks in advance. Samir Jul 21 '08 #1
12 Replies

 P: n/a Samir wrote: Hi Everyone, I am relatively new to Python so please forgive me for what seems like a basic question. Assume that I have a list, a, composed of nested lists with string representations of integers, such that a = [['1', '2'], ['3'], ['4', '5', '6'], ['7', '8', '9', '0']] I would like to convert this to a similar list, b, where the values are represented by integers, such as b = [[1, 2], [3], [4, 5, 6], [7, 8, 9, 0]] I have unsuccessfully tried the following code: n = [] for k in a: n.append([int(v) for v in k]) print n Does anyone know what I am doing wrong? Thanks in advance. Samir -- http://mail.python.org/mailman/listinfo/python-list You didn't tell us how it failed for you, so I can't guess what's wrong. However, your code works for me: >>a = [['1', '2'], ['3'], ['4', '5', '6'], ['7', '8', '9', '0']]n = []for k in a: .... n.append([int(v) for v in k]) .... >>print n [[1, 2], [3], [4, 5, 6], [7, 8, 9, 0]] (Although you seem to have confused variables b and n.) Gary Herron Jul 21 '08 #2

 P: n/a On Jul 21, 3:20*pm, Gary Herron >a = [['1', '2'], ['3'], ['4', '5', '6'], ['7', '8', '9', '0']] *>>n = [] *>>for k in a: ... * *n.append([int(v) for v in k]) ... *>>print n [[1, 2], [3], [4, 5, 6], [7, 8, 9, 0]] (Although you seem to have confused variables b and n.) Gary Herron- Hide quoted text - - Show quoted text - Hi Gary, Thanks for your quick response (and sorry about mixing up b and n). For some reason, the logic I posted seems to work ok while I'm using the Python shell, but when used in my code, the program just hangs. It never outputs the results. Below is the code in its entirety. Is there a problem with my indendentation? a = n = [] t = """ 1 2 3 4 5 6 7 8 9 0 """ d = t.split("\n") for x in range(1,len(d)-1): a.append(d[x].split(" ")) print a for k in a: n.append([int(v) for v in k]) print n Thanks again. Samir Jul 21 '08 #3

 P: n/a On Jul 22, 6:11 am, Samir ') > print n Jul 21 '08 #4

 P: n/a Samir wrote: On Jul 21, 3:20 pm, Gary Herron Samir wrote: >>Hi Everyone,I am relatively new to Python so please forgive me for what seems likea basic question.Assume that I have a list, a, composed of nested lists with stringrepresentations of integers, such thata = [['1', '2'], ['3'], ['4', '5', '6'], ['7', '8', '9', '0']]I would like to convert this to a similar list, b, where the valuesare represented by integers, such asb = [[1, 2], [3], [4, 5, 6], [7, 8, 9, 0]]I have unsuccessfully tried the following code:n = []for k in a: n.append([int(v) for v in k])print nDoes anyone know what I am doing wrong?Thanks in advance.Samir--http://mail.python.org/mailman/listinfo/python-list You didn't tell us how it failed for you, so I can't guess what's wrong.However, your code works for me: > >>a = [['1', '2'], ['3'], ['4', '5', '6'], ['7', '8', '9', '0']]n = []for k in a: ... n.append([int(v) for v in k])... > >>print n [[1, 2], [3], [4, 5, 6], [7, 8, 9, 0]](Although you seem to have confused variables b and n.)Gary Herron- Hide quoted text -- Show quoted text - Hi Gary, Thanks for your quick response (and sorry about mixing up b and n). For some reason, the logic I posted seems to work ok while I'm using the Python shell, but when used in my code, the program just hangs. It never outputs the results. Below is the code in its entirety. Is there a problem with my indendentation? Aha. There's the problem, right there in the first line. a = n = [] This sets a and n to the *same* empty list. This line creates one empty list and binds both n and a to that list. Note carefully, there is only one empty list here, but it can be accessed under two names Later in your code, for k in a: runs through that list, and n.append(...) append to the end of the same list. Thus the loop never get to the end of the (continually growing) list. Solve it by creating two different empty lists: a = [] n = [] Gary Herron t = """ 1 2 3 4 5 6 7 8 9 0 """ d = t.split("\n") for x in range(1,len(d)-1): a.append(d[x].split(" ")) print a for k in a: n.append([int(v) for v in k]) print n Thanks again. Samir -- http://mail.python.org/mailman/listinfo/python-list Jul 21 '08 #5

