python for-loop only executes once?

Question

import numpy as np
def Vin(t):
    inputs = []
    for i in range (1000):
        if (-1)**(np.floor( 2 * t[i] )) == 1:
            Vin = (1)
            inputs.append(Vin)
        else:
            Vin = (-1)
            inputs.append(Vin)

        return inputs

when I use this function on a range of t values, I only get one result,

i.e.

input1=Vin(tpoints)
print (input1)

only gives [1], whereas I want the function to do it for every t value.

As soon as your function hits a `return`, the function is finished. If you want to run the whole loop before you return, put the `return` statement after the loop instead of inside it. — khelwood, Jan 30 '17 at 10:16

score 3 · Answer 1 · answered Jan 30 '17 at 10:20

3

As others said, there's an indentation error in your return statement. I couldn't resist rewriting your code in a more pythonic way, avoiding all that cumbersome loops and the resulting errors:

return [1 if (-1)**(np.floor( 2 * t[i] )) == 1 else -1 for i in range (1000)]

that list comprenhension + ternary construction statement creates your -1,1 array in 1 line, much faster than you'd write with a loop.

answered Jan 30 '17 at 10:20

Jean-François Fabre

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The function can be reduced even further ,check my answer – Sarath Sadasivan Pillai Jan 30 '17 at 10:37

Sarath Sadasivan Pillai · Accepted Answer · 2017-01-30T10:42:14.960

You are returning from with in the for loop,So you would return from the function on first iteration of the loop.

you may re-writ e the function as following

Code

def Vin(t):
    inputs = []
    for i in range (1000):
        if (-1)**(np.floor( 2 * t[i] )) == 1:
            inputs.append(1)
        else:
            inputs.append(-1)
    return inputs

Check the indentation of return inputs in the function here .

BTW, your function can be reduced to as more pythonic and efficient code

def Vin(t):
 reduce map(lambda x:int((-1)**(np.floor( t[i] < 1))), range (1000))

score 0 · Answer 3 · answered Jan 30 '17 at 10:16

try:

 for i in range (1000):
        if (-1)**(np.floor( 2 * t[i] )) == 1:
            Vin = (1)
            inputs.append(Vin)
        else:
            Vin = (-1)
            inputs.append(Vin)

 return inputs

Current indentation of return , exits the for loop after 1st iteration

python for-loop only executes once?

3 Answers3

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