如何在列表中打印字符串及其索引在同一行？

Question

This probably has a very simple solution, but I'm just a beginner.

What I have is:

def content(L):  
    for i in range(len(L)):  
        print (i), (L[i])

Right now it only prints the index and not the string. I'm not entirely sure what the issue is, but when I switch the ordering in the last line so that (L[i]) comes first then it prints the string but not the index. I want to print both on the same line.

Answer 1

In Python 3.x, print is not a statement as in Python 2.x, but rather a function. You can use it like this:

print(i, L[i])

Additionally, enumerate is great for what you're trying to do:

for i, c in enumerate(L):
    print(i, c)

Note that you can use print as a Python 3.x-style function in Python 2 if you import it from the __future__ package (for Python 2.6 and up):

from __future__ import print_function

Answer 2

In Python 3.x, print is a function. So, you have to pass both the values as parameters like this

print (i, L[i])

When you say

print (i), (L[i])

Python 3.x, accepts i as the parameter to print and considers (L[i]) as a separate expression and evaluates it, but that doesn't get passed to the print function. That is why always the first element is getting printed.

Apart from that, your entire statement gets evaluated to the following tuple

(None, L[i])

To understand that, try something like this, in the Python REPL

>>> print (1), (2)
1
(None, 2)

When the inner print with 1 getting executed, it will return None and the next element is 2 . In Python, when you write two expressions separated by comma, in parenthesis, they will become a tuple.

You can confirm the same with Python's generated bytecodes, like this

from dis import dis
code1 = compile("print (1), (2)", "<string>", "exec")
dis(code1)

Output

  1           0 LOAD_NAME                0 (print) 
              3 LOAD_CONST               0 (1) 
              6 CALL_FUNCTION            1 (1 positional, 0 keyword pair) 
              9 LOAD_CONST               1 (2) 
             12 BUILD_TUPLE              2 
             15 POP_TOP              
             16 LOAD_CONST               2 (None) 
             19 RETURN_VALUE         

BUILD_TUPLE corresponds to the tuple construction and as we don't use it, it gets discarded immediately.

Answer 3

def content(L):  
    for ind, val in enumerate(L):  
        print (ind, val)

Answer 4

As others said, print is now a function, so what python is doing is interpreting the line: print(i), (L[i]) as a tuple. Do print(i, L[i]) instead.

You'll also want to use enumerate:

for i,line in enumerate(L):
    print(i, line)