Np Array values are being changed without doing stuff

March 4, 2022

Why does :

print(np.delete(MatrixAnalytics(Cmp),[0],1))
MyNewMatrix = np.delete(MatrixAnalytics(Cmp),[0],1)
print("SecondPrint")
print(MyNewMatrix)

returns :

[[ 2.  2.  2.  2.  2.]
 [ 1.  2.  2.  2.  2.]
 [ 1.  2.  0.  2.  2.]
 [ 2.  2.  2.  2.  2.]
 [ 2.  2.  2.  0.  0.]
 [ 1.  2.  2.  0.  2.]
 [ 1.  2.  2.  2.  2.]
 [ 1.  2.  2.  2. nan]
 [ 2.  2.  2.  2.  2.]
 [ 2.  2.  2.  2. nan]]
Second Print
[[-1.  0.  0.  0.  0.]
 [-1.  0.  0.  0.  0.]
 [-1.  0. -1.  0.  0.]
 [-1.  0.  0.  0.  0.]
 [-1.  0.  0. -1.  0.]
 [-1.  0.  0. -1.  0.]
 [-1.  0.  0.  0.  0.]
 [-1.  0.  0.  0. nan]
 [-1.  0.  0.  0.  0.]
 [-1.  0.  0.  0. nan]]

This is weird, and can’t figure this out. Why Would the values change without any line of code between 3 print ?

def MatrixAnalytics(DataMatrix):
    AnalyzedMatrix = DataMatrix
    for i in range(len(AnalyzedMatrix)): #Browse Each Column
        for j in range(len(AnalyzedMatrix[i])): #Browse Each Line
            if j>0:
                if AnalyzedMatrix[i][j] > 50:
                    if AnalyzedMatrix[i][j] > AnalyzedMatrix[i][j-1]:
                        AnalyzedMatrix[i][j] = 2
                    else:
                        AnalyzedMatrix[i][j] = 1
                else:
                    if AnalyzedMatrix[i][j] <50:
                        if AnalyzedMatrix[i][j] > AnalyzedMatrix[i][j-1]:
                            AnalyzedMatrix[i][j] = 0
                        else:
                            AnalyzedMatrix[i][j] = -1                          
    return AnalyzedMatrix

The input array is :

[[55.  57.6 57.2 57.  51.1 55.9]
 [55.3 54.7 56.1 55.8 52.7 55.5]
 [55.5 52.  52.2 49.9 53.8 55.6]
 [54.9 57.8 57.6 53.6 54.2 59.9]
 [47.9 50.7 53.3 52.5 49.9 45.8]
 [57.  56.2 58.3 55.4 47.9 56.5]
 [56.6 54.2 57.6 54.7 50.1 53.6]
 [54.7 53.4 52.  52.  50.9  nan]
 [51.4 51.5 51.2 53.  50.1 50.1]
 [55.3 58.7 59.2 56.4 53.   nan]]

It seems that it call again the function MatrixAnalytics But I don’t understand why

Doing this works :

MyNewMatrix = np.delete(MatrixAnalytics(Cmp),[0],1)
print(MyNewMatrix)
MyNewMatrix = np.delete(MatrixAnalytics(Cmp),[0],1)
print("SecondPrint")
print(MyNewMatrix)

>Solution :

I think I got the issue.
In this code :

def MatrixAnalytics(DataMatrix):
    AnalyzedMatrix = DataMatrix
    ...
    ...                      
    return AnalyzedMatrix

AnalyzedMatrix is not a copy of DataMatrix, it’s referencing to the same object in memory !

So on the first call of MatrixAnalytics, your are actually modifying the object behind the reference given as argument (because arrays are mutable).
In the second call, your are giving the same reference as argument so the array behind it has already been modified.

note : return AnalyzedMatrix statement just returns the a new reference to the object referenced by the DataMatrix argument (not a copy).

Try to replace this line :