max() function with for loops

Question

I am currently studying this Job Shop Problem from Google OR-Tools and I need your help to understand this one,

jobs_data = [  # task = (machine_id, processing_time).
        [(0, 3), (1, 2), (2, 2)],  # Job0
        [(0, 2), (2, 1), (1, 4)],  # Job1
        [(1, 4), (2, 3)]  # Job2
    ]

    machines_count = 1 + max(task[0] for job in jobs_data for task in job)
    all_machines = range(machines_count)

I want to understand this line:

machines_count = 1 + max(task[0] for job in jobs_data for task in job)

Thanks.

Possible duplicate of [Double Iteration in List Comprehension](https://stackoverflow.com/questions/1198777/double-iteration-in-list-comprehension) — Shishdem, Oct 09 '19 at 07:51
Dupe: [python list comprehension double for](//stackoverflow.com/q/17657720) — Aran-Fey, Oct 09 '19 at 07:53
"take the maximum of the values taken from `task[0]` for each `job in jobs_data` and for each `task in job`" what's hard to comprehend? — Giacomo Alzetta, Oct 09 '19 at 07:53
Possible duplicate of [python list comprehension double for](https://stackoverflow.com/questions/17657720/python-list-comprehension-double-for) — Giacomo Alzetta, Oct 09 '19 at 07:53

score 3 · Accepted Answer · answered Oct 09 '19 at 07:55

task[0] for job in jobs_data for task in job can be transferred into the following

new_list = []

for job in jobs_data: # for each Job
    for task in job: # for each task 
        print(task[0]) 
        new_list.append(task[0]) # get the id

max() just select the maximum.

score 0 · Answer 2 · answered Oct 09 '19 at 07:59

The line you are referring to can be translated into the following (hopefully more understandable) code:

count = []
for job in jobs_data:          #job is a list of tuples
    for task in job:           #task is one tuple
        count.append(task[0])  #task[0] is the first item of the tuple

machines_count = 1 + max(count)

max() function with for loops

2 Answers2