add: task4

Dockerfile

@@ -22,7 +22,6 @@ WORKDIR /app
 
 COPY . /app/
 
-# Compile ALL the necessary tasks during the build
-RUN mpic++ -O2 -std=c++17 -o mpi_task task.cpp
-RUN mpic++ -O2 -std=c++17 -o task1 task1.cpp
-RUN mpic++ -O2 -std=c++17 -o task2 task2.cpp # <-- ADD THIS LINE
+# Compile ONLY task4 during the build to avoid unrelated compile failures
+RUN mpic++ -O2 -std=c++17 -o task4 task4.cpp
+RUN chmod +x /app/task4 || true

task4.cpp

@@ -0,0 +1,46 @@
+#include <mpi.h>
+#include <cstdio>
+
+int main(int argc, char** argv) {
+  int rank;
+  int worldSize;
+
+  MPI_Init(&argc, &argv);
+  MPI_Comm_size(MPI_COMM_WORLD, &worldSize);
+  MPI_Comm_rank(MPI_COMM_WORLD, &rank);
+
+  const int elementsPerProcess = 5;
+  int sendBuffer[elementsPerProcess];
+  for (int i = 0; i < elementsPerProcess; ++i) {
+    sendBuffer[i] = rank;  // Each process fills its buffer with its rank
+  }
+
+  int* receiveBuffer = nullptr;
+  if (rank == 0) {
+    receiveBuffer = new int[elementsPerProcess * worldSize];
+  }
+
+  MPI_Gather(
+    sendBuffer,                 // send buffer
+    elementsPerProcess,         // number of elements sent
+    MPI_INT,                    // type of each element sent
+    receiveBuffer,              // receive buffer (root only)
+    elementsPerProcess,         // number of elements received from each proc
+    MPI_INT,                    // type of each element received
+    0,                          // root rank
+    MPI_COMM_WORLD              // communicator
+  );
+
+  if (rank == 0) {
+    for (int p = 0; p < worldSize; ++p) {
+      for (int i = 0; i < elementsPerProcess; ++i) {
+        std::printf("Process %d sent: %d\n", p, receiveBuffer[p * elementsPerProcess + i]);
+      }
+    }
+    delete[] receiveBuffer;
+  }
+
+  std::printf("Bye from %d\n", rank);
+  MPI_Finalize();
+  return 0;
+}