Filtering rows according to conditions (filtering)
Filter the rows of the matrix with a value of 5 in column 7
B = A[ A[:,6] == 5]
Filter the rows of the matrix whose 7th column is greater than 5
B = A[ A[:,6] > 5]
Numpy Basic Operations
Taking values based on row and column numbers (query)
Take the number in row 2, column 2
import numpy as np # Produce a matrix with 3 rows and 4 columns x=(0,12) x=((3,4)) print(x) y=x[1,1] print(y)
Intercept the first few rows and columns
import numpy as np # Produce a matrix with 3 rows and 4 columns x=(0,12) x=((3,4)) print(x) #[a:b,c:d] means take rows a-b, columns c-d. a, c can be omitted, means start from 0 # Take the first two rows of x (all columns) y=x[:2] # Equivalent to y=x[:2,:] Equivalent to y=x[0:2,...] print(y) # Take the first two columns of x (of all rows) z=x[:,:2] # is equivalent to z=x[:,0:2], which is equivalent to z=x[... ,0:2] print(z)
Intercepting certain rows and columns
import numpy as np # Produce a matrix with 3 rows and 4 columns x=(0,12) x=((3,4)) print(x) #[a:b,c:d] means take rows a-b, columns c-d. a, c can be omitted, means start from 0 # Take the 2nd row and 2nd-3rd columns of x. y=x[1,1:3]
Determine the row and column numbers from the values (locate)
Output the index of the column where the maximum value of a row is located
import numpy as np # Produce a matrix with 3 rows and 4 columns x=(0,12) x=((3,4)) print(x) # Take the index corresponding to the maximum value of the element in x and search for the maximum value in the direction of a[1] in a[0][1], i.e., the row direction. y = (x, 1) print(y)
Output the row index where the maximum value of a column is located
# Fetch the line number corresponding to the maximum value of the element in x y = (x, 0) print(y)
The row and column number where the maximum value is located
x=(0,12)
x=((3,4))
print(x)
# where returns a tuple of length 2, the first element holds the row number, the second element holds the column number
y = (x == (x))
print(y)
print("Maximum value in row:",y[0],"Column with maximum value:",y[1])Summing by row/column (summation)
Summing by rows, adding the sum to the last column of the matrix
x=(0,12) x=((3,4)) print(x) y = (x, axis=1) print(y) xy = ((x, ([0], -1))) print(xy)
Summing by column, adding the sum to the last row of the matrix
x=(0,12) x=((3,4)) print(x) z = (x, axis=0) print(z) xz = ((x, z)) print(xz)
assign a value to something
conditional assignment
x=(0,12) x=((3,4)) print(x) x[x<=5]=0 # Assigning numbers less than 5 to 0 print(x)
where()
x=(0,12) x=((3,4)) print(x) # results = (condition, x, y) # When the condition is true, the corresponding position returns the value in x. If the condition is not true, the value in y is returned. y = (x>5,x,0) #Values that satisfy greater than 5 are set to x and those that do not are set to 0 print(y)
logical operation
x=(0,12) x=((3,4)) print(x) print(x>5) # x>5traveler's dutyTure，or elseFalse
Assigns a value to the specified row/column/[row, column].
x=(0,12) x=((3,4)) print(x) x[1,1] = 999 # Assignment of specified row and column numbers print(x) x[1] = 123 # Assigning a value to a specific line print(x) x[:,1] = 321 # Assign values to specified columns print(x)
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