How to fix IndexError: invalid index to a scalar variable

Cause of How to fix IndexError: invalid index to a scalar variable

The error of How to fix IndexError: invalid index to a scalar variable occurs when there is no index related to the scalar variable. The problem here is that the scalar variable is not iterable in nature which is the main cause behind this error. 

Individual fixed-size data items, such as integers and pointers, are represented by scalar variables. Fixed-size objects made up of one or more primitive or composite types can also be stored in scalar variables. It also offers the ability to build composite structures as well as object arrays.

for traincv, testcv in cv:
    y_test = np.expm1(forest.fit(X_train[traincv], y_train[traincv]).predict(X_train[testcv]))
    results.append(RMSPE(np.expm1(y_train[testcv]), [y[1] for y in y_test]))

Error

IndexError: invalid index to scalar variable.

Solution of error 

One of the solutions to this problem can be by expanding the list by using list comprehension. 

To iterate over each element in the Python list, a comprehension is made out of brackets. These brackets carry the expression, which is then run for each element. A considerably shorter syntax is available in Python List comprehension for building a new list from the values of an existing list.

List comprehension benefits
1. more cost- and space-effective than loops.
2. entail writing less code.
3. creates a formula out of an iterative statement.

List comprehensions convert the conventional for loop iteration strategy into a straightforward formula, making them simple to employ. The method for utilizing list comprehension to iterate is through a list, string, tuple, etc. 

[results.append(..., y) for y in y_test]

Another solution can be by simply using for loop. 

for y in y_test:
    results.append(..., y)

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