The use of .join() in this particular code

Question

The code is from an UVA. It goes like:

Consider a list (list = []). You can perform the following commands:

 insert, print, remove, append, sort, pop, reverse.

Initialize your list and read in the value of followed by lines of commands where each command will be of the types listed above. Iterate through each command in order and perform the corresponding operation on your list.

Inputs: The first line contains an integer, denoting the number of commands. Each line of the subsequent lines contains one of the following commands.

Sample Input

12
insert 0 5
insert 1 10
insert 0 6
print 
remove 6
append 9
append 1
sort 
print
pop
reverse
print

Sample Output

[6, 5, 10]
[1, 5, 9, 10]
[9, 5, 1]

A neat solution I found is:

n = input()
l = []
for _ in range(n):
    s = raw_input().split()
    cmd = s[0]
    args = s[1:]
    if cmd !="print":
        cmd += " ("+ ",".join(args) +") "
        eval("l."+cmd)
    else:
        print l

I don't really understand how the part " ("+ ",".join(args) +") " works. Especially why the + in the beginning and at the end. An explanation would be great. Thanks.

Also, you should avoid using `eval` If the source of the string is trusted, then it's fine, but it's likely not a good habit to get into. There are better ways to accomplish this. — juanpa.arrivillaga, Apr 01 '17 at 10:20
@juanpa.arrivillaga Could you please explain some of such ways? thanks. — Rio1210, Apr 01 '17 at 10:20
@juanpa.arrivillaga I think I get what the `+` signs at the end at start mean. They are adding different strings. I thought, somehow they were "inside" the function or something. — Rio1210, Apr 01 '17 at 10:30
I thought they were part of the " .join " or something. I am new to python and this format of functions `.something()`. — Rio1210, Apr 01 '17 at 10:33
If you don't understand code that this course is giving you, you should be reading the instructional materials that surely accompanied it. You could also read the documentation and Google for even more resources. Asking a question here should be your last resort, not your first. — TigerhawkT3, Apr 01 '17 at 10:42
@TigerhawkT3 I'll keep that in mind going forward. Thank you. — Rio1210, Apr 01 '17 at 10:43
@Rio1210 [here](https://gist.github.com/juanarrivillaga/ccccc7c2a15bf000692531d0dc950c4c) is a sketch of the approach that doesn't require `eval`. — juanpa.arrivillaga, Apr 01 '17 at 10:45
@juanpa.arrivillaga Thanks a lot for this. Learnt a lot from this code. — Rio1210, Apr 01 '17 at 11:08

Neil · Answer 1 · 2017-04-01T10:35:47.073

.split will divide your your string into an array. Each space will create a new element, so if you have "I like food", it will become ["I","like","food"]. Now we can look at our .join. The .join is doing the reverse. So if we were to call ','.join(["I","like","food"]), we'd get "I,like,food". In your function, we are using .join to combine a series of list elements into a series of string arguments, then we invoke eval and it executes.

And, eval is not recommended. So, instead, we can write a chain for if and elif statements:

inpu = """12
insert 0 5
insert 1 10
insert 0 6
print 
remove 6
append 9
append 1
sort 
print
pop
reverse
print"""

li = []

for line in inpu.split("\n"):
    args = line.split()
    if args[0] == "insert":
        li.insert(int(args[1]),int(args[2]))
    elif args[0] == "print":
        print(li)
    elif args[0] == "remove":
        li.remove(int(args[1]))
    elif args[0] == "sort":
        li.sort()
    elif args[0] == "reverse":
        li = li[::-1]
    elif args[0] == "append":
        li.append(int(args[1]))
    elif args[0] == "pop":
        li.pop()

The use of **.join()** in this particular code

1 Answers1

The use of .join() in this particular code