Numpy lets us create arrays in multiple ways, most of the time in consonancy with core Python and other libraries like Pandas.
We can get Numpy vector and a matrix rapidly from a Python list:
We can quickly create matrices of ones and zeros:
Or any other value:
We can create ndarrays initialized with values:
Or generate ndarrays of random values:
We can observe some attributes from an ndarray, with the following operations, for an array defined as
- itemsize: Will return
4, that is the number of bytes used by the array.
int32is the result, that is the type of the elements in the array. (we can have np.int32, np.float16, np.float32, or np.float64)
- ndim: Number if dimensions of de array, 1 for vector, 2 for matrix etc..
These are the data types we can use in numpy:
|bool_||Boolean||Stored as a byte|
|int_||Default integer type||either int64 or int32|
|intc||C int||Either int32 or int64|
|intp||C ssize_t||Either int32 or int64|
|int8||Byte||-128 to 127|
|int16||Integer||-32768 to 32767|
|int32||Integer||-2147483648 to 2147483647|
|int64||Integer||-9223372036854775808 to 9223372036854775807|
|uint8||Unsigned integer||0 to 255|
|uint16||Unsigned integer||0 to 65535|
|uint32||Unsigned integer||0 to 4294967295|
|uint64||Unsigned integer||0 to 18446744073709551615|
|float_||float64||Shorthand for float64.|
|float16||Half precision float||Sign bit, 5 bits exponent, 10 bits mantissa|
|float32||Single precision float||Sign bit, 8 bits exponent, 23 bits mantissa|
|float64||Double precision float||Sign bit, 11 bits exponent, 52 bits mantissa|
|complex_||complex128||Shorthand for complex128.|
|complex64||Complex number||Represented by two 32-bit floats|
|complex128||Complex number||Represented by two 64-bit floats|
We mentioned that numpy and ndarrays are fast and compact because they have homogeneus data on them, but htey might be cases when we need heterogeneous data on them, we can do this using Panda’s Dataframes, but can we do it using just Numpy?
Yes, we have structured arrays that can include multipe data types:
- U10 means: Unicode string of maximum length 10 .
- i4 means: 4-byte (i.e., 32 bit) integer
- f8 means: 8-byte (i.e., 64 bit) float.
Slice and dice
Let´s create a 5x5 simple matrix and start slicing it!
The logic can be seen in the following highlighted image:
Stacking and tiling
Sometimes we’ll need to copy horitontally or vertically our matrix or vector, numpy can do this for us:
When there is no ambiguity the smaller array will be expanded to match the shape of the bigger array.