Example of how a module works (simple)

//updated 10:48pm 22/12 added few comments
//updated 4:28pm 23/12 comment of how to use save load
//updated 4:54pm added function declaration and function with parameters (with and without pointers)

#include <stdio.h>

typedef struct{
    char orderId[6], foodId[6], memberId[6];
    int quantity;
} Order;

//Global variable (you can change it to local, but need to pass by reference to each function)
//Structure list (for faster read/write)
Order orderList[100];
//How many structure inside the list?
int orderCount=0;

//function declaration
void del();
void search();
void modify();
void add()
void display();
int userIsConfirm(char *print);
void load();
void save();
void getUserInputOrderWithPointers(int isModify, Order *order);
Order getUserInputOrder(int isModify);

void main(){
    int choice;
    load(); //read data once enough, then no need read again, only write it back
    //load data to orderlist, then we modify the list, and save it back
    //so no need to read it as the data is stored in the list
    do{
        printf("\n\n1. Add\n2. Display\n3. Search\n4. Modify\n5. Delete\n6. Quit\nEnter: ");
        rewind(stdin);
        scanf("%d", &choice);
        if(choice == 1){
            add();
        }else if(choice == 2){
            display();
        }else if(choice == 3){
            search();
        }else if(choice == 4){
            modify();
        }else if(choice == 5){
            del();
        }
    }while(choice != 6);
}

void del(){
    //i use order to store data, so dont need to declare many string
    Order orderTemp;
    char userConfirm;
    int orderIndex, isFound, i;

    //initialize
    isFound=0; //0 = false, rest = true

    //display to user so they know which one to delete
    display();

    //get user input for which order they want to delete
    printf("Enter record of order ID to delete : ");
    rewind(stdin);
    gets(orderTemp.orderId);

    //find data from list
    for(i=0; i<orderCount; i++){
        //if order ID match
        if(strcmp(orderList[i].orderId, orderTemp.orderId) == 0){
            //save current index to orderIndex, for further reference
            orderIndex = i;
            //isFound = 1 or "true"
            isFound = 1;
            //this one is optional, but for long array (>10000) it is better to do this
            break;
        }
    }

    //found it! Get user input to update data
    if(isFound){
        if(userIsConfirm("Confirm to delete (Y/N)? ")){
            //reduced 1 structure
            orderCount--;
            for(i=orderIndex; i<orderCount; i++){
                //moving data one step forward, eg: 1 2 3 4 5, remove 2, become 1 3 4 5
                orderList[i] = orderList[i+1];
            }
        }
    }else{
        printf("No data found :(\n");
    }
}

void search(){
    Order orderTemp;
    int i;
    //give user option to which column they want to find
    //optional: use if/switch statement to get which column they want to find
    printf("Leave blank to skip\n");
    printf("Enter record of order ID to search: ");
    rewind(stdin);
    gets(orderTemp.orderId);
    printf("Enter record of food ID to search: ");
    rewind(stdin);
    gets(orderTemp.foodId);
    printf("Enter record of member ID to search: ");
    rewind(stdin);
    gets(orderTemp.memberId);

    //find data from list
    printf("ORDER ID | FOOD ID | MEMBER ID | QUANTITY\n");
    printf("+++++++++|+++++++++|+++++++++++|+++++++++\n");
    for(i=0; i<orderCount;i++){
        //check if it is not empty, if it is not empty, then check memberId is same with what user want
        //if it is empty, then the first character in the string is '\0'
        if((orderTemp.orderId[0] != '\0' && strcmp(orderList[i].orderId, orderTemp.orderId) == 0) ||
           (orderTemp.foodId[0] != '\0' && strcmp(orderList[i].foodId, orderTemp.foodId) == 0) ||
           (orderTemp.memberId[0] != '\0' && strcmp(orderList[i].memberId, orderTemp.memberId) == 0)){
            printf(" %-7s | %-7s | %-9s | %d\n", orderList[i].orderId, orderList[i].foodId, orderList[i].memberId, orderList[i].quantity);
        }
    }
}

void modify(){
    //i use order to store data, so dont need to declare many string
    Order orderTemp;
    char userConfirm;
    int orderIndex, isFound, i;

    //initialize
    isFound=0; //0 = false, rest = true

    //display to user so they know which one to modify
    display();

    //get user input for which order they want to modify
    printf("Enter record of order ID to modify : ");
    rewind(stdin);
    gets(orderTemp.orderId);

    //find data from list
    for(i=0; i<orderCount; i++){
        //if order ID is match
        if(strcmp(orderList[i].orderId, orderTemp.orderId) == 0){
            //save current index to orderIndex, for further reference
            orderIndex = i;
            //isFound = 1 or "true"
            isFound = 1;
            //this one is optional, but for long array (>10000) it is better to do this
            break;
        }
    }

