JOIV : Int. J. Inform. Visualization, 5(4) - December 2021 415-421 INTERNATIONAL JOURNAL ON INFORMATICS VISUALIZATION INTERNATIONAL JOURNAL ON INFORMATICS VISUALIZATION journal homepage : www.joiv.org/index.php/joiv Feature-reduction Fuzzy c-means Clustering for Basketball Players Positioning Yessica Nataliani a,* a Department of Information System, Satya Wacana Christian University, Diponegoro 52-60, Salatiga, 50711, Indonesia Corresponding author: *yessica.nataliani@uksw.edu Abstract— One of the best-known clustering methods is the fuzzy c-means clustering algorithm, besides k-means and hierarchical clustering. Since FCM treats all data features as equally important, it may obtain a poor clustering result. To solve the problem, feature selection with feature weighting is needed. Besides feature selection by assigning feature weights, there is also feature selection by assigning feature weights and eliminating the unrelated feature(s). THE Feature-reduction FCM (FRFCM) clustering algorithm can improve the FCM clustering result by weighting the features and discarding the unrelated feature(s) during the clustering process. Basketball is one of the famous sports, both international and national. There are five players in basketball, each with a different position. A player can generally be in guard, forward, or center position. Those three general positions need different characteristics of players’ physical conditions. In this paper, FRFCM is used to select the related physical feature(s) for basketball players, consisting of height, weight, age, and body mass index. to determine the basketball players’ position. The result shows that FRFCM can be applied to determine the basketball players’ position, where the most related physical feature is the player’s height. FRFCM gets one incorrect player’s position, so the error rate is 0.0435. As a comparison, FCM gets five incorrect player’s positions, with an error rate of 0.2174. This method can help the coach decide the basketball new player’s position. Keywords— Feature-reduction; clustering; fuzzy c-means; basketball; position. Manuscript received 24 Aug. 2021; revised 22 Sep. 2021; accepted 26 Nov. 2021. Date of publication 31 Dec. 2021. International Journal on Informatics Visualization is licensed under a Creative Commons Attribution-Share Alike 4.0 International License. Furthermore, currently, high-dimensional data is widely used for data analysis. Of course, high-dimensional data needs more computation time. Therefore, the use of feature selection is very important [6]. Feature weight can be used to support feature selection, where each feature has its weight [7], [8]. Feature-weighted has been used for clustering algorithms, for example, sparse k-means, entropy-weighted kmeans, feature-weighted k-means, feature-weighted FCM, weighted FCM with feature-weight learning, etc. [9]. Some literature applied feature selection and featured weight in clustering to discard unrelated features. There are featurereduction FCM [9], feature-reduction k-means for multi-view data [10], feature-reduction scheme for possibilistic c-means [11], and dimensionality-reduction using PCA and k-means [12]. Basketball is one of the famous sports, both international and national. The National Basketball Association (NBA) is a well-known international basketball event. In Indonesia, one big event is named the Indonesian Basketball League (IBL). 12 teams are competing in IBL. One of them is Satya Wacana Saints [13]. This team consists of 23 students of Satya Wacana Christian University in Salatiga. I. INTRODUCTION In pattern recognition, cluster analysis is unsupervised learning. Clustering is a method to find groups, such that the similar characteristics data will be in the same cluster, and data with different characteristics will be in the other clusters. Many areas use cluster analysis, such as business, image processing, education, etc. k-Means is the most popular clustering method, where each data point belongs to exactly one cluster. k-Means is extended to be fuzzy c-means (FCM), where each data point can be included into several clusters, depending on the cluster membership values [1], [2]. A dataset is defined by a number of points and dimensions (attributes, features). In general, the clustering process treats all features to be equally important. However, some features may be unrelated and affect the clustering performance. It is important to find the related features to obtain a better clustering result [3]. Feature selection can solve the problem by removing unrelated and redundant data to reduce the computation time, and the clustering performance can be improved [4], [5]. 