Python 2.7 plotting query

Question

Bit of a newbie question here. I've plotted some data in Python and I need to know where, on the x axis, the first time the corresponding y-values drop below a certain number. I want to ignore any further increases in y values.

I've tried using a zip function like:

for x_val, y_val in zip(xvalues, yvalues):
    if 100 < y_val < 150:
    edge1 = x_val

 But I can't seem to get my threshold values right and I would like a technique that just selects the first time the y values drop below a certain number.

Thanks. Helen

Answer 1

Once the Y value is below the threshold you have to break from the loop, otherwise the loop will continue and the X value will get overwritten if the Y value is below the threshold another time.

for x_val, y_val in zip(xvalues, yvalues):
    if 100 < y_val < 150:
        edge1 = x_val
        break

Or you could use itertools.dropwhile to skip all the values not in that range:

next(itertools.dropwhile(lambda t: not 100 < t[1] < 150, zip(xvalues, yvalues)))

Answer 2

Try

for x_val, y_val in zip(xvalues, yvalues):
    if 100 < y_val < 150:
        edge1 = x_val

Your code doesn't work because the statement inside the if is not executed because it is not properly indented or if you want to all the x_val which satisfies the the if statement

x=[]
for x_val, y_val in zip(xvalues, yvalues):
    if 100 < y_val < 150:
        x.append(x_val)