How to use a string as a name of array in python, so that this can be used in for-loops

Question

Let's say

t1=loadtxt("t\\1.txt")
t2=loadtxt("t\\2.txt")

Assume inside 1.txt is

1 2 3
4 5 6

Assume also inside 2.txt is

1 2 3
4 5 6

#

t1[0,0]=1
t1[0,1]=2
t1[0,2]=3
t2[0,0]=1
t2[0,1]=2
t2[0,2]=3
st=0


for i in range(2):
    for j in range(3):
        a='t'+str(i+1)

        st=st+a

I got "TypeError: string indices must be integers"

What I want this piece of code to do is st=t1[0,0]+t1[0,1]+t1[0,2]+t2[0,0]+t2[0,1]+t2[0,2]

Then how to fulfill this goal? How to represent the array name with regular numbers in for loop? Instead of summing each value one by one.

Your code example does not include what t1 and t2 are. t1[0, 0]=1 tries to assign the value 1 to the index (0, 0) of t1. But what is t1? — Viktor Fougstedt, Dec 11 '15 at 15:10
Yes, actually t1=loadtxt("t\1.txt") , similar for t2. I have added this part in the problem statement. — user3737702, Dec 11 '15 at 15:16

Philip Feldmann · Accepted Answer · 2015-12-11T15:48:00.190

2

Well you could either use a dictionary with all your t's or use the exec built in function. Python 2.x looks like this:

t1 = [1,2,3]
t2 = [1,2,3]
st=0

for i in range(2):
    for j in range(3):
        exec "a = t"+str(i+1)+"["+str(j)+"]"
        st+=a
print st

In Python 3.x you want to use () for the built in functions. Makes the code look like this:

t1 = [1,2,3]
t2 = [1,2,3]
st=0

    for i in range(2):
        for j in range(3):
            exec("a = t"+str(i+1)+"["+str(j)+"]")
            st+=a
    print(st)

The exec function will take a string and act like it was a normal line of code and execute it. This way you can assemble your command and let the interpreter do the rest. (Using String Formatting instead of connecting the string with + might be a better approach though)

edited Dec 11 '15 at 15:48

answered Dec 11 '15 at 15:14

Philip Feldmann

7,887
6
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But when I run your code, it tells me that " exec "a = t"+str(i+1) ^ SyntaxError: invalid syntax" – user3737702 Dec 11 '15 at 15:24
Obviously, because I was giving an example assuming you are using normal variables, not tables like you actually do. I updated my example to fit your a code using tables. – Philip Feldmann Dec 11 '15 at 15:33
Somehow the same warning appeared again, " exec "a = t"+str(i+1)+"["+str(j)+"]" ^ SyntaxError: invalid syntax" Is it because the version of python, I am using python 3.4.0 – user3737702 Dec 11 '15 at 15:45
Sorry I was assuming you are using pythong 2.x. Python 3.x uses () for all built in functions like print and exec. Just to make it really clear I edited my post for 2.x and 3.x . Hope this helps. – Philip Feldmann Dec 11 '15 at 15:49
Wonderful solution!! Although a bit slow, I can have a good sleep tonight~ – user3737702 Dec 11 '15 at 15:51

score 1 · Answer 2 · answered Dec 11 '15 at 15:14

1

There are ways to do what you want (see link in the comment to your question), but why bother when you can simply do:

st = 0
for t in (t1,t2):
    for i in range(3):
        st += t[0,i]

answered Dec 11 '15 at 15:14

soungalo

1,106
2
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oops, open my mind, but what if I have t1, t2.... t100? – user3737702 Dec 11 '15 at 15:19
I think in this case it would be easier to just store the t's as dictionary keys rather than variables. – soungalo Dec 11 '15 at 15:20
I will learn more later~ – user3737702 Dec 11 '15 at 15:49

How to use a string as a name of array in python, so that this can be used in for-loops

2 Answers2