C - place integer in a char array and extract it - c

Question

String manipulation in C is the bane of my existence, I have tried to understand how to manipulate char arrays and fall short of grasping it every time. I simply want to do this in C.
Method1 passes ["1298","9"]
Method2 receives and converts them both to ints
int method1(){
char *values[2];
int i = 1298;
int j = 9;
//sprintf(values,"%d" "%d",i,j);
sprintf(&values[0],"%d" ,i);
sprintf(&values[1],"%d" ,j);
method2(values,2);
}
int method2(char *strings, int num_values){
int i, j;
char buffer[256];
sscanf(buffer, "%d" "%d", &i, &j);
printf("i: %d, j: %d\n",i,j);
return 0;
}
The result of this is 1 and 9; but i want 1298 and 9
Please even after providing an answer, can you explain why it is the way it is?

Answer 1

char *values[2];
The above line is parsed as char *(values[2]);, i.e., [] take precedence over *. This means that values is an array with 2 elements. The type of the elements is char *. So, values[0] and values[1] are of type char *. Since you haven't initialized the array elements, the two items in the array have unknown, random values.
//sprintf(values,"%d" "%d",i,j);
Although you have commented this out, it seems like you were trying to store the string representation of i and j into values using one sprintf call. As you probably found out, this is impossible.
sprintf(&values[0],"%d" ,i);
Here, you are calling sprintf to store the string representation of i into the memory location pointed to by values[0]. But as noted above, values[0] is not pointing anywhere useful, since you haven't initialized it. This is bad.
sprintf(&values[1],"%d" ,j);
Same as above.
method2(values, 2);
Here, you are calling method2, but you haven't declared the signature of method2. As it turns out, method2 takes a char * and an int. You are passing values and 2. values is an array of char *, which gets converted to a pointer in this context, so method2 should take a char ** and an int.
int method2(char *strings, int num_values){
This should be: int method2(char **strings, int num_values) {
int i, j;
char buffer[256];
sscanf(buffer, "%d" "%d", &i, &j);
I am not sure what you are trying to do here. You are ignoring strings, but instead using a buffer variable. buffer is uninitialized, so trying to interpret the data in it as two integers is pointless.
I am assuming you are writing this program to understand strings, pointers, and functions. As such, there is no single correct way to do what you are trying to do. But here is one approach:
#include <stdio.h>
/* Define method2 before method1, so that its definition is visible
in method1 */
/* Assume that method2 will be called with one char * containing two integer
values. I also changed the return type to void since you weren't really
using the int return value. Same for method1. */
void method2(const char *strings)
{
int i, j;
sscanf(strings, "%d %d", &i, &j);
printf("i: %d, j: %d\n",i,j);
}
void method1(void)
{
/* Changed values to a char array that has enough space */
char values[256];
int i = 1298;
int j = 9;
sprintf(values, "%d %d", i, j);
method2(values);
}
int main(void)
{
method1();
return 0;
}
The above outputs:
i: 1298, j: 9
There are a lot of moving parts in your program. I recommend that you learn the basics about pointers, arrays etc., in C, before passing them around in functions.

Answer 2

The result of your program is called UNDEFINED BEHAVIOR. You MUST use compiler warnings if you are learning.
Your code has many issues, you should READ about strings in c instead of trying stuff just to see if they work.
You are using sprintf() wrong, you should pass a pointer to char as the first parameter, that points to valid data. You are passing uninitialized pointers instead which leads to undefined behavior.
Function method1() (please note, that methods are for classes or objects, in c there are no methods, just functions) is also not passing anything anywhere. The array of pointer values is local to the function and is not accessible from outside the function. To pass something to a function you should do it through the parameters instead and it would be the other way around since you can't call main() from a c program because it is undefined behavior.
You are using sscanf() where you apparently what fscanf() or just scanf().
If you want to read input from the user use
if (scanf("%d %d", &i, &j) == 2)
printf("i: %d, j: %d\n", i, j);
else
printf("Invalid input\n");
you can substitute scanf() with fscanf(stdin, ... or another FILE * object to read from a file.
In your code, your program is expecting buffer to contain a string (a sequence of bytes terminated with a '\0'). You are passing an uninitialized array instead which invokes undefined behavior.
If you want to test how sscanf() works, do this
char buffer[] = "1 2";
if (sscanf(buffer, "%d %d", &i, &j) ...
the rest is the same.
I would say, that you want this
int
main(void)
{
char buffer[256];
int i;
int j;
sprintf(buffer, "%d %d", 1289, 9); // - This is not necessay, but for illustration
// - This is also, unsafe as it could cause
// a buffer overflow.
if (sscanf(buffer, "%d %d", &i, &j) == 2) // Would be surprised if this is false,
printf("values: %d %d\n", i, j); // but you MUST always check to make
else // sure, because this kind of situation
printf("invalid input?\n"); // is very unusual in real code.
}
I hope you can see, why your code is SO wrong. You can also move the part you want to the function. But it should be the right part.

Answer 3

Here's something I wrote up quickly; you can pass an array of strings to this function along with it's length and it will dynamically allocate an array of integers with their converted values.
The pitfall here is that with strtol: if it fails to convert one of the strings it will default to 0. So if an element in your array of strings happens to contain some non-numeric characters you may not get what you expect; and an error won't be reported as you'd have to also confirm that the string itself wasn't 0.
#include <stdlib.h>
#include <stdio.h>
int *convert_to_int_array(const char **strings, size_t length) {
int *numbers = malloc(length * sizeof(int));
size_t i = 0;
if(numbers == NULL)
return NULL;
for(i = 0; i < length; i++) {
numbers[i] = strtol(strings[i], NULL, 10);
}
return numbers;
}
int main(int argc, char **argv) {
const char *strings[2];
strings[0] = "100";
strings[1] = "50";
int *numbers;
size_t length = 2, i;
numbers = convert_to_int_array(strings, length);
if(numbers == NULL) {
fprintf(stderr, "failed to initialize numbers\n");
}
for(i = 0; i < length; i++) {
printf("%d\n", numbers[i]);
}
return EXIT_SUCCESS;
}

Answer 4

Try this:
char values[10];
int i = 1298;
int j = 9;
sprintf(values,"%d %d",i,j);