415 cluster center, with ∑ = 1, ∀", is the " #$ point, is the % #$ cluster center, and 1 < ( < ∞ is the fuzzy exponent. The Euclidean distance, , = ‖ − ‖, is the #$ #$ distance between the " point and the % cluster center. Since In basketball, there are five players, where each player can be on the shooting guard, point guard, small forward, power forward, or center position. Each position needs different players’ physical conditions, e.g., height, weight, age, and body mass index (BMI). Some experts said that anthropometric or body measurements could determine the athlete's career, especially in basketball [14], [15]. Physical condition is important for coaches to choose a new player, besides his/her talent. Players’ physical conditions affect the players’ position in basketball. Unsuitable positions allow players not to play optimally [16], [17]. The players’ physical condition can generally determine the position. If the player is tall and big, he can be in the center or power forward position. If the player is small and agile, he can be on the guard position [18]. In general, the players’ positions can be divided into three positions, i.e., guard, forward, and center. Players in the guard position are more often outside the paint area. The team puts the smallest and most agile players for this position. Guards have less physical contact with opposing players than the forward and center positions. Guards usually are the brain of attack on a team. This position consists of two kinds, point guard and shooting guard. The second position is forward. A player in this position is a player whose job is to see an open position near the paint area, to break through the opponent's defense, or in other words, to drive inside. A forward is usually tall and strong because his main job is to defend and rebound. Players in this position must have a medium level of shooting accuracy. This position consists of two types, small forward and power forward. The last position is center. Players in this position are often called the big man, who is in charge of guarding their paint area and attacking the opponent's paint area. The center position is more likely to physically collide with opposing players in scoring or blocking the opponent's center position. This position is held by the tallest and biggest player [19]. Clustering can be used to find a suitable position for each player. Players with similar physical conditions will be grouped into the same position, and players with different physical conditions will be in different positions. In this paper, the grouping of the positions of Satya Wacana Saint basketball team based on their physical conditions with FCM will be discussed. The physical conditions used here are referred to as height, weight, age, and BMI. Furthermore, the feature-reduction method with FCM will be used to find the most important features that can determine the player’s position, so it can help the coach decide whether a new basketball player is more suitable in what position. There are some variants of feature-weighted clustering. Some of them are modifying the k-means and FCM clustering objective function into a new objective function by adding feature weight. Before some feature-weighted clustering is presented, FCM clustering will be described first. , = , ∑- - − - , where is the number of attributes (features or dimension), then Eq. (1) can be written as in Eq (2). ∑ ∑, =∑ (2) -− - The updating formula for the membership value and the cluster center are shown in Eqs. (3) and (4), = :8<;:8 6 .∑178 /01 2341 5 9 :8<;:8 ∑>=78.∑6 /01 23=1 5 9 178 - = ; ∑@ 078 ?04 /01 ; ∑@ 078 ?04 (3) (4) where " = 1, … , ; % = 1, … , ; A = 1, … , . The FCM clustering algorithm can be described as follows, Input: points , number of clusters , and threshold B > 0. D 1) Initialize random cluster centers, , % = 1, … , . 