How to implement Scanf while using pointers in C

Question

#include<stdio.h>
#include<stdlib.h>

void south_east(int *lat, int *lon){                       // lat and lon are the pointer variables which take addresses as input  , int *lat = &latitude
  *lat = *lat - 1 ;                                       // Using the * operator to update the values at the address.
  *lon = *lon + 1;
}

int main(int argc, char const *argv[]) {
  int latitude;
  int longitude;
  scanf("Enter Latitude and Longitude %i\n %i\n", latitude, longitude);
  south_east(&latitude,&longitude);                                        // We pass the addresses so that the function above can easily modify them
  printf("Move to [%i, %i]\n", latitude, longitude );
  return 0;
}

ships.c: In function 'main':
ships.c:14:40: warning: format '%i' expects argument of type 'int *', but argument 2 has type 'int' [-Wformat=]
   scanf("Enter Latitude and Longitude %i\n %i\n", latitude, longitude);
                                        ^
ships.c:14:45: warning: format '%i' expects argument of type 'int *', but argument 3 has type 'int' [-Wformat=]
   scanf("Enter Latitude and Longitude %i\n %i\n", latitude, longitude);
                                             ^
ships.c:14:3: warning: 'longitude' is used uninitialized in this function [-Wuninitialized]
   scanf("Enter Latitude and Longitude %i\n %i\n", latitude, longitude);
   ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
ships.c:14:3: warning: 'latitude' is used uninitialized in this function [-Wuninitialized]

What's wrong with this? When I hardcode the values of latitude and Longitude, I get the desired output but when I use scanf() to get inputs, it gives me warinings!

scanf needs pointers to variables to write the result into them — Raildex
– Raildex, Commented Nov 20, 2020 at 19:16
You are not asking about implementing scanf. You're just asking how to use it. — William Pursell
– William Pursell, Commented Nov 20, 2020 at 19:22
You need to add compiler flags that will generate warning that tell you exactly what is wrong with your code. — William Pursell
– William Pursell, Commented Nov 20, 2020 at 19:23
Just as a side note: It is unsafe to use scanf without checking the return value. See this page for further information: A beginners' guide away from scanf() — Andreas Wenzel
– Andreas Wenzel, Commented Nov 20, 2020 at 19:39

Pawel Blaszczyk · Accepted Answer · 2020-11-20 19:30:00Z

1

Change this line to send addresses, not values. Also - in scanf define only what you want to read, not the message

printf("Enter Latitude and Longitude ");
scanf("%i %i", &latitude, &longitude);

edited Nov 20, 2020 at 19:30

answered Nov 20, 2020 at 19:18

Pawel Blaszczyk

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1 Comment

technextgen Over a year ago

when I enter the values as 21 and 12 I get this as output 'Move to [4194431, 3805185]' . I think it is outputing the address!

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