What is the meaning of axis=-1 in keras.argmax?

Question

I am a beginner in Keras and need help to understand keras.argmax(a, axis=-1) and keras.max(a, axis=-1). What is the meaning of axis=-1 when a.shape = (19, 19, 5, 80)? And also what will be the output of keras.argmax(a, axis=-1) and keras.max(a, axis=-1)?

Answer 1

This means that the index that will be returned by argmax will be taken from the last axis.

Your data has some shape (19,19,5,80). This means:

• Axis 0 = 19 elements
• Axis 1 = 19 elements
• Axis 2 = 5 elements
• Axis 3 = 80 elements

Now, negative numbers work exactly like in python lists, in numpy arrays, etc. Negative numbers represent the inverse order:

• Axis -1 = 80 elements
• Axis -2 = 5 elements
• Axis -3 = 19 elements
• Axis -4 = 19 elements

When you pass the axis parameter to the argmax function, the indices returned will be based on this axis. Your results will lose this specific axes, but keep the others.

See what shape argmax will return for each index:

• K.argmax(a,axis= 0 or -4) returns (19,5,80) with values from 0 to 18
• K.argmax(a,axis= 1 or -3) returns (19,5,80) with values from 0 to 18
• K.argmax(a,axis= 2 or -2) returns (19,19,80) with values from 0 to 4
• K.argmax(a,axis= 3 or -1) returns (19,19,5) with values from 0 to 79