2) Let the iteration rate, E = 1. F 3) Compute the membership values, using Eq. (3). F 4) Update the cluster centers, using Eq. (4). F F2 G < B, then STOP, else go back to step 5) If G − c and E = E + 1. Output: cluster points I , % = 1, … , [1]. The stopping condition used in this algorithm is when there is no significant difference between the cluster center in the next iteration. B. Feature-weighted Clustering Some extensions from k-means for feature-weighted clustering are presented. Weighted k-means added the feature weights during clustering iteration processes. The objective L ∑ ∑function is , ,J = ∑ K-− - , #$ where K- is the feature weight of the A feature and M is a constant. Entropy weighted k-means is also an extension of kmeans by adding a weight entropy term. The objective is to minimize the within-cluster distance and maximize the negative weight entropy. Its objective function is , ,J = ∑ ∑ ∑K - -− - + γ ∑ ∑- OK - log K - S, where K - is the feature weight of the A #$ feature in the % #$ cluster and T are a constant [9]. Sparse k-means is also used to select features by assigning feature weights in the interval [0,1] at first and then updating the feature's weight (s) with small weights into 0. Whether a feature weight is small or not is determined based on a threshold [20]. Besides k-means, FCM is also extended into some methods for feature-weighted clustering. Weighted FCM added feature-weight learning to improve the FCM performance, where the objective function is , ,J = ∑ ∑ ∑K- - − - [9]. The other method is simultaneous clustering and attributes discrimination (SCAD1). In SCAD1, each cluster has a different feature weight. SCAD1’s objective function is , ,J = A. FCM Clustering Let = ,…, be a dataset on ℝ with is the number of data and is the number of dimensions. The FCM objective function is defined in Eq. (1). ∑ , =∑ , (1) where is the cluster number, is the number of points, = [0,1] is the membership value of the " #$ points and the % #$ 416 ∑ ∑ ∑∑- U K - , K - - − - +∑ where U is a constant which indicates the importance of feature weights in each cluster [21]. All these methods consider feature-weighted clustering, so selecting the related feature(s) needs to be done manually. Another modification from FCM objective function is for feature-reduction. The feature-reduction idea is to eliminate the unrelated feature(s) automatically. Some methods have been proposed by modifying the FCM clustering objective function. For example, feature-reduction for FCM clustering algorithm, where the objective function is , ,J = ∑ ∑ ∑U- K- O - − - S + ∑- OK- log U- K- S [9]. Another method is feature-reduction fuzzy co-clustering algorithm (FRFCoC). The objective function is , ,J = ∑ ∑ ∑V - U- K- O - − - S + α1 ∑- OK- log U- K- S + α2 ∑%=1 ∑"=1O "% log "% S + α3 ∑%=1 ∑A=1OV%A log V%A S, where V - is the feature membership for the % #$ cluster center and the A #$ feature, α , α , αZ are constants. Its objective function consists of four terms, one term is a modification from the FCM objective function and three terms are the entropy terms of feature weight, fuzzy membership, and feature membership [22]. , ,J = ∑ ∑- ∑ ∑- U- KOK- log U- K- S -− - + (5) where is the number of features, - is the cluster center of the % #$ cluster and the A #$ feature, K- = [0,1] is the feature weight of the A #$ feature, with ∑- K- = 1. Here, a constant U- is used to handle the dispersion and variation of each feature. The formula of U- is shown in Eq. (6), \]^_ / U- = − [ `^a / [ ∑@ 078 /0 (6) - ∑@ 078 /0 2 hi / 5 where mean = and var = , ∀A. 2 The updating formula for the membership, the cluster center, and the feature weight are computed as in Eqs. (7), (8), and (9), respectively, = :8<;:8 6 .∑178 j1 k1 /01 2341 5 9 ∑>=78.∑6 K- = (7) ; ∑@ 078 ?04 /01 (8) 5 > @ ; 8 o:> ∑478 ∑078 p04 l1.q01 :r419 s ]mn l1 @ (9) - = C. Basketball Players’ Position Analysis Some analysis about basketball players’ position has been presented in the literature. Pion et al. [23] found that artificial neural networks can provide a specific position characteristic in basketball. They mentioned that multivariate variance analysis could not predict the specific players’ position accurately. Zhang et al. [24] used clustering to see the performance of NBA players according to their anthropometric features and their playing performance. Erga and Nataliani have researched feature selection with FCM for basketball players’ positioning. They combined four physical conditions, i.e., height, weight, age, and BMI, one by one to get the best clustering result, which was measured by the accuracy rate. Height and BMI are the best combinations to determine the player’s position [19]. :8<;:8 j k / 23=1 5 9 178 1 1 01 ; ∑@ 078 ?04 5 > @ ; 8 o:> ∑478 ∑078 p04 l1.q01:r41 9 s ]mn l1 @ ∑6 t78 The FRFCM clustering algorithm is described as follows, Input: points , number of clusters , and threshold B > 0. D 1) Initialize random cluster centers, , % = 1, … , and D define the initialization of feature weight, J- = [1⁄ ] × , A = 1, … , . 