C API example (#4333)

demo/c-api/Makefile

@@ -0,0 +1,19 @@
+SRC=c-api-demo.c
+TGT=c-api-demo
+
+cc=cc
+CFLAGS ?=-O3
+XGBOOST_ROOT ?=../..
+INCLUDE_DIR=-I$(XGBOOST_ROOT)/include -I$(XGBOOST_ROOT)/dmlc-core/include -I$(XGBOOST_ROOT)/rabit/include
+LIB_DIR=-L$(XGBOOST_ROOT)/lib
+
+build: $(TGT)
+
+$(TGT): $(SRC) Makefile
+	$(cc) $(CFLAGS) $(INCLUDE_DIR) $(LIB_DIR) -o $(TGT) $(SRC) -lxgboost
+
+run: $(TGT)
+	LD_LIBRARY_PATH=$(XGBOOST_ROOT)/lib ./$(TGT)
+
+clean:
+	rm -f $(TGT)

demo/c-api/c-api-demo.c

@@ -0,0 +1,89 @@
+/*!
+ * Copyright 2019 XGBoost contributors
+ *
+ * \file c-api-demo.c
+ * \brief A simple example of using xgboost C API.
+ */
+
+#include <stdio.h>
+#include <stdlib.h>
+#include <xgboost/c_api.h>
+
+#define safe_xgboost(call) {                                            \
+int err = (call);                                                       \
+if (err != 0) {                                                         \
+  fprintf(stderr, "%s:%d: error in %s: %s\n", __FILE__, __LINE__, #call, XGBGetLastError()); \
+  exit(1);                                                              \
+}                                                                       \
+}
+
+int main(int argc, char** argv) {
+  int silent = 0;
+  int use_gpu = 0;  // set to 1 to use the GPU for training
+  
+  // load the data
+  DMatrixHandle dtrain, dtest;
+  safe_xgboost(XGDMatrixCreateFromFile("../data/agaricus.txt.train", silent, &dtrain));
+  safe_xgboost(XGDMatrixCreateFromFile("../data/agaricus.txt.test", silent, &dtest));
+  
+  // create the booster
+  BoosterHandle booster;
+  DMatrixHandle eval_dmats[2] = {dtrain, dtest};
+  safe_xgboost(XGBoosterCreate(eval_dmats, 2, &booster));
+
+  // configure the training
+  // available parameters are described here:
+  //   https://xgboost.readthedocs.io/en/latest/parameter.html
+  safe_xgboost(XGBoosterSetParam(booster, "tree_method", use_gpu ? "gpu_hist" : "hist"));
+  if (use_gpu) {
+    // set the number of GPUs and the first GPU to use;
+    // this is not necessary, but provided here as an illustration
+    safe_xgboost(XGBoosterSetParam(booster, "n_gpus", "1"));
+    safe_xgboost(XGBoosterSetParam(booster, "gpu_id", "0"));
+  } else {
+    // avoid evaluating objective and metric on a GPU
+    safe_xgboost(XGBoosterSetParam(booster, "n_gpus", "0"));
+  }
+
+  safe_xgboost(XGBoosterSetParam(booster, "objective", "binary:logistic"));
+  safe_xgboost(XGBoosterSetParam(booster, "min_child_weight", "1"));
+  safe_xgboost(XGBoosterSetParam(booster, "gamma", "0.1"));
+  safe_xgboost(XGBoosterSetParam(booster, "max_depth", "3"));
+  safe_xgboost(XGBoosterSetParam(booster, "verbosity", silent ? "0" : "1"));
+  
+  // train and evaluate for 10 iterations
+  int n_trees = 10;
+  const char* eval_names[2] = {"train", "test"};
+  const char* eval_result = NULL;
+  for (int i = 0; i < n_trees; ++i) {
+    safe_xgboost(XGBoosterUpdateOneIter(booster, i, dtrain));
+    safe_xgboost(XGBoosterEvalOneIter(booster, i, eval_dmats, eval_names, 2, &eval_result));
+    printf("%s\n", eval_result);
+  }
+
+  // predict
+  bst_ulong out_len = 0;
+  const float* out_result = NULL;
+  int n_print = 10;
+
+  safe_xgboost(XGBoosterPredict(booster, dtest, 0, 0, &out_len, &out_result));
+  printf("y_pred: ");
+  for (int i = 0; i < n_print; ++i) {
+    printf("%1.4f ", out_result[i]);
+  }
+  printf("\n");
+
+  // print true labels
+  safe_xgboost(XGDMatrixGetFloatInfo(dtest, "label", &out_len, &out_result));
+  printf("y_test: ");
+  for (int i = 0; i < n_print; ++i) {
+    printf("%1.4f ", out_result[i]);
+  }
+  printf("\n");
+
+  // free everything
+  safe_xgboost(XGBoosterFree(booster));
+  safe_xgboost(XGDMatrixFree(dtrain));
+  safe_xgboost(XGDMatrixFree(dtest));
+  return 0;
+}