How to edit .csv files in C - c

Question

I'm new at programming, and I need help in my C project. I have to search for a city, confirm it exists in the first file (city.csv), and take its id from there. Then I have to match that id with the corresponding one in the second file (meteo.csv), and then edit its weather information, that is in that second file. However, I don't know how I can take the city id from the first file, and then how to edit the second file after obtaining all the new weather informations. Here is the code:
void addInfo() {
FILE * fp;
char id_city[100];
char city[100];
char humidity[100];
char temp_max[100];
char temp_min[100];
char pressure[100];
char date[100];
printf("Name of the city: ");
scanf("%s", city);
// I think it's here that I have to write the code for take the city's id from the first file
if (id_city != NULL) {
printf("Maximun temperature: ");
scanf("%s", temp_max);
printf("Minimun temperature: ");
scanf("%s", temp_min);
printf("Humidity: ");
scanf("%s", humidity);
printf("Pressure: ");
scanf("%s", pressure);
printf("Date, in the format YYYY-MM-DD: ");
scanf("%s", date);
fp = fopen ("meteo.csv", "a");
fprintf(fp, "%s, %s, %s, %s, %s \n", temp_max, temp_min, humidity, pressure, date); //I think there's something wrong here too...
fclose(fp);
printf("Information edited successfully");
}
The file city.csv has 152 lines and 4 columns:
(id_city,city,county,district)
such as
(56,Lisbon,Lisbon,Lisbon)
The file meteo.csv has 152 lines and 7 columns:
(id_meteo_city,id_city,temp_max,temp_min,humidity,pressure,date)
such as
(56,56,14,5,62,1025,2018-02-12)

Answer 1

The first thing I would do is encapsulate the data in a struct, that makes it
easier to map a line of a CSV file into an object representing a line.
If both files city.csv and meteo.csv have different columns, I'd create a
different struct for each file. If both files have the same columns, you could
use the struct. I assume that both files are different and that city has the
format meteo_id,city_id,name.
typedef struct city_t {
int meteo_id;
int city_id;
char name[100]; // no city should have
// longer than 100 chars
} city_t;
typedef struct meteo_t {
int meteo_id;
int city_id;
int tempt_max;
int tempt_mix;
double humidity;
double preassure;
char date[11];
} meteo_t;
Let's assume that both files are well formatted, otherwise you would have to
write code that checks for errors and handles them, that would be the next step
in the exercise, so I'm going to write only the basic version with basic error
recognition.
#include <stdio.h>
#include <string.h>
#include <errno.h>
// takes 2 params, the filename and a pointer
// to size_t where the number of cities is stored
city_t *read_cities(const char *filename, size_t *len)
{
if(filename == NULL || len == NULL)
return NULL;
FILE *fp = fopen(filename, "r");
if(fp == NULL)
{
fprintf(stderr, "Could not open %s: %s\n", filename, strerror(errno));
return NULL;
}
city_t *arr = NULL, *tmp;
*len = 0;
// assuming that no line will be longer than 1023 chars long
char line[1024];
while(fgets(line, sizeof line, fp))
{
tmp = realloc(arr, (*len + 1) * sizeof *arr);
if(tmp == NULL)
{
fprintf(stderr, "could not parse the whole file %s\n", filename);
// returning all parsed cities so far
if(*len == 0)
{
free(arr);
arr = NULL;
}
return arr;
}
arr = tmp;
// %99[^\n] is to read up to 99 characters until the end of the line
if(sscanf(line, "%d,%d,%99[^\n]", &(arr[*len].meteo_id),
&(arr[*len].city_id), arr[*len].name) != 3)
{
fprintf(stderr, "Invalid line format (skipping line):\n%s\n", line);
// skip this line, and decrement *len
(*len)--;
continue;
}
// incrementing only when parsing of line was OK
(*len)++;
}
fclose(fp);
// file is empty or
// all lines have wrong format
if(*len == 0)
{
free(arr);
arr = NULL;
}
return arr;
}
void print_cities(city_t *cities, size_t len, FILE *fp)
{
if(cities == NULL || fp == NULL)
return;
for(size_t i = 0; i < len; ++i)
fprintf(fp, "%d,%d,%s\n", cities[i].meteo_id, cities[i].citiy_id,
cities[i].name);
}
Now I've written the read and write functions for the file citiy.csv assuming the
format meteo_id;city_id;name. The print_cities allows you to print the CSV
content on the screen (passing stdout as the last argument) or to a file
(passing a FILE object as the last argument).
You can use these functions as templates for reading and writing meteo.csv, the
idea is the same.
You can use these function as follows:
int main(void)
{
size_t cities_len;
city_t *cities = read_cities("city.csv", &cities_len);
// error
if(cities == NULL)
return 1;
do_something_with_cities(cities, cities_len);
// update csv
FILE *fp = fopen("city.csv", "w");
if(fp == NULL)
{
fprintf(stderr, "Could not open city.csv for reading: %s\n",
strerror(errno));
free(cities);
return 1;
}
print_cities(cities, cities_len, fp);
fclose(fp);
free(cities);
return 0;
}
Now for your exercise: write a similar function that parses meteo.csv (using
my function as a template shouldn't be that difficult) and parse both files. Now
that you've got them in memory, it's easy to manipulate the data (insert,
update, delete). Then write the files like I did in the example and that's it.
One last hint: how to search for a city:
// returns the index in the array or -1 on error or when not found
int search_for_city_by_name(city_t *cities, size_t len, const char *name)
{
if(cities == NULL || name == NULL)
return -1;
for(size_t i = 0; i < len; ++i)
if(strcmp(name, cities[i].name) == 0)
return i;
// not found
return -1;
}
Now I have given you almost all parts of the assignment, all you have to do is
stick them together and write the same functions for the meteo.csv file.

Answer 2

To edit one field:
void _ERR(char a) {
if (a == "f") printf("\n\tError File !!\n\n");
if (a == "m") printf("\n\tError Memory !!\n\n");
exit(1); }
char* stmm(const char* src) {
char* dst = malloc(strlen(src) + 1);
if (dst == NULL) return NULL;
strcpy(dst, src);
return dst; }
const char* getfield(char* line, int num) {
const char* tok;
for (tok = strtok(line, ",");
tok && *tok;
tok = strtok(NULL, ",\n"))
{
if (!--num)
return tok;
}
return NULL; }
void edit_file(char* FName, char* NewValue, int row, int col) {
int i, r = 0, c;
char line[1024];
FILE* fr, * fw;
fr = fopen(FName, "r");
fw = fopen(FName, "r+");
if (fr == NULL|| fw == NULL) _ERR("f");
while (fgets(line, 1024, fr))
{
char* tmp = stmm(line);
if (tmp == NULL) _ERR("m");
for (i = 0, c = 1; i < strlen(tmp); i++) {
if (tmp[i] == 44) c++;
}
for (i = 0; i < c; i++) {
if (r == row && i+1 == col) {
fprintf(fw,"%s", NewValue);
} else {
free(tmp);
tmp = stmm(line);
if (tmp == NULL) _ERR("m");
fprintf(fw,"%s", getfield(tmp, i + 1));
}
(i < c - 1) ? fprintf(fw,",") : fprintf(fw,"\n");
}
free(tmp);
r++;
}
fclose(fr);
fclose(fw); }
edit_file(".\FileName.csv","NewValue",Row,Column);