2) Let the iteration rate, E = 1. 3) Compute U- using Eq. (6) F 4) Compute the membership values, using Eq. (7). F 5) Update the cluster centers, using Eq. (8). F 6) Update the feature weights, J- using Eq. (9). 7) Discard the feature(s) if the feature weight is less than 1⁄w . 8) Adjust J- II. MATERIALS AND METHOD ∑- Some datasets may contain unimportant features that affect the clustering results. These unimportant features should be discarded to make a better clustering result. Yang and Nataliani [9] proposed a method of feature-reduction for FCM clustering algorithm, abbreviated with FRFCM. In the FRFCM clustering, the features are weighted with feature weights. Feature(s) with small feature weights must be discarded during the clustering process. In this way, FRFCM clustering method improves FCM clustering. FRFCM automatically computes different feature weights of each feature by modifying the objective function of FCM in Eq. (2) and adding a feature-weighted entropy term, ∑- OK- log U- K- S. This algorithm can eliminate the unrelated features with small feature weights, such that a better clustering result can be obtained and computation time can be decreased. The objective function of FRFCM clustering is defined in Eq. (5). F K- = 1. using K-x = k1 6 @yz ∑t78 kt , in order to keep F F2 G{ < B, then STOP, else go back 9) If {GJ- G − GJ- hk hk to step 3) with = and E = E + 1. Since is F F obtained, then points, and the cluster center, need to be updated. Output: cluster points and feature weight [9]. The stopping condition used in this algorithm is when there is no significant difference between the feature weight of the current iteration and the next iteration. According to the algorithm, the flowchart of FRFCM algorithm is shown in Fig. 1. This paper applies FRFCM to find the important and related feature(s) on basketball players' positioning. There are four features used to determine the position of a player, i.e., height, weight, age, and BMI. 417 TABLE I BASKETBALL PLAYERS’ DATA Player’s Name Height Weight Age BMI Anjas Rusadi Putra Antoni Erga Ardian Ariadi Aldi Falentino Alexander Franklyn Bryan Adha Elang David Liberty Nuban Elyakim Tampa'i Febrianus Gregory Fransiscus Bryan Henry Cornelis Lakay Raymond Putra Fajar Randy Ady Prasetya Mas Kahono Alif Rian Sanjaya Janson Kurniawan M. Yassir Alkatiri Martin Steven Ray Jody Sebastian Peter Surjantoro Fauji Ridho Pamungkas 190 179 180 171 184 196 190 175 181 183 196 195 202 192 178 178 184 179 178 204 171 186 189 75 76 83 70 82 98 80 75 76 80 96 130 77 79 73 69 74 76 75 110 72 85 85 23 20 26 20 20 22 22 23 21 20 22 21 23 19 22 21 19 21 21 21 19 22 24 20 23 26 24 25 37 22 25 23 24 25 34 19 21 23 22 22 23 24 26 24 25 25 Actual Position Forward Guard Guard Guard Guard Center Forward Guard Guard Guard Center Center Center Forward Guard Guard Forward Guard Guard Center Guard Forward Forward Fig. 2 Basketball players’ data All computations in this paper use the fuzzy exponent of ( = 2. The error rate (ER) is used to measure the clustering performance. The formula of ER is ER = 1 − ∑ I , where I is the number of incorrect points in cluster %. The smaller the ER indicates, the better the clustering performance. In the FCM clustering process, the first step is to determine the clusters, where = 3, consisting of guard, forward, and center positions. After the cluster centers are initialized, then the membership is computed. The calculation of the cluster center and membership value are updated continuously until the stop condition is reached. Table II shows the final membership values of FCM clustering result. From Table II, for example, the first player, Anjas Rusadi Putra, according to FCM clustering result, he is more suitable on Cluster 2, with the membership value of 0.7130, than on Cluster 1 (with the membership value of 0.2635), and even more, does not Fig. 1 Flowchart of FRFCM clustering algorithm III. RESULTS AND DISCUSSION The data was collected