making an array within an array in python

Question

I am trying to make in python an array within an array that looks like this:

[[3,0,-3,-4],[6,0,-2.44,-4]]

which I could use in a vector plot in matplotlib.

I tried doing this with the following program:

data = np.loadtxt(sys.argv[1], dtype='str',delimiter=',', skiprows=1, usecols=(0,1,2,3,4,5,6,7))

x = data[:,0].astype(float)
u = data[:,6].astype(float)
v = data[:,7].astype(float)

soa = []
for t in range(0,2):
    print "At time ",x[t]," U is ",u[t]," and V is ",v[t]
    result = [x[t],0,u[t],v[t]] 
    soa = np.append(soa, [result])


print "soa is ",soa

When I run the program I get the output:

soa is [ 3. 0. -3. -4. 6. 0. -2.44 -4. ]

This cannot be plotted as a vector plot in matplotlib. How can I tweak the above script to get the array into the format:

 [[3,0,-3,-4],[6,0,-2.44,-4]]

where [3,0,-3,-4] and [6,0,-2.44,-4] are vectors that I could plot in matplotlib?

quick fix, soa = soa.reshape((2,-1))

juanpa.arrivillaga
– juanpa.arrivillaga

2017-02-02 22:25:35 +00:00
Commented Feb 2, 2017 at 22:25 — juanpa.arrivillaga
– juanpa.arrivillaga, Commented Feb 2, 2017 at 22:25

ForRealHomie · Accepted Answer · 2017-02-02 22:26:10Z

1

np.append is different than the noraml array append.

try:

for t in range(0,2):
    print "At time ",x[t]," U is ",u[t]," and V is ",v[t]
    result = [x[t],0,u[t],v[t]] 
    soa.append(result)

answered Feb 2, 2017 at 22:26

ForRealHomie

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Comments

ImportanceOfBeingErnest · Accepted Answer · 2017-02-02 22:36:10Z

1

The easiest way to get the desired array is to simply stack the columns from the data and put a zero column in the middle

data = np.loadtxt(sys.argv[1], dtype='str',delimiter=',', skiprows=1, 
                  usecols=(0,1,2,3,4,5,6,7))

x = data[:,0].astype(float)
u = data[:,6].astype(float)
v = data[:,7].astype(float)

soa = np.vstack((x,np.zeros(len(x)),u,v)).T

print "soa is ",soa

answered Feb 2, 2017 at 22:36

ImportanceOfBeingErnest

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