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I'm trying to understand how functional programming languages work, and I decided to take the approach of program in a functional way in python, as it is the language in which I feel more comfortable.

I'm trying to, given an array of arrays and a function with 2 params, get two sentences with the two elements in each array.

I cannot figure out how to do it without nesting lambdas, but even using them:

def sentence( x, y):
    return " this string contains %s and %s" % (x,y)
matrix = [['a','b'],['c','d']]

output = map(lambda a: map(lambda b: map(lambda c,d: sentence(c,d),b),a),matrix)

Of course, because a I am a old fashioned imperative programmer, I try to get the output with the good old for loop. Sure there's a better way but...

#print(output)
for i in output:
    #print(i)
    for j in i:
        #print(j)
        for k in j:
            print(k)

At the end I only get this result:

  File "fp.py", line 12, in <module>
    for k in j:
TypeError: <lambda>() missing 1 required positional argument: 'd'

So yes, I guess I'm doing something wrong passing the values to the function, but I cannot guess why.

Any ideas?

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3
  • Can you show what your input and output are supposed to look like? I am having trouble seeing exactly what you want. Commented Jan 12, 2018 at 21:19
  • If your aim is education, consider giving coconut-lan.org a try. Also take a look at toolz. Commented Jan 12, 2018 at 21:22
  • Probably my fault, as I'm not a English native speaker. The input is the matrix variable, containing two arrays a,b and c,d. The output should be an array with two strings , " this string contains a and b" and " this string contains c and d". Commented Jan 12, 2018 at 21:23

5 Answers 5

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4
  1. Python style guide recommends using list comprehensions instead of map/reduce
  2. String formatting using percent operator is obsolete, consider using format() method

  3. the code you need is this simple one-liner

    output = [" this string contains {} and {}".format(x, y) for (x, y) in matrix]

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Comments

2

You have a couple of issues, these structures aren't nested deeply enough to warrant the nested loops.

You need 1 map for each level of list you wish to process, so if you want to process a list, you need a map, if you want to process a list of lists, you need 2 and so on.

In this case you most likely only want to process the top level (effectively this is because you want each list in the top level to become a sentence).

def sentence( x, y):
return " this string contains %s and %s" % (x,y)
matrix = [['a','b'],['c','d']]

output = map(lambda a: sentence(a[0],a[1]), matrix)

# Print the top level
for i in output:
    print(i)
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2 Comments

In Python 2, you can even unpack the tuple in place: lambda (a0, a1): sentence(a0, a1). Now it's obvious that you just need to uncurry the call, and eliminate the lambda, calling sentence(*a); itertools.stramap does this.
Wow, simple and straightforward. Thank you!
1

You do it too difficult

def sentence( x, y):
    return " this string contains %s and %s" % (x,y)
matrix = [['a','b'],['c','d']]
# c will get sublists consequently
output = [sentence(*c)  for c in matrix]
print(output) # [' this string contains a and b', ' this string contains c and d']

You can avoid for in code above by 

output = list(map(lambda c: sentence(*c), matrix))
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1 Comment

It is deliberated, I'm forcing myself to avoid for and while loops, as I want to learn and practice pure functional programming workflows.
0

Your last lambda will not receive two arguments, because it's fed the elements of b. It must receive one argument.

Such a ladder of lambdas does not look nice in any language. With named functions, it's much more readable.

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Comments

-2

You don't need lambdas at all, but you do need to unpack your arguments, which has changed in recent versions of python. The code below will work in older and newer versions both:

def sentence(x_y):
    x, y = x_y
    return "Here is a sentence with {} and {}".format(x, y)

matrix = [['a', 'b'], ['c', 'd']]
output = list(map(sentence, matrix))
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