Why my code cannot find maximum number in string of numbers (space separated)

Question

I gave this string to my python code from which I wanted to find the maximum.

>>> a = ("4 5 29 54 4 0 -214 542 -64 1 -3 6 -6")
>>> a = a.split(" ")
>>> max(a)
>>> 6

Can someone please explain me why it is not returning 542? At last, I found the correct code:

>>> a = a.split(" ");
>>> a = [int(i) for i in a];
>>> return str(max(a))

But could someone please teel me my foolishness in the former code.

After the split, `a` will still be a list of strings. Map it to a list of ints before calling `max` — sshashank124, May 17 '18 at 14:36
Try printing out the value of `a` after the split line and you'll see — sshashank124, May 17 '18 at 14:37
**incorrect** copy paste in your question. `>>> max(a)` in the REPL will output `'6'`, not `>>> 6`! And that should serve as a hint :) — Antti Haapala -- Слава Україні, May 17 '18 at 14:50

score 2 · Answer 1 · answered May 17 '18 at 14:53

2

You can also find the maximum string as if interpreted as an integer by passing a key

max(a.split(), key=int)

'542'

And make it an int afterwards

int(max(a.split(), key=int))

542

answered May 17 '18 at 14:53

piRSquared

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That's nice. Do you know why it is faster to use `key=float`? `%timeit max(a.split(), key=int)` gives `100000 loops, best of 3: 4.68 µs per loop` and `%timeit max(a.split(), key=float)` gives `100000 loops, best of 3: 2.43 µs per loop`. – Cleb May 17 '18 at 15:06
I don't know. Interesting though. – piRSquared May 17 '18 at 15:07
Seems to hold also for much larger lists. – Cleb May 17 '18 at 15:11
Python 3.6, can't see much difference really. 3.77 vs 3.67... that one looks strange – Antti Haapala -- Слава Україні May 17 '18 at 15:16
Seems to be a base ambiguity issue: https://stackoverflow.com/questions/21232733/why-is-float-faster-than-int – sshashank124 May 17 '18 at 15:17

Attersson · Answer 2 · 2018-05-17T15:01:43.633

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max([int(x) for x in a.split()])

This will convert a into a list of ints and find the correct max. Strings are compared char by char, comparing charcodes.

max(int(x) for x in a.split())

The above works too, without creating a list, as a generator variant.

edited May 17 '18 at 15:01

answered May 17 '18 at 14:38

Attersson

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You don't need the list comprehension with `[` and `]`. `max` can directly take a generator expression – sshashank124 May 17 '18 at 14:39
Alternatively, `max(map(int, a))` works for Python 3.x & 2.7, and is lazy in 3.x. – jpp May 17 '18 at 14:40
@sshashank124 Thanks for the comment, I edited my answer – Attersson May 17 '18 at 14:41
`max` does not do double generation. could you please provide a source on that? – sshashank124 May 17 '18 at 14:45
@sshashank124 Apparently I can't find any. I had in mind a similar case where I read that generators indeed double up the number of generations. There was a popular StackOverflow entry about that. However this might have been a similar case. In the meantime I will definitely amend this answer. – Attersson May 17 '18 at 14:55
I'm not claiming that it definitely doesn't but I tried searching it and skimming the src code and I couldn't find it. I think it would be safer to keep it out of the discussion in the answer itself haha. – sshashank124 May 17 '18 at 14:58
@sshashank124 the gen expr is very slightly slower. `test_list = list(np.random.randint(0, 500, 100000).astype(str))`. From `timeit` - list: `29.7 ms ± 375 µs per loop (mean ± std. dev. of 7 runs, 10 loops each)`, gen expr: `32.7 ms ± 593 µs per loop (mean ± std. dev. of 7 runs, 10 loops each)` – roganjosh May 17 '18 at 15:01
@Cleb added a.split(). roganjosh thanks for the benchmark (anyway OP already wrote `a= a.split()` but it is ok to specify) – Attersson May 17 '18 at 15:03

score 0 · Answer 3 · answered May 18 '18 at 06:44

0

max(map(int, a.split(" ")))
# 542

answered May 18 '18 at 06:44

Amitkumar Karnik

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Why my code cannot find maximum number in string of numbers (space separated)

3 Answers3