from all 23 players of the Satya Wacana Saints Salatiga basketball team in 2021. There are two kinds of data collected, data of the physical conditions and data of the position of each player. Physical condition data consists of four features, features of height, weight, age, and BMI. Physical condition data is used to cluster players' positions using FCM, consisting of three positions, namely guard, forward, and center, while player position data compares the clustering results with actual conditions. Table I shows each player's physical condition and actual position, and Fig. 2 shows the matrix of scatter plots by a group for the basketball players’ position according to their physical conditions. 418 suitable on the Cluster 3 (with the membership value of 0.0234). For the final cluster center of FCM clustering result, as shown in Table III, three cluster centers consist of four features. According to Table II, each data cluster can be determined by choosing the highest value of membership value. The clustering results of FCM are shown in Table IV, where 11 players are in the guard position, nine players are in the forward position, and three players are in the center position. Table IV shows that there are five players with incorrect positions (see the bold font), i.e., Alexander Franklyn, Fransiscus Bryan, Henry Cornelis Lakay, Randy Ady Prasetya, and M. Yassir Alkatiri. Therefore, the ER of the FCM clustering result is 0.2174. Next, FRFCM is applied for this basketball player’s position to see what feature(s) are related to determining the player’s basketball position. Since the data has four features, then for the initialization, the feature weight is defined by D J- = [0.25 0.25 0.25 0.25]. For the computation of FCM, the same initialization of cluster centers with FCM, D , is used. After U- , , and are computed, then the TABLE II MEMBERSHIP VALUES OF FCM CLUSTERING RESULT Player’s name Anjas Rusadi Putra Antoni Erga Ardian Ariadi Aldi Falentino Alexander Franklyn Bryan Adha Elang David Liberty Nuban Elyakim Tampa'i Febrianus Gregory Fransiscus Bryan Henry Cornelis Lakay Raymond Putra Fajar Randy Ady Prasetya Mas Kahono Alif Rian Sanjaya Janson Kurniawan M. Yassir Alkatiri Martin Steven Ray Jody Sebastian Peter Surjantoro Fauji Ridho Pamungkas Cluster 1 0.2635 0.9673 0.4992 0.8587 0.2754 0.1612 0.0290 0.9387 0.8965 0.4623 0.1611 0.0601 0.2238 0.1061 0.9835 0.8962 0.6974 0.9733 0.9963 0.0333 0.8693 0.1220 0.0653 Cluster 2 0.7130 0.0304 0.4632 0.1207 0.7050 0.3517 0.9672 0.0548 0.0980 0.5204 0.5278 0.0854 0.6915 0.8773 0.0151 0.0932 0.2877 0.0249 0.0034 0.0646 0.1120 0.8583 0.9195 Cluster 3 0.0234 0.0023 0.0377 0.0206 0.0196 0.4871 0.0037 0.0065 0.0056 0.0173 0.3111 0.8545 0.0847 0.0166 0.0014 0.0106 0.0149 0.0018 0.0003 0.9022 0.0187 0.0197 0.0152 feature weight, J- , is updated. The result for the first iteration shows that the features weight of weight, age, and BMI are close to 0, while the feature weight of height is close to 1. The threshold for discarding features is defined by 1⁄w = 1⁄w 23 4 = 0.1043. Therefore, the weight, age, and BMI features are discarded from the clustering process. The next step is adjusting J- , where K-x = k1 6 @yz ∑t78 kt Height 177.9485 189.6042 199.0460 Weight 74.4266 81.9087 115.3002 Age 20.9808 21.8875 21.1878 BMI 23.4568 23.2556 30.3886 TABLE IV FCM CLUSTERING RESULT Player’s name Anjas Rusadi Putra Antoni Erga Ardian Ariadi Aldi Falentino Alexander Franklyn Bryan Adha Elang David Liberty Nuban Elyakim Tampa'i Febrianus Gregory Fransiscus Bryan Henry Cornelis Lakay Raymond Putra Fajar Randy Ady Prasetya Mas Kahono Alif Rian Sanjaya Janson Kurniawan M. Yassir Alkatiri Martin Steven Ray Jody Sebastian Peter Surjantoro Fauji Ridho Pamungkas Clustering results Cluster Position 2 Forward 1 Guard 1 Guard 1 Guard 2 Forward 3 Center 2 Forward 1 Guard 1 Guard 2 Forward 2 Forward 3 Center 2 Forward 2 Forward 1 Guard 1 Guard 1 Guard 1 Guard 1 Guard 3 Center 1 Guard 2 Forward 2 Forward = [1.00]. The new J- is used for the next iteration, along with the new cluster center and the new number of features. This process is repeated until the stop condition is reached. Table V shows the feature weights for each feature in each iteration. Tables VI and VII show the final membership values and the final cluster center of FRFCM clustering results, respectively. The final cluster center consists of