I found this question on HackerRank and I am unable to understand the code(solution) that is displayed in the discussions page.
The question is:
Consider a list (list = []). You can perform the following commands:
insert i e: Insert integer at position .
print: Print the list.
remove e: Delete the first occurrence of integer .
append e: Insert integer at the end of the list.
sort: Sort the list.
pop: Pop the last element from the list.
reverse: Reverse the list.
Even though I have solved the problem using if-else, I do not understand how this code works:
n = input()
slist = []
for _ in range(n):
s = input().split()
cmd = s[0]
args = s[1:]
if cmd !="print":
cmd += "("+ ",".join(args) +")"
eval("slist."+cmd)
else:
print slist
Well, the code takes advantage of Python's eval function. Many languages have this feature: eval, short for "evaluate", takes a piece of text and executes it as if it were part of the program instead of just a piece of data fed to the program. This line:
s = input().split()
reads a line of input from the user and splits it into words based on whitespace, so if you type "insert 1 2", s is set to the list ["insert","1","2"]. That is then transformed by the following lines into "insert(1,2)", which is then appended to "slist." and passed to eval, resulting in the method call slist.insert(1,2) being executed. So basically, this code is taking advantage of the fact that Python already has methods to perform the required functions, that even happen to have the same names used in the problem. All it has to do is take the name and arguments from an input line and transform them into Python syntax. (The print option is special-cased since there is no method slist.print(); for that case it uses the global command: print slist.)
In real-world code, you should almost never use eval; it is a very dangerous feature, since it allows users of your application to potentially cause it to run any code they want. It's certainly one of the easier features for hackers to use to break into things.
It's dirty code that's abusing eval.
Basically, when you enter, for example, "remove 1", it creates some code that looks like sList.remove(1), then gives the created code to eval. This has Python interpret it.
This is probably the worst way you could solve this outside of coding competitions though. The use of eval is entirely unnecessary here.
Actually I Find some error in the code, but I came to an understanding of how this code runs. here is it:
input :
3
1 2 3
cmd = 1 + ( 2 + 3)
then eval(cmd) i.e., eval("1 + (2 + 3)") which gives an output 6
another input:
4
4 5 6 2
cmd = 4 + ( 5 + 6 + 2)
eval(cmd)
if __name__ == '__main__':
N = int(raw_input())
lst=[]
for _ in range(N):
cmd, *line = input().split()
ele= list(map(str,line))
if cmd in dir(lst):
exec('lst.'+cmd+'('+','.join(ele)+')')
elif cmd == 'print':
print(lst)
else:
print('wrong command', cmd)
Related
How to print variable name by input, Example:
a = 1
b = 2
what_variable = input('Which Variable?: ') #User for example introduces 'b'
Console: 2
You can write
print(globals()[what_variable])
but it's not a good approach. Use a dict instead
You can use exec:
var = input('Which Variable?: ')
exec("print(" + var + ")")
Output:
Which Variable?: b
2
>>
Just do the following:
print(eval(input('Which Variable?: ')))
You can also do
print(globals()[input('Which Variable?: ')])
While the other answers seem to address the obvious solution, it's not very 'Pythonic'. The main issues with these is, by far, safety. Let's say that your user inputs apiKey, and you happen to have a variable by that name... let's just say your bank statement is probably looking at a slight increase in magnitude. What most people in these answers don't realise is that using .globals()[input()] is no safer than eval(input()), because, shockingly, people store private info in variables. Alternatively, if it points to a method, e.g
a = print
b = os.system
eval(input())()
I could enter any function name there, and the damage would be done before the second () executes.
Why? Well, let's take a look at how exec and eval work (I won't go into the difference here, see this question for that). All they do is evaluate the string as Python code, and (simplifying here) return the value of the evaluation:
var1 = 3
print(eval("var1"))
# ====is equal to====
var1 = 3
print(var1)
(where var1 as a string obviously comes from the input typed in)
But if someone enters something malicious, this is essentially the basis of an SQL injection:
(where userInput is substituted by a user's input into an input())
userInput = "a + os.system('reboot now')"
print(eval(userInput))
# ====is equal to====
print(a + os.system('shutdown now')
and you suddenly find your computer's off.
Therefore, we'd either use a:
Dictionary (or object): x={a:1, b:2}, then do x[input()]
Array x=[1, 2], then do x[["a", "b"].index(input())]
Simply don't. Find a way to work around it. What's wrong with an if/else set? It's not good practise, because of the safety concerns outlined above. What most people seem to miss about dictionaries (or my array option) is that if you enter a malformed input (i.e not a or b), it would result in either uncaught errors being thrown, or undefineds being thrown around. And if you're going to do input validation, you're using an if statement anyway, so why not do it from the onset?
