Function of adding two lists in Python

Question

whenever I run following code of adding two lists in python below mentioned error appears

    def add_lists(L1, L2):
        R = []
        for i in range(0, len(L1)):
            R.append(L1[i]+L2[i])
        return R

    L2 = [3, 3, 3, 3]
    L1 = [1, 2, 3, 4]
    add_lists(L1, L2)
    print("Resultant list of: ", str(L1), '& ' + str(L2), 'is', R)

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this code yields NameError: name 'R' is not defined

Answer 1

The variable R is local to your function and so is not accessible to your print statement. (Generally, this is good. It makes the function self-contained and avoids dependencies on what global variables may or may not exist )

To print the result of the function, assign the result to an in-scope variable and use that.

def add_lists(L1, L2):
    R = []
    for i in range(0, len(L1)):
        R.append(L1[i]+L2[i])
    return R

L2 = [3, 3, 3, 3]
L1 = [1, 2, 3, 4]
res = add_lists(L1, L2)  # assigns the result of the function call to a variable we can access
print("Resultant list of: ", str(L1), '& ' + str(L2), 'is', res)

Answer 2

You defined R in a local scope "it looks like inside a function" while you are trying to use it outside of that scope in the last line.

Try moving the initialization of "R = []" to the outer scope.

Answer 3

its because R = [] is inside the function and print is outside the function... so try this:

R = []
def add_lists(L1, L2):
        
        for i in range(0, len(L1)):
            R.append(L1[i]+L2[i])
        return R

L2 = [3, 3, 3, 3]
L1 = [1, 2, 3, 4]
add_lists(L1, L2)
print("Resultant list of: ", str(L1), '& ' + str(L2), 'is', R)