 P: n/a On Jul 21, 4:44*pm, Gary Herron Hi Everyone, >I am relatively new to Python so please forgive me for what seems likea basic question. >Assume that I have a list, a, composed of nested lists with stringrepresentations of integers, such that >a = [['1', '2'], ['3'], ['4', '5', '6'], ['7', '8', '9', '0']] >I would like to convert this to a similar list, b, where the valuesare represented by integers, such as >b = [[1, 2], [3], [4, 5, 6], [7, 8, 9, 0]] >I have unsuccessfully tried the following code: >n = []for k in a:* * n.append([int(v) for v in k])print n >Does anyone know what I am doing wrong? >Thanks in advance. >Samir--http://mail.python.org/mailman/listinfo/python-list You didn't tell us how it failed for you, so I can't guess what's wrong. However, your code works for me: *>>a = [['1', '2'], ['3'], ['4', '5', '6'], ['7', '8', '9', '0']] *>>n = [] *>>for k in a: ... * *n.append([int(v) for v in k]) ... *>>print n [[1, 2], [3], [4, 5, 6], [7, 8, 9, 0]] (Although you seem to have confused variables b and n.) Gary Herron- Hide quoted text - - Show quoted text - Hi Gary, Thanks for your quick response (and sorry about mixing up b and n). For some reason, the logic I posted seems to work ok while I'm using the Python shell, but when used in my code, the program just hangs. It never outputs the results. *Below is the code in its entirety. *Is there a problem with my indendentation? Aha. *There's the problem, right there in the first line. a = n = [] This sets a and n to the *same* empty list. * *This line creates one empty list and binds both n and a to that list. *Note carefully, *there is only one empty list here, but it can be accessed under two names Later in your code, * for k in a: runs through that list, and * n.append(...) append to the end of the same list. *Thus the loop never get to the endof the (continually growing) list. Solve it by creating two different empty lists: * a = [] * n = [] Gary Herron t = """ 1 2 3 4 5 6 7 8 9 0 """ d = t.split("\n") for x in range(1,len(d)-1): * * a.append(d[x].split(" ")) print a for k in a: * * n.append([int(v) for v in k]) print n Thanks again. Samir -- http://mail.python.org/mailman/listinfo/python-list- Hide quoted text - - Show quoted text -- Hide quoted text - - Show quoted text - Gary, That did the trick! I didn't realize that the way I initialized my lists would lead to the behavior that I observed. After doing something similar to what John had suggested I did indeed discover that I created an endless loop. I'm glad I learned something today. Thanks for your help. Samir Jul 21 '08 #6

 P: n/a Samir wrote: On Jul 21, 3:20 pm, Gary Herron Samir wrote: >>Hi Everyone,I am relatively new to Python so please forgive me for what seems likea basic question.Assume that I have a list, a, composed of nested lists with stringrepresentations of integers, such thata = [['1', '2'], ['3'], ['4', '5', '6'], ['7', '8', '9', '0']]I would like to convert this to a similar list, b, where the valuesare represented by integers, such asb = [[1, 2], [3], [4, 5, 6], [7, 8, 9, 0]]I have unsuccessfully tried the following code:n = []for k in a: n.append([int(v) for v in k])print nDoes anyone know what I am doing wrong?Thanks in advance.Samir--http://mail.python.org/mailman/listinfo/python-list You didn't tell us how it failed for you, so I can't guess what's wrong.However, your code works for me: > >>a = [['1', '2'], ['3'], ['4', '5', '6'], ['7', '8', '9', '0']]n = []for k in a: ... n.append([int(v) for v in k])... > >>print n [[1, 2], [3], [4, 5, 6], [7, 8, 9, 0]](Although you seem to have confused variables b and n.)Gary Herron- Hide quoted text -- Show quoted text - Hi Gary, Thanks for your quick response (and sorry about mixing up b and n). For some reason, the logic I posted seems to work ok while I'm using the Python shell, but when used in my code, the program just hangs. It never outputs the results. Below is the code in its entirety. Is there a problem with my indendentation? a = n = [] t = """ 1 2 3 4 5 6 7 8 9 0 """ d = t.split("\n") for x in range(1,len(d)-1): a.append(d[x].split(" ")) print a for k in a: n.append([int(v) for v in k]) print n Thanks again. Samir -- http://mail.python.org/mailman/listinfo/python-list I think this will work better, a sub-list comprehension of sorts: n = [[int(i) for i in k] for k in a] here is an ipython interactive session using it: In [1]: a = n = [] In [2]: t = """ ...: 1 2 ...: 3 ...: 4 5 6 ...: 7 8 9 0 ...: """ In [3]: In [4]: d = t.split("\n") In [5]: for x in range(1,len(d)-1): ...: a.append(d[x].split(" ")) ...: ...: In [6]: a Out[6]: [['1', '2'], ['3'], ['4', '5', '6'], ['7', '8', '9', '0']] In [7]: n = [[int(i) for i in k] for k in a] In [8]: n Out[8]: [[1, 2], [3], [4, 5, 6], [7, 8, 9, 0]] -- Andrew Jul 21 '08 #7

