For loop and zip in python

Question

I have a code that I am trying to understand and I need help.

import numpy as np
Class_numbers=np.array(['a','b','c'])
students_per_class=np.array([10,20,30])
print("Students counts per class:\n{}".format(
{x: y for x, y in zip(Class_numbers, students_per_class)}))

output:

Students counts per class:
{'a': 10, 'b': 20, 'c': 30}

What I understand: 1- we use {} and .format(...) to replace {} with ...

Here are my questions:

Q1- I do not understand "for x, y in zip(Class_numbers, students_per_class)". Is it like a 2d for loop? why we need the zip? Can we have 2d loop with out zip function?

Q2-I am not understanding how x:y works! the compile understand automatically that the definition of x and y (in "x:y") is described in the rest of the line(e.g. for loop)?

P.S: I am expert in MATLAB but I am new to python and it is sometimes very confusing!

Ehsan

self-proclaimed experts usually claim they know more than they do — eagle
– eagle, Commented Apr 11, 2018 at 20:04

Freddy · Accepted Answer · 2018-04-11 20:11:58Z

25

zip allows you to iterate two lists at the same time, so, for example, the following code

letters = ['a', 'b', 'c']
numbers = [1, 2, 3]

for letter, number in zip(letters, numbers):
    print(f'{letter} -> {number}')

gives you as output

a -> 1
b -> 2
c -> 3

when you iterate with zip, you generate two vars, corresponding to the current value of each loop.

answered Apr 11, 2018 at 20:11

Freddy

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Comments

Dric512 · Accepted Answer · 2018-04-11 20:09:42Z

8

Q1: zip is used to merge 2 lists together. It returns the first element of each list, then 2nd element of each list, etc. This is a trick to consider the two lists as key and data to create a dictionary.

Q2: this is a dictionary (hash), using a method called "dict comprehension". It creates the dictionary shown as the output. If assigned to a variable d, d['a'] = 10, etc.

answered Apr 11, 2018 at 20:09

Dric512

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Comments

sai_subramani · Accepted Answer · 2021-02-02 10:12:18Z

2

zip is useful to enumerate the elements in the 2 different list and make a single list

example :

 x: y for x, y in zip(Class_numbers, students_per_class)

explanation:

the element form Class_number is assigned to x which is a "Key" for the dictionary and the students_per_class is assigned to y which is a "value
until the loop end it will iterate to dictionary.

answered Feb 2, 2021 at 10:12

sai_subramani

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Comments

shahaf · Accepted Answer · 2018-04-11 20:12:06Z

0

just print the line you don't understand to make it clearer...

first we define 2 arrays

Class_numbers=np.array(['a','b','c'])
['a','b','c']

students_per_class=np.array([10,20,30])
[10,20,30]

zip just add them together like

[('a', 10), ('b', 20), ('c', 30)]

now we iterate on each element with x and y and create a dict object (hash map) { key : value}

{'a': 10, 'c': 30, 'b': 20}

answered Apr 11, 2018 at 20:12

shahaf

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Comments

oxal · Accepted Answer · 2021-08-06 05:38:50Z

0

there is a trick in using zip:

letters = ('A', 'B', 'C')
numbers = ('1', '2', '3')
let_num = zip(letters, numbers)

use the list() function to display zip:

print(list(let_num))

above code will produce

[('A', '1'), ('B', '2'), ('C', '3')]

please note that, if you want to print again, this will not print anything since we already consumed the zip

print(list(let_num))

edited Aug 6, 2021 at 5:38

answered Aug 6, 2021 at 5:27

oxal

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