    //found it! Get user input to update data
    if(isFound){
        //get all data into temporary variable (here is orderTemp)
        //we use pointer to edit as the orderId is in ordertemp
        getUserInputOrderWithPointers(1, &orderTemp);

        //get confirmation to change
        if(userIsConfirm("Confirm to modify (Y/N)? ")){
            //structure can use =
            orderList[orderIndex] = orderTemp;
            save();//save after updated
            printf("Successfully updated list!\n");
        }
    }else{
        printf("No data found :(\n");
    }
}

void add(){
    Order order;

    //using function with parameter to get user input
    order = getUserInputOrder(0);

    //put in the order to order list
    orderList[orderCount] = order;

    //how above line works:
    //imagine currently has 0 order, so orderCount is 0
    //if you want to store something, you would store it at the first index (0), as there is no data
    //which we store it at orderList[0] = order;
    //and we can directly do orderList[orderCount] = order;

    //add 1 to total count, eg: now has 0, added, then become 1
    orderCount++;
    //now has 1 order, and orderCount is 1
    //when next time you want to store a data, you would store it at the second index (1), as there is no data
    //which we store it at orderList[1] = order;
    //and we can directly do orderList[orderCount] = order;

    //or a shorter code, combing both line become orderList[orderCount++] = order;

    save();//save after updated
}

void getUserInputOrderWithPointers(int isModify, Order *order){
    //using pointers to directly modify the order
    //if is not modify, then it is new, so we get user input for order
    if(isModify == 0){
        printf("Enter order ID: ");
        rewind(stdin);
        gets(order->orderId); //or gets((*order).orderId);
    }
    printf("Enter %sfood ID: ", isModify ? "new " : "");
    rewind(stdin);
    gets(order->foodId);  //or gets((*order).foodId);
    printf("Enter %smember ID: ", isModify ? "new " : "");
    rewind(stdin);
    gets(order->memberId); //or gets((*order).memberId);
    printf("Enter %squantity: ", isModify ? "new " : "");
    scanf("%d", &(order->quantity)); //or scanf("%d", &(*order).quantity);
}

Order getUserInputOrder(int isModify){
    //return a new order back to the line where it call
    Order order;
    //if is not modify, then it is new, so we get user input for order
    if(isModify == 0){
        printf("Enter order ID: ");
        rewind(stdin);
        gets(order.orderId);
    }
    //the isModify ? "new " : "" is same as if(isModify){ "new" } else { "" } in a simplier form
    printf("Enter %sfood ID: ", isModify ? "new " : "");
    rewind(stdin);
    gets(order.foodId);
    printf("Enter %smember ID: ", isModify ? "new " : "");
    rewind(stdin);
    gets(order.memberId);
    printf("Enter %squantity: ", isModify ? "new " : "");
    scanf("%d", &order.quantity);
    return order;
}

void display(){
    int i;
    printf("ORDER ID | FOOD ID | MEMBER ID | QUANTITY\n");
    printf("+++++++++|+++++++++|+++++++++++|+++++++++\n");
    for(i=0;i<orderCount;i++){
        printf(" %-7s | %-7s | %-9s | %d\n", orderList[i].orderId, orderList[i].foodId, orderList[i].memberId, orderList[i].quantity);
    }
}

void save(){
    FILE *fptr=fopen("order.txt", "w");
    int i;
    if(fptr==NULL){
        printf("Error! Unable open file!\n");
        return;
    }
    //write data from index 0 to orderCount-1 (which is from first to last)
    for(i=0; i<orderCount;i++){
        fprintf(fptr, "%s|%s|%s|%d\n", orderList[i].orderId, orderList[i].foodId, orderList[i].memberId, orderList[i].quantity);
    }
    fclose(fptr);
}

void load(){
    FILE *fptr=fopen("order.txt", "r");
    if(fptr==NULL){
        printf("Error! Unable open file!\n");
        return;
    }
    orderCount = 0;
    //read data
    while(fscanf(fptr, "%[^|]|%[^|]|%[^|]|%d\n", orderList[orderCount].orderId, orderList[orderCount].foodId, orderList[orderCount].memberId, &orderList[orderCount].quantity)!=EOF){
        //orderCount increase by one with each data has been written to orderList
        orderCount++;
    }
    fclose(fptr);
}

int userIsConfirm(char *print){
    char userConfirm;
    do{
        printf("%s", print);
        rewind(stdin);
        userConfirm = tolower(getchar());
        if(userConfirm != 'y' && userConfirm != 'n')
            printf("Error! Invalid input! Either Y or N!\n");
    }while(userConfirm != 'y' && userConfirm != 'n');
    return userConfirm == 'y';
}