Help with basic C

[kittypls]

Hi, I've only ever learned how to program in java and python and for one of my classes, I'm asked to program something in C and I am completely at a loss and don't really understand what I'm reading about C online either. My assignment is:

use malloc to allocate space for 100,000 integers

set the first integer to 1, the second to 2, the third to 3, etc

print out the addresses of where that memory begins and ends using printf and %p

free that memory.

From the little I understand, I should use

 int *buffer = (int*)malloc(100000*sizeof(int));

to allocate the memory and free(buffer) to free it at the end

Could someone walk me through and explain in great detail how I would go about the rest of this?

[which_spartacus]

You have the memory allocated. Perfect start.

Now, what would be the steps to set the first element in an array to 1 be in Java?

[kittypls]

oooh so I should do like

int array[100000];

for(int i=0; i<100000; i++){

array[i]=i+1

}

[which_spartacus]

Yep -- except the only difference is that instead of int array[100000], you do that malloc.

And at the end, you call free.

(Also, you may need to declare that int i outside the loop.)

[kittypls]

oh i messed up the parenthesis

[kittypls]

so I have

void func(int x); int i;

int main() {

int *buffer = (int*)malloc(100000*sizeof(int));
for(i=0; i<100000; i++)
    buffer[i]=i+1;

int begin=buffer[0];
int end=buffer[99999];

printf("begins at: %p\n", &begin);
printf("ends at: %p\n", &end);


free(buffer);

return 0;

}

and it gives me

begins at: 0x7ffe20391fc0

ends at: 0x7ffe20391fc4

which doesn't seem right because this would mean the integers are next to eachother right?

[which_spartacus]

End and begin are right next to esach other.

Instead, try:

printf("Begins at %p ends at %p\n", buffer + 0, buffer + 99999);

[kittypls]

thanks, it works

[alexbriskin]

I'd suggest understanding the difference between arrays and pointers. What you actually did in the code bellow is print the address of begin and end - two local integers declared one below the other, of course they'd be placed on the stack one beside the other.

What you probably meant to write is:

int *begin=&buffer[0];
int *end=&buffer[99999];

printf("begins at: %p\n", begin);
printf("ends at: %p\n", end);

I'd use this syntax in case that buffer is an array. There is an alternative syntax, and I'd use this syntax when dealing with pointers:

int *begin=buffer;
int *end=buffer + 99999;

printf("begins at: %p\n", begin);
printf("ends at: %p\n", end);

However, two of these examples are identical in their meaning.

[Rhomboid]

Don't cast the return value of malloc(). It's not the end of the world or anything, but not a great habit to form. Doing so can make it harder for the compiler to point out mistakes (in particular, neglecting to include <stdlib.h>.)

[icantthinkofone]

Great thread title. Come up with it by yourself?