Program for First Fit algorithm in Memory Management

Prerequisite : Partition Allocation Methods
In the first fit, the partition is allocated which is first sufficient from the top of Main Memory.
Example : 

Input : blockSize[]   = {100, 500, 200, 300, 600};
        processSize[] = {212, 417, 112, 426};
Output:
Process No.    Process Size    Block no.
   1               212            2
   2               417            5
   3               112            2
   4               426        Not Allocated

• Its advantage is that it is the fastest search as it searches only the first block i.e. enough to assign a process.
• It may have problems of not allowing processes to take space even if it was possible to allocate. Consider the above example, process number 4 (of size 426) does not get memory. However it was possible to allocate memory if we had allocated using best fit allocation [block number 4 (of size 300) to process 1, block number 2 to process 2, block number 3 to process 3 and block number 5 to process 4].

Implementation:
1- Input memory blocks with size and processes with size.
2- Initialize all memory blocks as free.
3- Start by picking each process and check if it can
   be assigned to current block. 
4- If size-of-process <= size-of-block if yes then 
   assign and check for next process.
5- If not then keep checking the further blocks.

Below is an implementation of above steps.
 

// C++ implementation of First - Fit algorithm
#include<bits/stdc++.h>
using namespace std;

// Function to allocate memory to 
// blocks as per First fit algorithm
void firstFit(int blockSize[], int m, 
              int processSize[], int n)
{
    // Stores block id of the 
    // block allocated to a process
    int allocation[n];

    // Initially no block is assigned to any process
    memset(allocation, -1, sizeof(allocation));

    // pick each process and find suitable blocks
    // according to its size ad assign to it
    for (int i = 0; i < n; i++)
    {
        for (int j = 0; j < m; j++)
        {
            if (blockSize[j] >= processSize[i])
            {
                // allocate block j to p[i] process
                allocation[i] = j;

                // Reduce available memory in this block.
                blockSize[j] -= processSize[i];

                break;
            }
        }
    }

    cout << "\nProcess No.\tProcess Size\tBlock no.\n";
    for (int i = 0; i < n; i++)
    {
        cout << " " << i+1 << "\t\t" 
             << processSize[i] << "\t\t";
        if (allocation[i] != -1)
            cout << allocation[i] + 1;
        else
            cout << "Not Allocated";
        cout << endl;
    }
}

// Driver code
int main()
{
    int blockSize[] = {100, 500, 200, 300, 600};
    int processSize[] = {212, 417, 112, 426};
    int m = sizeof(blockSize) / sizeof(blockSize[0]);
    int n = sizeof(processSize) / sizeof(processSize[0]);

    firstFit(blockSize, m, processSize, n);

    return 0 ;
}

// C implementation of First - Fit algorithm
#include<stdio.h>

// Function to allocate memory to
// blocks as per First fit algorithm
void firstFit(int blockSize[], int m, int processSize[], int n)
{
    int i, j;
    // Stores block id of the
    // block allocated to a process
    int allocation[n];

    // Initially no block is assigned to any process
    for(i = 0; i < n; i++)
    {
        allocation[i] = -1;
    }
  
    // pick each process and find suitable blocks
    // according to its size ad assign to it
    for (i = 0; i < n; i++)        //here, n -> number of processes
    {
        for (j = 0; j < m; j++)        //here, m -> number of blocks
        {
            if (blockSize[j] >= processSize[i])
            {
                // allocating block j to the ith process
                allocation[i] = j;

                // Reduce available memory in this block.
                blockSize[j] -= processSize[i];

                break;    //go to the next process in the queue
            }
        }
    }

    printf("\nProcess No.\tProcess Size\tBlock no.\n");
    for (int i = 0; i < n; i++)
    {
        printf(" %i\t\t\t", i+1);
        printf("%i\t\t\t\t", processSize[i]);
        if (allocation[i] != -1)
            printf("%i", allocation[i] + 1);
        else
            printf("Not Allocated");
        printf("\n");
    }
}

// Driver code
int main()
{
    int m;    //number of blocks in the memory
      int n;  //number of processes in the input queue
      int blockSize[] = {100, 500, 200, 300, 600};
    int processSize[] = {212, 417, 112, 426};
    m = sizeof(blockSize) / sizeof(blockSize[0]);
    n = sizeof(processSize) / sizeof(processSize[0]);

    firstFit(blockSize, m, processSize, n);

    return 0 ;
}

// Java implementation of First - Fit algorithm

// Java implementation of First - Fit algorithm
class GFG 
{
    // Method to allocate memory to 
    // blocks as per First fit algorithm
    static void firstFit(int blockSize[], int m, 
                         int processSize[], int n)
    {
        // Stores block id of the 
        // block allocated to a process
        int allocation[] = new int[n];
    
        // Initially no block is assigned to any process
        for (int i = 0; i < allocation.length; i++)
            allocation[i] = -1;
    
        // pick each process and find suitable blocks
        // according to its size ad assign to it
        for (int i = 0; i < n; i++)
        {
            for (int j = 0; j < m; j++)
            {
                if (blockSize[j] >= processSize[i])
                {
                    // allocate block j to p[i] process
                    allocation[i] = j;
    
