How would I plot a graph from a large array?

Question

I have a numpy array [(a1, x1), (a2, x2), ..., (a100, x100)] and I need to plot it such that a is my y-axis and x is my x-axis.

I know that it would be easier to have [a1, a2, ..., a100] and [x1, x2, ..., x100] as separate arrays to plot, but I'm doing a project, and I've been specifically told to do it this way, but am struggling.

Attempt 1 gave me a graph, but I think it's plotted a graph of a and x values against their numbers of order in the array.

Attempt 2, which was suggested by a matplotlib website just gave me a long error message.

# attempt 1 
plt.plot(array,'r.') 
plt.show()
plt.close()

# attempt 2 
plt.plot(array_1[0], array_1[1:],'r.') 
plt.show()
plt.close()

Any advice would be greatly appreciated. Thank you!

Just reshape and unpack to get the quantities you want: a, x = array.T followed by plt.plot(a,x), or unpack with the * operator: plt.plot(*array.T). In this sequence T is for transpose and loosely stated *(a,b,c,d) = a,b,c,d — kevinkayaks
– kevinkayaks, Commented Mar 31, 2019 at 18:50

Christian Sloper · Accepted Answer · 2019-03-31 19:31:21Z

1

Assuming your array is a numpy array with shape (100,2) you can do:

plt.plot(arr[:,1],arr[:,0])

The way to read this is:

[ first dimension, second dimension ] where : is "all of the dimension" and the indices 1 and 2 specify position in the dimension.

So the code arr[:,1] means "all the rows", "second element"

edit: After discussing with OP it appears that his/her array shape is actually (202,)

You can fix this by reshape.

array_1 = array_1.reshape(-1,2)
plt.plot(arr[:,1],arr[:,0])

edited Mar 31, 2019 at 19:31

answered Mar 31, 2019 at 18:13

Christian Sloper

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18 Comments

Christian Sloper Over a year ago

can you type arr.shape and report the result?

Christian Sloper Over a year ago

i just need the values of "type(arr)" and "arr.shape"

Christian Sloper Over a year ago

still not giving me the values for array_1.shape

Christian Sloper Over a year ago

all numpy arrays have a shape attribute so you can type print(array_1.shape) at the end of your code

Christian Sloper Over a year ago

print it out and report here.. otherwise i have to guess.

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