I want to use two for loop to iterate two large lists, however nohting happens.
Here is the code
pfam = open(r'D:\RPS_data\pfam_annotations.tbl', 'r')
number = []
description = []
for j in pfam:
number.append(j.split('\t')[0])
description.append(j.split('\t')[-2])
print len(number),len(description)
for j in number:
for r in description:
if j==r:
print 'ok'
else:
print 'not match'
The result is:
D:\Pathyon\pathyon2.7\python.exe "D:/Biopython/Pycharm/Python Learning/war_battle/AUL-prediction/trash.py"
16230 16230
Process finished with exit code 0
My question is to find the same fraction in number and description. It is obvious that python does not run the for loop. Is it because I get two large lists? Anyone knows how to overcome this?
I modified your script to run a test. This is what I did: I inserted a dummy text blob as an io.StringIO file, and printed the resulting columns.
import io
annotations=u"""
1024\tmiddle\tkay\tend
3192\tmiddle\tdiana\tend
8675309\tmiddle\tjenny\tend
""".strip()
#pfam = open(r'D:\RPS_data\pfam_annotations.tbl', 'r')
pfam = io.StringIO(annotations)
number = []
description = []
for j in pfam:
number.append(j.split('\t')[0])
description.append(j.split('\t')[-2])
print len(number),len(description)
print number, description
for j in number:
for r in description:
if j==r:
print 'ok'
else:
print 'not match'
When I run this, the output is:
$ python2.7 test.py
3 3
[u'1024', u'3192', u'8675309'] [u'kay', u'diana', u'jenny']
not match
I'll argue that (1) your initial code appears to be working fine; and (2) Python is, in fact, running the second loop.
I am suspicious of your second loop. You are comparing every number against every description, using a direct comparison (==). Those both seem to be possible sources of error.
For example, are you sure you want to compare a number from line 1 (1024) with a description from line 3 (jenny)?
For example, are you sure you want to compare using equality? Are you looking for directly-equals, or containment. A "description" might contain several words, one of which was the number in question.
How to pass a string to a variable if an index error is found? Consider the code:
for l1, l2 in zip(open('file1.list'), open ('file2.list')):
a=fasta1[int(l1)]
b=fasta2[int(l2)]
alignments = pairwise2.align.globalxx(a,b)
top_aln = alignments[0]
aln_a, aln_b, score, begin, end = top_aln
print aln_a+'\n'+aln_b
outfast1 = aln_a
outfast2 = aln_b
A number of these functions must be imported (pairwise2 align),
but the file.lists are single column text files with one sequence id (text and numbers) per line, that are used to extract from the fasta1 and fasta2 text files.
Basically, I want to try: each list command ( a=fasta1[int(l1)]) and if there is no error (the id is in range), do as normal (assign variables a and b for that iteration), but if NOT, assign the 'a' variable some placeholder text like 'GGG':
for l1, l2 in zip(open('file1.list'), open ('file2.list')):
try:
a=fasta1[int(l1)]
except IndexError,e:
a="GGG"
continue
try:
b=fasta2[int(l2)]
except (IndexError):
b="CCC"
continue
This code doesn't quite work (when integrated with above code), which isn't surprising given my lack of python prowess, but I don't quite know why. I actually get no text output, despite the print calls... Am I thinking about this right? If there is NO error in the index, I just want it to go on and do the pairwise alignment (with the first a and b variables) and then print some text to stdout.
Any ideas?
Python's conditional (aka ternary) expressions can one-line this for you. They're often criticized for lack of readability, but I think this example reads well enough.
a = fasta1[int(l1)] if int(l1) < len(fasta1) else "GGG"
You don't need continue, because it will skip that iteration of the loop. Consider the following:
for l1, l2 in zip(open('file1.list'), open ('file2.list')):
a = 'GGG'
b = 'CCC'
try:
a = fasta1[int(l1)]
b = fasta2[int(l2)]
except IndexError:
pass
Can I write the following code in single line in python?
t=int(input())
while t:
t-=1
n=int(input())
a=i=0
while not(n&1<<i):
i+=1
while n&1<<i:
n^=1<<i
a=a*2+1
i+=1
print(n^1<<i)+a/2
If not, How can I write this piece of code in minimum possible lines?(PS: I could reduce this in 6 lines, can it be any better)My Solutiont=int(input())
while t:
t-=1;n=int(input());a=i=0
while not(n&1<<i):i+=1
while n&1<<i:n^=1<<i;a=a*2+1;i+=1
print(n^1<<i)+a/2Thanks
Since pythons list comprehensions are turing complete and require no line breaks, any program can be written as a python oneliner.
