# R ```library(reshape2)
library(ggplot2)

set.seed(7)
m <- matrix(0, 100, 100)
n <- 1000
m[sample(length(m), size = n)] <- rep(-1:1, length=n)
m

ggplot(melt(m), aes(Var1,Var2, fill=value)) + geom_raster() + scale_fill_gradient2(low='red', high='black', mid='white') + theme_bw() + xlab("x1") + ylab("x2")```

# Matplotlib ```'''
Shows a way of plotting a sparse matrix

Sparse matrix is a matrix that contains many zero entries
Zero values are plotted as regular values mapped through colormap.
NaN on the other hand are not mapped through colormap
For plotting purposes it might be useful to replace 0 with NaN

Another alternative is to use masks; as described in
http://stackoverflow.com/questions/2578752/how-can-i-plot-nan-values-as-a-special-color-with-imshow-in-matplotlib

@see: http://matplotlib.sourceforge.net/api/pyplot_api.html#matplotlib.pyplot.imshow

@author: Neil Rubens
'''

from pylab import *

Z = rand(10,10)

# note that 0 != NaN (in this case it will just take the minimum value
Z[0,:] = 0
Z[1,:] = np.nan

# colormap
cmap = cm.jet
# set NaN values as white
im = imshow(Z, cmap=cmap, interpolation='nearest')
colorbar()
show()```

Simpler method with fewer configurations:

```from pylab import *

Z = rand(10,10)
Z[0,:] = 0
Z[1,:] = np.nan
matshow(Z)
show()```

Alternative way that uses masking (as described in this post):

```import numpy as np
import matplotlib.pyplot as plt
f = plt.figure()
a = np.arange(25).reshape((5,5)).astype(float)
a[3,:] = np.nan
ax.imshow(a, interpolation='nearest')
f.canvas.draw()```
```masked_array = np.ma.array (a, mask=np.isnan(a))
cmap = matplotlib.cm.jet

Matplotlib allows to create/generate random sparse matrix by:

```matplotlib.mlab.get_sparse_matrix(M, N, frac=0.10000000000000001)

# Return a M x N sparse matrix with frac elements randomly filled.
# http://matplotlib.sourceforge.net/api/mlab_api.html```

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