                    // Reduce available memory in this block.
                    blockSize[j] -= processSize[i];
    
                    break;
                }
            }
        }
    
        System.out.println("\nProcess No.\tProcess Size\tBlock no.");
        for (int i = 0; i < n; i++)
        {
            System.out.print(" " + (i+1) + "\t\t" + 
                             processSize[i] + "\t\t");
            if (allocation[i] != -1)
                System.out.print(allocation[i] + 1);
            else
                System.out.print("Not Allocated");
            System.out.println();
        }
    }
    
    // Driver Code
    public static void main(String[] args)
    {
        int blockSize[] = {100, 500, 200, 300, 600};
        int processSize[] = {212, 417, 112, 426};
        int m = blockSize.length;
        int n = processSize.length;
        
        firstFit(blockSize, m, processSize, n);
    }
}

# Python3 implementation of First-Fit algorithm 

# Function to allocate memory to 
# blocks as per First fit algorithm 
def firstFit(blockSize, m, processSize, n):
    
    # Stores block id of the 
    # block allocated to a process 
    allocation = [-1] * n 

    # Initially no block is assigned to any process

    # pick each process and find suitable blocks 
    # according to its size ad assign to it 
    for i in range(n):
        for j in range(m):
            if blockSize[j] >= processSize[i]:
                
                # allocate block j to p[i] process 
                allocation[i] = j 

                # Reduce available memory in this block. 
                blockSize[j] -= processSize[i] 

                break

    print(" Process No. Process Size      Block no.")
    for i in range(n):
        print(" ", i + 1, "         ", processSize[i], 
                          "         ", end = " ") 
        if allocation[i] != -1: 
            print(allocation[i] + 1) 
        else:
            print("Not Allocated")

# Driver code 
if __name__ == '__main__': 
    blockSize = [100, 500, 200, 300, 600] 
    processSize = [212, 417, 112, 426]
    m = len(blockSize)
    n = len(processSize)

    firstFit(blockSize, m, processSize, n)
    
# This code is contributed by PranchalK

// C# implementation of First - Fit algorithm
using System;

class GFG 
{
    // Method to allocate memory to 
    // blocks as per First fit algorithm
    static void firstFit(int []blockSize, int m, 
                         int []processSize, int n)
    {
        // Stores block id of the block 
        // allocated to a process
        int []allocation = new int[n];
    
        // Initially no block is assigned to any process
        for (int i = 0; i < allocation.Length; i++)
            allocation[i] = -1;
    
        // pick each process and find suitable blocks
        // according to its size ad assign to it
        for (int i = 0; i < n; i++)
        {
            for (int j = 0; j < m; j++)
            {
                if (blockSize[j] >= processSize[i])
                {
                    // allocate block j to p[i] process
                    allocation[i] = j;
    
                    // Reduce available memory in this block.
                    blockSize[j] -= processSize[i];
    
                    break;
                }
            }
        }
    
    Console.WriteLine("\nProcess No.\tProcess Size\tBlock no.");
        for (int i = 0; i < n; i++)
        {
            Console.Write(" " + (i+1) + "\t\t" + 
                          processSize[i] + "\t\t");
            if (allocation[i] != -1)
            Console.Write(allocation[i] + 1);
            else
                Console.Write("Not Allocated");
            Console.WriteLine();
        }
    }
    
    // Driver Code
    public static void Main()
    {
        int []blockSize = {100, 500, 200, 300, 600};
        int []processSize = {212, 417, 112, 426};
        int m = blockSize.Length;
        int n = processSize.Length;
        
        firstFit(blockSize, m, processSize, n);
    }
}

// This code is contributed by nitin mittal.

// Javascript implementation
<script>

const firstFit = (blockSize,m,processSize,n) => {

        // Stores block id of the 
        // block allocated to a process
        const allocation = [];
    
        // Initially no block is assigned to any process
        for (let i = 0; i < allocation.length; i++)
            allocation[i] = -1;
    
        // pick each process and find suitable blocks
        // according to its size ad assign to it
        for (let i = 0; i < n; i++)
        {
            for (let j = 0; j < m; j++)
            {
                if (blockSize[j] >= processSize[i])
                {
                    // allocate block j to p[i] process
                    allocation[i] = j;
    
                    // Reduce available memory in this block.
                    blockSize[j] -= processSize[i];
    
                    break;
                }
            }
         }
         
         document.write("\nProcess No.\tProcess Size\tBlock no.");
        for (let i = 0; i < n; i++)
        {
            let s = "Not Allocated"
            if (allocation[i] > -1)
                document.write(" " + (i+1) + "\t\t\t\t" + 
                             processSize[i] + "\t\t\t\t"+ (allocation[i] + 1));
            else
                document.write(" " + (i+1) + "\t\t\t\t" + 
                             processSize[i] + "\t\t\t\tNot Allocated");
            
        }
    }
        
        
    const blockSize = [100, 500, 200, 300, 600];
        const processSize = [212, 417, 112, 426];
        let m = blockSize.length;
        let n = processSize.length;
        
        firstFit(blockSize, m, processSize, n);

// This code is contributed by ashishsingh13122000.
</script>

Output : 
 

Process No.    Process Size    Block no.
1              212             2
2              417             5        
3              112             2
4              426             Not Allocated

Time complexity of First Fit algorithm is O(n*m), where n is the number of processes and m is the number of memory blocks. The outer for loop runs for n times and the inner for loop runs for m times.
Auxiliary Space of First Fit algorithm is O(n), where n is the number of processes. The allocation array is used to store the block number allocated to the process, which takes a space of O(n).
 

First Fit, Best Fit, Next Fit and Worst Fit Algorithms in OS

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