Python: While loop inside a for loop only running for the first instance of for variable

Question

So here's my problem: I have a for loop with a variable k running from 1 to 31. Inside the for loop there is a while loop that seemingly only runs for the very first k and no others.

from numpy import exp

a = 0.0
N = 1
x = 0.1

def f(t):
    return exp(-t**2)

def int_trap(x,N):
    h = (x-a)/N
    s = 0.5*f(a) + 0.5*f(x)
    for i in range(1,N):
        s += f(a + i*h)
    return h*s

new_value = 1.0
old_value = 0.0

for k in range(1,11):
    x = k/10
    while abs(new_value - old_value) > 10**-6:
        old_value = new_value
        N = N*2
        new_value = int_trap(x,N)
        print(N,'\t',x,'\t',abs(new_value - old_value))
    print(x)

The print(x) at the end is there to confirm that that the code is running through the k's.

And here's the output:

2        0.1     0.900373598036
4        0.1     3.09486672713e-05
8        0.1     7.73536466929e-06
16       0.1     1.93372859864e-06
32       0.1     4.83425115119e-07
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1.0

You while loop checks abs(new_value - old_value) against 10**-6 - the last time it prints you had 4.83425115119e-07 which is smaller than the 10**-6 so it won't run again. — doctorlove
– doctorlove, Commented Nov 13, 2017 at 12:25
Are you using python2? Division of integers gives an integer in python2, so your problem is most likely in the line x = k/10 — user2261062
– user2261062, Commented Nov 13, 2017 at 12:25

Pedro von Hertwig Batista · Accepted Answer · 2017-11-13 12:32:36Z

The for loop runs fine through all k values. It doesn't run through the while loop, perhaps because you don't reset the new_value and old_value variables inside the for loop. If we add some things to print to the original loop:

for k in range(1,11):
    x = k/10
    while abs(new_value - old_value) > 10**-6:
        old_value = new_value
        N = N*2
        new_value = int_trap(x,N)
        print(N,'\t',x,'\t',abs(new_value - old_value), 'In while for x={} and k={}'.format(x, k))
    print(x, '\tThis is me completing the loop for k=', k)

We see that it is correctly running for all k values:

2    0.1     0.900373598036 In while for x=0.1 and k=1
4    0.1     3.09486672713e-05 In while for x=0.1 and k=1
8    0.1     7.73536466929e-06 In while for x=0.1 and k=1
16   0.1     1.93372859864e-06 In while for x=0.1 and k=1
32   0.1     4.83425115119e-07 In while for x=0.1 and k=1
0.1     This is me completing the loop for k= 1
0.2     This is me completing the loop for k= 2
0.3     This is me completing the loop for k= 3
0.4     This is me completing the loop for k= 4
0.5     This is me completing the loop for k= 5
0.6     This is me completing the loop for k= 6
0.7     This is me completing the loop for k= 7
0.8     This is me completing the loop for k= 8
0.9     This is me completing the loop for k= 9
1.0     This is me completing the loop for k= 10

So try the following:

for k in range(1,11):
    x = k/10
    new_value = 1.0
    old_value = 0.0
    while abs(new_value - old_value) > 10**-6:
        old_value = new_value
        N = N*2
        new_value = int_trap(x,N)
        print(N,'\t',x,'\t',abs(new_value - old_value), 'In while for x={} and k={}'.format(x, k))
    print(x, '\tThis is me completing the loop for k=', k)

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