I have a numpy array of numbers like this:cols = np.arange(1, 6). I want to append the letter 't' in front of every number in cols. I write the followind loc:
f = lambda x: 't' + str(x)
temp = f(cols)
print(temp)
I get an output like this:
t[1 2 3 4 5].
I need the output to be ['t1', 't2', 't3'...]. I need to do it for 1000s of numbers. What am I doing wrong?
You can use np.core.char.add:
np.core.char.add('t', np.arange(1,6).astype(str))
#array(['t1', 't2', 't3', 't4', 't5'],
# dtype='|S12')
It's faster then list(map(...)) for large arrays:
%timeit np.core.char.add('t', np.arange(1,100000).astype(str))
# 10 loops, best of 3: 31.7 ms per loop
f = lambda x: 't' + str(x)
%timeit list(map(f, np.arange(1,100000).astype(str)))
# 10 loops, best of 3: 38.8 ms per loop
82.6ms for np.core.char.add while 163ms for list(map(...)).You can to do that with list comprehension:
['t' + str(x) for x in cols]
['t1', 't2', 't3', 't4', 't5', 't6']
This will append 't' for every element x in collection cols.
lambda function is an anonymous function, so you usually pass it into functions that expects a callable object.
Your problem is that your applying your function to the entire array. Your basically doing:
't' + str(cols)
Which of course does not work. You need to apply it element-wise:
list(map(f, cols)) # No need for list() if using Python 2
