3.7.1.3. Exercise: Looping using while 3#
Using a force plate, we recorded the vertical ground reaction force during one step, at a sampling frequency of 20 Hz:
grf = [
0.5, 0.9, 0.2, 0.0, 0.3, 0.2, 0.4, 0.7, 5.5, 7.1, 4.5, 37.7, 180.8,
400.2, 603.1, 598.4, 584.2, 620.6, 635.5, 602.7, 246.0, 14.5, -1.2,
-0.7, 0.4, 0.5, 0.8, 0.6
]
We consider that the step begins when the force exceeds a given threshold, and that it ends when the force stops exceeding this threshold. Write the function calculate_step_time that calculates the duration of the step, then test it with a threshold of 50 N.
def calculate_step_time(grf, threshold, sampling_frequency):
"""
Calculate the step time using vertical ground reaction forces.
Parameters
----------
grf : list[float]
A list of vertical ground reaction forces in newtons. There must
be only one step in this list.
threshold : float
A value above which we consider the step begins, and below which
the step ends.
sampling_frequency :
The sampling frequency in Hz.
Returns
-------
float
The duration of the step.
"""
Tip
Start by finding at which index the step begins.
From this index, find at which index the step ends.
Calculate the step time using the difference between the indexes and the sampling frequency.
Show code cell content
def calculate_step_time(grf, threshold, sampling_frequency):
# Find i_begin
current_index = 0
while grf[current_index] < threshold:
current_index += 1
i_begin = current_index
# Find i_end
while grf[current_index] >= threshold:
current_index += 1
i_end = current_index
# Calculate and return the time step
return (i_end - i_begin) / sampling_frequency
# Test it
calculate_step_time(grf, threshold=50, sampling_frequency=20)
0.45