 P: n/a On Jul 21, 6:15*pm, Andrew Freeman Hi Everyone, >I am relatively new to Python so please forgive me for what seems likea basic question. >Assume that I have a list, a, composed of nested lists with stringrepresentations of integers, such that >a = [['1', '2'], ['3'], ['4', '5', '6'], ['7', '8', '9', '0']] >I would like to convert this to a similar list, b, where the valuesare represented by integers, such as >b = [[1, 2], [3], [4, 5, 6], [7, 8, 9, 0]] >I have unsuccessfully tried the following code: >n = []for k in a:* * n.append([int(v) for v in k])print n >Does anyone know what I am doing wrong? >Thanks in advance. >Samir--http://mail.python.org/mailman/listinfo/python-list You didn't tell us how it failed for you, so I can't guess what's wrong. However, your code works for me: *>>a = [['1', '2'], ['3'], ['4', '5', '6'], ['7', '8', '9', '0']] *>>n = [] *>>for k in a: ... * *n.append([int(v) for v in k]) ... *>>print n [[1, 2], [3], [4, 5, 6], [7, 8, 9, 0]] (Although you seem to have confused variables b and n.) Gary Herron- Hide quoted text - - Show quoted text - Hi Gary, Thanks for your quick response (and sorry about mixing up b and n). For some reason, the logic I posted seems to work ok while I'm using the Python shell, but when used in my code, the program just hangs. It never outputs the results. *Below is the code in its entirety. *Is there a problem with my indendentation? a = n = [] t = """ 1 2 3 4 5 6 7 8 9 0 """ d = t.split("\n") for x in range(1,len(d)-1): * * a.append(d[x].split(" ")) print a for k in a: * * n.append([int(v) for v in k]) print n Thanks again. Samir -- http://mail.python.org/mailman/listinfo/python-list I think this will work better, a sub-list comprehension of sorts: n = [[int(i) for i in k] for k in a] here is an ipython interactive session using it: In [1]: a = n = [] In [2]: t = """ * *...: 1 2 * *...: 3 * *...: 4 5 6 * *...: 7 8 9 0 * *...: """ In [3]: In [4]: d = t.split("\n") In [5]: for x in range(1,len(d)-1): * *...: * * a.append(d[x].split(" ")) * *...: * * * *...: * * In [6]: a Out[6]: [['1', '2'], ['3'], ['4', '5', '6'], ['7', '8', '9', '0']] In [7]: n = [[int(i) for i in k] for k in a] In [8]: n Out[8]: [[1, 2], [3], [4, 5, 6], [7, 8, 9, 0]] -- Andrew- Hide quoted text - - Show quoted text - Andrew, Thanks for the tip, though the syntax makes my head spin a bit in trying to comprehend it. For my small list, I didn't notice a discernible increase in speed, but I may have to try it with a larger list size. Incidentally, I had never heard of iPython but from their web site, it looks like an interesting tool. I'll have to check it out. Thanks. Samir Jul 21 '08 #8