If you enforce arbitrary restrictions (like "order of the statements" - what does that even mean? Execution order? First apperarance in sourcecode?), then the answer is: you can eliminate some linebreaks, but not all.
instead of
if x:
do_stuff()
you can do:
if x: do_stuff()
instead of
x = 23
y = 42
you can do:
x,y = 23, 42
and instead of
do_stuff()
do_more_stuff()
you can do
do_stuff; do_more_stuff()
And if you really, really have to, you can exec a multi-line python program in one line, so your program becomes something like:
exec('''t=int(input())\nwhile t:\n t-=1;n=int(input());a=i=0\n while not(n&1<<i):i+=1\n while n&1<<i:n^=1<<i;a=a*2+1;i+=1\n print(n^1<<i)+a/2\n''')
But if you do this in "real" code, e.g. not just for fun, kittens die.
It's not recommended to collapse lines in Python very often, because you lose Python's famous simplicity and clarity that way. And you often cannot collapse lines, because indentation levels are used to define block structure / nesting.
But if you really want a condensed version:
print "s0"
while True:
print "s1"; print "s2"
while True: print "s3"
while True: print "s4"; print "s5"; print "s6"
print "s7"
(Where your expressions have been replaced with True for simplicity`)
I want to print the result of the equation in my if statement if the input is a digit and print "any thing" if it is a letter.
I tried this code, but it's not working well. What is wrong here?
while 1:
print '\tConvert ciliuse to fehrenhit\n'
temp = input('\nEnter the temp in C \n\t')
f = ((9/5)*temp +32)
if temp.isdigit():
print f
elif temp == "quit" or temp == "q" :
break
elif temp.isalpha() :
print ' hhhhhhh '
You need to go through your code line by line and think about what type you expect each value to be. Python does not automatically convert between, for example, strings and integers, like some languages do, so it's important to keep types in mind.
Let's start with this line:
temp = input('\nEnter the temp in C \n\t')
If you look at the documentation for input(), input() actually calls eval() on what you type in in Python 2.x (which it looks like you're using). That means that it treats what you type in there as code to be evaluated, just the same as if you were typing it in the shell. So if you type 123, it will return an int; if you type 'abc', it will return a str; and if you type abc (and you haven't defined a variable abc), it will give you an error.
If you want to get what the user types in as a string, you should use raw_input() instead.
In the next line:
f = ((9/5)*temp +32)
it looks like you're expecting temp to be a number. But this doesn't make sense. This line gets executed no matter what temp is, and you're expecting both strings containing digits and strings containing letters as input. This line shouldn't go here.
Continuing on:
if temp.isdigit():
isdigit() is a string method, so here you're expecting temp to be a string. This is actually what it should be.
This branch of the if statement is where your equation should go, but for it to work, you will first have to convert temp to an integer, like this:
c = int(temp)
Also, to get your calculation to work out right, you should make the fraction you're multiplying by a floating-point number:
f = ((9/5.0)*c +32)
The rest of your code should be okay if you make the changes above.
A couple of things first - always use raw_input for user input instead of input. input will evaluate code, which is potentially dangerous.
while 1:
print "\tConvert ciliuse to fehrenhit\n"
temp = raw_input("\nEnter the temp in C \n\t")
if temp in ("quit", "q"):
break
try:
f = ((9.0 / 5.0) * float(temp) + 32)
except ValueError:
print "anything"
Instead of using isalpha to check if input is invalid, use a catch clause for ValueError, which is thrown when a non-numerical value is used.
Why isn't it working? Are you getting an error of any kind?
Straight away I can see one problem though. You are doing the calculation before you verify it as a number. Move the calculation to inside the if temp.isdigit().
Take a look at this for some examples:
http://wiki.python.org/moin/Powerful%20Python%20One-Liners
OK, this works. Only problem is when you quit, you get dumped out of the interpreter.
while 1: import sys; temp=raw_input('\nEnter the temp in C \n\t'); temp.isdigit() and sys.stdout.write('%lf' %((9./5)*float(temp)+32)) or temp=='q' and sys.exit(0) or sys.stdout.write(temp)