Help with this code

[José Manuel Suárez Sierra]

El lunes, 9 de enero de 2017, 14:09:09 (UTC+1), José Manuel Suárez Sierra  escribió:
> Hello, I am trying to make a code wich compares between 2 or several sequences (lists). It compares every element in a list with another list elements. For example, if we have a list_a=["a","b","c","d"] and list_b=["a","b"] I want to obtain a new list_c containing elements that match between these lists (a and b here), but, if for instance list_b were ["a","c"] the program must not store this data because they are not in same order.
> 
> Said this, I wrote this code but it doesnt work:
> 
> if __name__ == "__main__":
> 
> 
>     def compare(a, b):
> 
>         i = 0
>         j = 0
>         c1 = []
> 
>         while a[i] == b[j]:
>             c1.append(a[i])
>                 j = j+1
>                 i=i+1
> 
>         return c1
> 
> 
> 
> 
>     cadena_1=raw_input("Introduce list 1 \n")
>     cadena_2 = raw_input("Introduce list 2 \n")
> 
> 
>     transf1=list(cad_1) 
>     transf2 = list(cad_2)
> 
> 
>     print compare(transf1,transf2)
> 
> 
> 
> 
> Thank you

Thanks for your reply,
I wrote this code:
    def comparar(cad1, cad2):
        i = 0
        j = 0
        cad3 = []
        k = True

        for cad1[i] in cad1:
            k = True
            for cad2[j] in cad2:
                while cad1[i] == cad2[j] and i<= len(cad1) and j <= len(cad2):
                    cad3.append(cad1[i])
                    i = i + 1
                    j = j + 1

                    if cad1[i] != cad2[j]:
                        k = False
            i = i + 1
        return cad3

This is a function, another important thing is the order of elements, it must be ordered as it appears in one of the lists, in your example, my function must return a list with elements 1,2,5 in that order.

I cant find my mistake in these function I wrote you above.

Thank you very much