Change a specific element of a list in python?

Question

I have three lists inside a list. I want to change a specific element of a specific list. So lets say the second list, first element. I tried it like this:

Here is the whole thing, I am just learning how to program so don't judge me :)

class Board:
#creates visible playing board
global rows
rows = []
def create_row(self):
    row1 = []
    while len(row1) <= 4:
        if len(row1) <= 3:
            row1.append("x")
        else:
            row1.append("\n")
    rows.append(row1)

def input(self):
    print "Enter a colum num"
    height = int(raw_input(">"))
    height = height - 1
    print "Enter a row num"
    width = raw_input(">")
    print rows.__class__
   # for i in rows[height]:
       # i[height] = 'a'




def display(self):
    for i in rows:
        for m in i:
            print m,



row1 = Board()
row1.create_row()

row2 = Board()
row2.create_row()

row3 = Board()
row3.create_row()

row4 = Board()
row4.create_row()

row4.display()
row4.input()
row4.display()

Can you show us what list you're using? The error is because you're trying to assign to an object that can't be changed. — Rafe Kettler
– Rafe Kettler, Commented Apr 17, 2011 at 1:21
rows is unlikely to be the problem. @bipolarpants, please show the whole traceback, which should confirm that the error is in the line i[height] = 'a'. Before that line, insert print type(i), repr(i) and show us what the result is. — John Machin
– John Machin, Commented Apr 17, 2011 at 4:01

Hugh Bothwell · Accepted Answer · 2011-04-17 20:12:40Z

2

Trace this through - assume we enter '3' when prompted, so height = 2; then

for i in rows[2]:

the first value of i is 'x'

    'x'[2] = 'a'

and this gives you your error - you cannot change a character in the string because strings are immutable (instead you have to construct a new string and replace the old one).

Try the following instead:

def getInt(msg):
    return int(raw_input(msg))

row = getInt('Which row? ')
col = getInt('Which col? ')

rows[row-1][col] = 'a'

Try the following instead:

def getInt(msg, lo=None, hi=None):
    while True:
        try:
            val = int(raw_input(msg))
            if lo is None or lo <= val:
                if hi is None or val <= hi:
                    return val
        except ValueError:
            pass

class GameBoard(object):
    def __init__(self, width=4, height=4):
        self.width  = width
        self.height = height
        self.rows   = [['x']*width for _ in xrange(height)]

    def getMove(self):
        row = getInt('Please enter row (1-{0}): '.format(self.height), 1, self.height)
        col = getInt('Please enter column (1-{0}): '.format(self.width), 1, self.width)
        return row,col

    def move(self, row, col, newch='a'):
        if 1 <= row <= self.height and 1 <= col <= self.width:
            self.rows[row-1][col-1] = newch

    def __str__(self):
        return '\n'.join(' '.join(row) for row in self.rows)

def main():
    bd = GameBoard()

    row,col = bd.getMove()
    bd.move(row,col)

    print bd

if __name__=="__main__":
    main()

edited Apr 17, 2011 at 20:12

answered Apr 17, 2011 at 3:59

Hugh Bothwell

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2 Comments

John Machin Over a year ago

"This gives you your error": The CPython error message changed in 2.5 from the fixed "object does not support item assignment" to variable -- in this case "'str' object does not ...". The OP says he got "can't assign to immutable object". How do you account for that?

Greg Hewgill Over a year ago

@John Machin: I don't think we know which version of which implementation the OP is using. For example, Jython contains the original error message.

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