Question

I am currently in an introductory c++ class and am working assignment that sorts data. We recently covered structs and I decided to use structs to approach the problem rather than create 3 arrays to hold the information from our data file.

The trouble i’m having is when i’m trying to pass my struct to my function.
Here is my error:

analyze_data.cpp: In function ‘int main()’:
analyze_data.cpp:76: error: conversion from ‘weather*’ to non-scalar type ‘weather’ requested
analyze_data.cpp: In function ‘int find_pos_of_smallest(weather, int, int)’:
analyze_data.cpp:110: error: no match for ‘operator[]’ in ‘data[pos]’
analyze_data.cpp:110: error: no match for ‘operator[]’ in ‘data[pos_of_smallest]’

I don’t understand the error from line 76. I have done some research about passing structs to functions, and what i found was adding the “&” in the type declarations. However, i have no idea what it does or why i would need to do that as we haven’t covered it in class. I also did try it, but i just got a different set of errors. So i figured i’d not post those and just start from what i know.

Here is my code

  /*
    Program name: Analyze data
    Program discription:  This program will read a data file named data.txt.
      This data file is expected to be formated in a specific way and contain
      specific weather information.  The program will analyize this data and
      return max, min temperatures, 'perfect days', how many cold fronts per
      year, 10 coldest  and hottest days in a year and finally find the 5
      median days of the year.

    Date:  10/1/2012
  */

  #include <iostream>
  #include <fstream>

  using namespace std;

  /*
  * Declare struct's here
  *
  */ 
  struct weather
  {
    string date;
    int high;
    int low;
  };

  /* 
  *  Forward declaration of a function.  This declares the function,
  *  but does not define it.  (Notice that there is no code, just
  *  a function header with a semicolon after it.
  *
  */
  int find_pos_of_smallest (weather data, int start_pos, int end_pos);

  /* Our main function.
  *
  * Parameters:
  *    none
  *
  * Return value:
  *    0 if we complete successfully, 1 if there was an error.
  */
  int main()
  {
    //read the data file
    ifstream weather_data("data.txt");
    //declare array size, and then create array using struct
    int days = 365;
    weather data[days];

    //store the data.txt in the array and then close the file
    for (int i=0; i<days; i++)
    {
      weather_data >> data[i].date;
      weather_data >> data[i].high;
      weather_data >> data[i].low;
    }
    weather_data.close();

    ofstream data_results ("results.txt");
    // create the first 3 lines of the output reults in the following formated
    data_results << "Assignment #5\n"
                << "CS 1410/2000\n"
                << "Jonathan Larsen\n";
    /*
    for(int i=0; i<3; i++)
    {
      cout<<data[i].date << " "<<data[i].high << " " << data[i].low<<endl;
    }
    cout<<endl;
    */  
    cout<<find_pos_of_smallest(data, 0, days)<<endl;


    return 0;  //no error so return a zero  
  }//end of program


  /**** FUNCTIONS ****/

  /* Write down exactly what the function will do (a postcondition).
  * Write down what is required to use the function (any preconditions).
  * Write down any other behavior or comments that will help a programmer.
  *
  * Parameters:   (list parameters by type and name, and explain them)
  *   int example -- an example parameter
  *
  * Returns:
  *   double -- an example return value
  */

  /* Returns the position of the smallest value found in the specified
  * subarray.  (Only the elements in the subarray
  * between start_pos and end_pos inclusive are checked.)
  *
  * Parameters:
  *    d - a data array
  *    start_pos - the first position to check
  *    end_pos - the last position to check
  */
  int find_pos_of_smallest (weather data, int start_pos, int end_pos)
  {
    int pos_of_smallest = start_pos;

    for (int pos = start_pos+1; pos <= end_pos; pos++)
      if (data[pos].low < data[pos_of_smallest].low)
        pos_of_smallest = pos;

    return pos_of_smallest;
  }

Answer 1

You need to pass your array as a pointer:

int find_pos_of_smallest (weather *data, int start_pos, int end_pos)

Declaring the parameter as weather data means just a single instance of the struct. Making it a pointer means you can pass an array (the pointer will get the address of the first element in the array, and when you index it with data[pos] you will get the relevant element in that array).