 P: n/a Samir wrote: On Jul 21, 6:15 pm, Andrew Freeman Samir wrote: >>On Jul 21, 3:20 pm, Gary Herron I am relatively new to Python so please forgive me for what seems likea basic question.>Assume that I have a list, a, composed of nested lists with stringrepresentations of integers, such that>a = [['1', '2'], ['3'], ['4', '5', '6'], ['7', '8', '9', '0']]>I would like to convert this to a similar list, b, where the valuesare represented by integers, such as>b = [[1, 2], [3], [4, 5, 6], [7, 8, 9, 0]]>I have unsuccessfully tried the following code:>n = []for k in a: n.append([int(v) for v in k])print n>Does anyone know what I am doing wrong?>Thanks in advance.>Samir--http://mail.python.org/mailman/listinfo/python-list>You didn't tell us how it failed for you, so I can't guess what's wrong.However, your code works for me: >>a = [['1', '2'], ['3'], ['4', '5', '6'], ['7', '8', '9', '0']] >>n = [] >>for k in a:... n.append([int(v) for v in k])... >>print n[[1, 2], [3], [4, 5, 6], [7, 8, 9, 0]](Although you seem to have confused variables b and n.)Gary Herron- Hide quoted text -- Show quoted text -Hi Gary,Thanks for your quick response (and sorry about mixing up b and n).For some reason, the logic I posted seems to work ok while I'm usingthe Python shell, but when used in my code, the program just hangs.It never outputs the results. Below is the code in its entirety. Isthere a problem with my indendentation?a = n = []t = """1 234 5 67 8 9 0"""d = t.split("\n")for x in range(1,len(d)-1): a.append(d[x].split(" "))print afor k in a: n.append([int(v) for v in k])print nThanks again.Samir--http://mail.python.org/mailman/listinfo/python-list I think this will work better, a sub-list comprehension of sorts:n = [[int(i) for i in k] for k in a]here is an ipython interactive session using it:In [1]: a = n = []In [2]: t = """ ...: 1 2 ...: 3 ...: 4 5 6 ...: 7 8 9 0 ...: """In [3]:In [4]: d = t.split("\n")In [5]: for x in range(1,len(d)-1): ...: a.append(d[x].split(" ")) ...: ...:In [6]: aOut[6]: [['1', '2'], ['3'], ['4', '5', '6'], ['7', '8', '9', '0']]In [7]: n = [[int(i) for i in k] for k in a]In [8]: nOut[8]: [[1, 2], [3], [4, 5, 6], [7, 8, 9, 0]]--Andrew- Hide quoted text -- Show quoted text - Andrew, Thanks for the tip, though the syntax makes my head spin a bit in trying to comprehend it. For my small list, I didn't notice a discernible increase in speed, but I may have to try it with a larger list size. Incidentally, I had never heard of iPython but from their web site, it looks like an interesting tool. I'll have to check it out. Thanks. Samir -- http://mail.python.org/mailman/listinfo/python-list If it helps look at this: n = [[int(i) for i in k] for k in a] like this: n = [] for k in a: for i in k: n.append(int(i)) It is more verbose and easier to read and they both do exactly the same thing! iPython is great, to install it you might try easy_install: http://peak.telecommunity.com/DevCenter/EasyInstall Then in a command line type: easy_install ipython Then, once it is complete, to use iPython type: ipython -- Andrew Jul 21 '08 #9

 P: n/a Samir wrote: For my small list, I didn't notice a discernible increase in speed, but I may have to try it with a larger list size. About speed, and memory consumption: List comprehensions (http://docs.python.org/tut/node7.htm...00000000000000) are just shortcuts for for-loops. I do not believe there is any speed benefit. However, there are generators, they basically load one part of an iterator (a list in this case) at a time, this can greatly reduce memory usage. Have a look at PEP 289: http://www.python.org/dev/peps/pep-0289/ Here is the list comprehension as a generator (actually 2): n = ((int(i) for i in k) for k in a) Note, you can't just print a generator, it only computes something when needed: >>print n You can, however iterate over it: In [2]: for k in n: ....: for i in k: ....: print i, ....: ....: 1 2 3 4 5 6 7 8 9 0 In [3]: n = ((int(i) for i in k) for k in a) In [49]: list(n) Out[49]: [, , , ] Each sub-list is a generator too! In [50]: n = ((int(i) for i in k) for k in a) In [51]: for i in list(n): # list() converts the variable n to a list ....: list(i) ....: ....: Out[51]: [1, 2] Out[51]: [3] Out[51]: [4, 5, 6] Out[51]: [7, 8, 9, 0] This is only going to make a difference if you were dealing with a *very* large data set. I thought I would show you even if you never user them, for learning purposes. Note: a generator is one way, redefine it every time you use it. -- Andrew Jul 21 '08 #10

 P: n/a >>a = [['1', '2'], ['3'], ['4', '5', '6'], ['7', '8', '9', '0']] >>[map(int, i) for i in a] [[1, 2], [3], [4, 5, 6], [7, 8, 9, 0]] On Jul 21, 9:06*pm, Samir

 P: n/a n = [] for k in a: * * n.append([int(v) for v in k]) print n Does anyone know what I am doing wrong? Thanks in advance. Samir Use extend instead of append: * Append -add the one item to the end of the list * Extend -add the list of items to the end of the list Jul 22 '08 #12

 P: n/a Wow! Thanks for all of the great additional feedback and responses since I last checked in. The help this group provides is amazing. I'm glad I found it. @Andrew -- Thanks for the clarification on the nested for loop and how to intrepret it. Also, thanks for the information on generators. I have never come across this before so I will look into it a bit more. The same thing goes for iPython. I have been looking for an IDE that comes with better debugging capabilities that the default Python shell. @dusans -- Just when I thought I couldn't simplify my code anymore, you provide come up with another way. Thank you. @ptn -- Thanks for the explanation between "append" and "extend". I guess I've gotten lazy and have always used "append", apparently even in situations where it may not be appropriate. Thanks again, everyone! Samir Jul 22 '08 #13

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