CODEWITHSUNDEEP
pythonlistloops
Droid-Bird
Droid-Bird

Reputation: 1505

How to loop through two lists and work on the match?

I have two lists. I want to be able to iterate using for loops, and want to do some operation on the element based on matched element. I cannot figure out what going wrong here,

lst1 = ["a1", "b1", "c1", "d1", "e1", "f1", "g1"] 
lst2 = ["c1", "d1", "e1"]

for l in lst1:
    for m in lst2:
        if l == m:
            v = l + "__match_found"
        else:
            v = l + "__no_match"
    print(v)

I am getting following output:

a1__no_match
b1__no_match
c1__no_match
d1__no_match
e1__match_found
f1__no_match
g1__no_match

I am expecting that each element should only be printed once, either with match, or no_match the end.

Upvotes: 1

Views: 2929

Answers (2)

not_speshal
not_speshal

Reputation: 23146

In your code, when l is c1, in each iteration of your inner loop:

  • m is c1: v = l + "__match_found"
  • m is d1: v = l + "__no_match"
  • m is e1: v = l + "__no_match"

You then print only the final value of v.

Instead, when you find a match, you should be using break to dis-continue the inner loop:

for l in lst1:
    for m in lst2:
        if l == m:
            v = l + "__match_found"
            break
        else:
            v = l + "__no_match"        
    print(v)

A cleaner way to do this would be using in:

for l in lst1:
    if l in lst2:
        print (l + "__match_found")
    else:
        print (l + "__no_match")

Upvotes: 1

Mad Physicist
Mad Physicist

Reputation: 114330

Since you print v in the outer loop, you will get whatever the last value of m assigned to it. This is not what you want: you want to break out of the inner loop as soon as you find a match:

for l in lst1:
    v = l + "__no_match"
    for m in lst2:
        if l == m:
            v = l + "__match_found"
            break
    print(v)

Now let's simplify the and make it more pythonic. First, the inner loop is just a check for containment. In python, that's usually done with the in operator:

for l in lst1:
    v = "__match_found" if l in lst2 else "__no_match"
    print(l + v)

Now if your only goal is to check for containment, you are much better off using set than list as your container. For example:

s1 = set(lst1)
s2 = set(lst2)

for i in s1 & s2:
    print(i + '__match_found')
for i in s1 - s2:
    print(i + '__no_match')

These operations are done with hashables, and are much faster than the linear lookup of lists.

Upvotes: 1

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