Unable to modify values in 2-dimensional matrix in python

Question

I am trying to modify data in a 2 dimensional matrix (represented by a sequence of smaller sequences) in python. The matrix values are first initialized to "0.0" (shown as "current" in code below). I have a second 2 dimensional matrix that contains the actual data points, however, these data points are represented by ".", "C", and "H". I have written the code below to basically translate these characters into the proper values, but I'm running into an error "str object does not support item assignment". I know strings are immutable, but I'm not changing individual characters of a string, I'm removing the whole string. Here is the code:

for rline in range(len(lineAppend)):
    for cline in range(len(lineAppend)):
        if lineAppend[rline][cline]==".":
            #print "is dot"
            lineAppend[rline][cline] == 0.0
            #lineAppend.remove(".")
            #ineAppend.insert("0.0")
        elif lineAppend[rline][cline]=="C":
            lineAppend[rline][cline] == 25.0
        elif lineAppend[rline][cline]=="H":
            lineAppend[rline][cline] == 100.0

lineAppend is a matrix that is filled by reading a text file. It simply reads each line and saves it as a sequence. I am trying to change individual indexes to the above values, but I keep running into the error that occurs because strings are immutable. Is there any other way to modify the data to the desired values?

Here is the traceback:

    Traceback (most recent call last):
    File "/home/Desktop/python_projects/temp.py", line 172, in <module>
main()
    File "/home/Desktop/python_projects/temp.py", line 169, in main
read_config("plate.txt")
    File "/home/Desktop/python_projects/temp.py", line 110, in read_config
    if lineAppend[rline][cline]==".":

TypeError: list indices must be integers, not str

There is no assignment taking place here; can you show us the full traceback? — Martijn Pieters
– Martijn Pieters, Commented Oct 18, 2013 at 17:48

Martijn Pieters · Accepted Answer · 2013-10-18 17:52:29Z

You are mixing equality tests with assignment:

if lineAppend[rline][cline]==".":
    lineAppend[rline][cline] == 0.0

Note how both lines use == to test for equality. Assignment, on the other hand, uses one =:

if lineAppend[rline][cline] == ".":
    lineAppend[rline][cline] = 0.0

You are doing this throughout your code; if you want to assign, use a single equals symbol.

Also, to loop over your matrix, you can loop directly. You can include an index when you need to assign to the inner lists:

for rline in lineAppend:
    for i, cline in enumerate(rline):
        if cline == ".":
           rline[i] = 0.0
        elif cline == "C":
           rline[i] = 25.0
        elif cline == "H":
           rline[i] = 100.0

If you were to use a mapping, the whole nested loop can be replaced by a list comprehension:

replacements = {'.': 0.0, 'C': 25.0, 'H': 100.0}
lineAppend = [[replacements.get(v, v) for v in rline] for rline in lineAppend]

thefourtheye · Accepted Answer · 2013-10-18 17:46:34Z

1

lineAppend[rline][cline] == 0.0

is not assigning value, but it is simply comparing. Change it to

lineAppend[rline][cline] = 0.0

answered Oct 18, 2013 at 17:46

thefourtheye

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3 Comments

Jeremy Fisher Over a year ago

When I change it to an assignment operator I still get the "str does not support item assignment" error

Martijn Pieters Over a year ago

@JeremyFisher: You'll need to show us the full traceback; your code, as posted, doesn't assign anything so it cannot be the cause of the exception.

Jeremy Fisher Over a year ago

I just added the traceback. Even after changing the double equal to single equal I still keep getting the error, so does that mean it could be an assignment error?

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