just one feature, i.e., height, where the guard players (Cluster 1) have an average height of 177.8513, the forward players (Cluster 2) have an average height of 188.0854, and the center players (Cluster 3) have an average height of 197.9934. TABLE III CLUSTER CENTER OF FCM CLUSTERING RESULT Cluster Cluster 1 Cluster 2 Cluster 3 , such that the new J- Real position Forward Guard Guard Guard Guard Center Forward Guard Guard Guard Center Center Center Forward Guard Guard Forward Guard Guard Center Guard Forward Forward TABLE V FEATURE WEIGHTS IN EACH I TERATION Iteration Initialization Height 0.25 Iteration 1 1 Iteration 2 1.00 Weight 0.25 2.1265e27 - Age 0.25 1.6183e44 - BMI 0.25 2.9226e43 - According to Table VI, the cluster of each data can be determined by choosing the highest value of membership value from Table VII. The clustering results of FRFCM is shown in Table VIII, where 14 players are on the guard position, five players are on the forward position, and four players are on the center position. As can be seen in Table VIII, there is just one player with incorrect position (see the bold font), i.e., M. Yassir Alkatiri. He tends to be tall but FRFCM put him on the guard position, so he will be more suitable to play on the forward position, because he has a tall and big body to break into the opponent's ring. Therefore, the ER of the FRFCM clustering result is 0.0435. 419 TABLE VI MEMBERSHIP VALUES OF FRFCM CLUSTERING RESULT Player’s name Anjas Rusadi Putra Antoni Erga Ardian Ariadi Aldi Falentino Alexander Franklyn Bryan Adha Elang David Liberty Nuban Elyakim Tampa'i Febrianus Gregory Fransiscus Bryan Henry Cornelis Lakay Raymond Putra Fajar Randy Ady Prasetya Mas Kahono Alif Rian Sanjaya Janson Kurniawan M. Yassir Alkatiri Martin Steven Ray Jody Sebastian Peter Surjantoro Fauji Ridho Pamungkas Cluster 1 0.0245 0.9985 0.9692 0.7782 0.3577 0.0068 0.0245 0.9047 0.8891 0.5583 0.0068 0.0196 0.0332 0.0528 0.9943 0.9943 0.3577 0.9985 0.9943 0.0541 0.7782 0.0772 0.0066 Cluster 2 0.9191 0.0012 0.0253 0.1561 0.5864 0.0329 0.9191 0.0709 0.0933 0.3919 0.0329 0.1114 0.0941 0.6323 0.0045 0.0045 0.5864 0.0012 0.0045 0.138 0.1561 0.8913 0.9835 combinations consist of one feature only, two features, three features, and all four features. The advantage of FRFCM is that the algorithm can automatically find the related feature(s) during the clustering process by updating the feature weight and discarding the unrelated feature(s). Cluster 3 0.0565 0.0003 0.0055 0.0657 0.0559 0.9604 0.0565 0.0243 0.0176 0.0498 0.9604 0.869 0.8727 0.3149 0.0012 0.0012 0.0559 0.0003 0.0012 0.808 0.0657 0.0315 0.0099 IV. CONCLUSION The FRFCM clustering algorithm can group the basketball players’ positions. FRFCM is done by weighting each feature with feature weight and discards the feature(s) with a small feature weight. The weighting and discarding processes are included in the clustering process simultaneously. There are four features of the players’ physical condition, i.e., height, weight, age, and BMI. FRFCM finds the height feature as the most related physical condition to determine the players’ position, especially for Satya Wacana Saints team. By comparing the clustering result with the actual position, FRFCM obtains only one incorrect position, such that the ER is 0.0435. Different from the previous research conducted by Erga and Nataliani [19], where height and BMI are the most important features, they found the related feature(s) by combining the related feature(s) one by one. This method can help the coach to decide the basketball player’s position. For future works, FRFCM can be implemented on highdimensional data related to more complex players’ features. The features used are not limited to the physical conditions, but other measurements, for example, jumping ability, shooting score, rebound skill, can also be used to determine the player’s position. TABLE VII CLUSTER CENTER OF FRFCM CLUSTERING RESULT Cluster Cluster 1 Cluster 2 Cluster 3 Height 177.8513 188.0854 197.9934 TABLE VIII FRFCM CLUSTERING RESULTS Player’s name Anjas Rusadi Putra Antoni Erga Ardian Ariadi Aldi Falentino Alexander Franklyn Bryan Adha Elang David Liberty Nuban Elyakim Tampa'i Febrianus Gregory Fransiscus Bryan Henry Cornelis Lakay Raymond Putra Fajar Randy Ady Prasetya Mas Kahono Alif Rian Sanjaya Janson Kurniawan M. 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