fscanf() in C - need help reading a file using pointer notation - c

Question

Spent an hour or two trying to debug this, but cannot figure out why it won't read my file correctly.
FILE *input;
int numAccounts = 7;
char **accountNames = malloc(sizeof(char)*numAccounts*10);
int *accountNumbers = malloc(sizeof(int)*numAccounts);
float *accountValues = malloc(sizeof(float)*numAccounts);
char *fileName = malloc(sizeof(char)*20);
input=fopen("input.txt","r");
int i;
for(i=0;i<numAccounts;i++) {
fscanf(input,"%s%d%f",*(accountNames+i),accountNumbers+i,accountValues+i);
printf("\n%s %d %f", *(accountNames+i), *(accountNumbers+i), *(accountValues+i));
}
fclose(input);
return 0;
And here is input.txt
Brown 1435 234.55
Dunn 2091 2011.75
Smith 8766 945.05
Stone 4530 0.0
Becker 9073 6235.75
Rich 1003 -42.00
Doe 6739 3655.80
Thank you!

Answer 1

You should start with non-dynamic allocation,
int main()
{
FILE *input = NULL;
int numAccounts = 7;
char accountNames[20] = "";
int accountNumbers = 0;
float accountValues = 0;
input=fopen("input.txt","r");
int i;
for(i=0;i<numAccounts;i++)
{
fscanf(input,"%s %d %f",accountNames,&accountNumbers,&accountValues);
printf("\n%s %d %f\n", accountNames, accountNumbers, accountValues);
}
fclose(input);
return 0;
}
If you must do dynamic allocation, then this pointer-to-pointer to char allocation is incorrect:
char **accountNames = malloc(sizeof(char)*numAccounts*10);
You'd need to allocate numAccounts of char * first, then for each char *, allocate 10 char. That can be done as follows:
char **accountNames = malloc(sizeof(char*)*numAccounts); // allocate numAccounts of char *
int i;
for( i=0; i < numAccounts; i++ )
{
*(accountNames + i) = malloc(sizeof(char)*10); // allocate 10 of char
}
So this should work:
#include <stdio.h>
#include <stdlib.h>
int main()
{
FILE *input = NULL;
int numAccounts = 7;
char **accountNames = malloc(sizeof(char*)*numAccounts); // allocate numAccounts of char *
int i;
for( i=0; i < numAccounts; i++ )
{
*(accountNames + i) = malloc(sizeof(char)*10); // allocate 10 of char
}
int *accountNumbers = malloc(sizeof(int)*numAccounts);
float *accountValues = malloc(sizeof(float)*numAccounts);
input=fopen("input.txt","r");
for(i=0; i<numAccounts; i++)
{
fscanf(input,"%s%d%f",*(accountNames+i), accountNumbers+i, accountValues+i);
printf("\n%s %d %f", *(accountNames+i), *(accountNumbers+i), *(accountValues+i));
}
fclose(input);
return 0;
}

Answer 2

If the account values are currency. You may want to save them as ints and divide by the lowest unity of that currency.
So instead of saving float accountValue = 23.25 as 23.25, save it as 2335 (23.25 * 100).
Then when you read it divide by 100 to get that value back.
float accountValueDollars;
int accountValueInCents;
fscanf(..., &accountValueInCents);
accountValueDollars = accountValueInCents / 100;
Also, you may want to consider saving in binary format, this way you don't have to worry about parsing the text.
typedef struct _account_t{
char name[50];
int account_number;
double amount;
} account_t;
int how_many = 0;
account_t * accounts = NULL;
// error checking skipped
fread(&how_many, 1, sizeof(int), input);
accounts = (account_t*)malloc(sizeof(account_t) * how_many);
for(int i = 0; i < how_many; i++){
fread(&accounts[i], 1, sizeof(account_t), input);
// print here or whatever
}
fclose(input);
NOTE: Untested code.