def transform(self, X):
        """Impute all missing values in X.

        Parameters
        ----------
        X : {array-like, sparse matrix}, shape (n_samples, n_features)
            The input data to complete.
        """
        check_is_fitted(self, 'statistics_')

        X = self._validate_input(X)

        statistics = self.statistics_

        if X.shape[1] != statistics.shape[0]:
            raise ValueError("X has %d features per sample, expected %d"
                             % (X.shape[1], self.statistics_.shape[0]))

        # Delete the invalid columns if strategy is not constant
        if self.strategy == "constant":
            valid_statistics = statistics
        else:
            # same as np.isnan but also works for object dtypes
            invalid_mask = _get_mask(statistics, np.nan)
            valid_mask = np.logical_not(invalid_mask)
            valid_statistics = statistics[valid_mask]
            valid_statistics_indexes = np.flatnonzero(valid_mask)

            if invalid_mask.any():
                missing = np.arange(X.shape[1])[invalid_mask]
                if self.verbose:
                    warnings.warn("Deleting features without "
                                  "observed values: %s" % missing)
                X = X[:, valid_statistics_indexes]

        # Do actual imputation
        if sparse.issparse(X):
            if self.missing_values == 0:
                raise ValueError("Imputation not possible when missing_values "
                                 "== 0 and input is sparse. Provide a dense "
                                 "array instead.")
            else:
                mask = _get_mask(X.data, self.missing_values)
                indexes = np.repeat(np.arange(len(X.indptr) - 1, dtype=np.int),
                                    np.diff(X.indptr))[mask]

                X.data[mask] = valid_statistics[indexes].astype(X.dtype,
                                                                copy=False)
        else:
            mask = _get_mask(X, self.missing_values)
            n_missing = np.sum(mask, axis=0)
            values = np.repeat(valid_statistics, n_missing)
            coordinates = np.where(mask.transpose())[::-1]

            X[coordinates] = values

        return X