Pointers

The mystery continues....

Strings\0

What are they?

How are they represented in C?

Strings in C

There are 3 ways to create a string in C.

1. const char *some = "I am a string!";
2. char some[] = "I am a string!";
3. char *some = malloc(15); strcpy(some, "I am a string!"); 

String input in C

#include <stdio.h>

int main() {
    char *input = malloc(100);
    scanf("%s", input);
    printf("You entered: %s\n", input);
    return 0;
}

scanf, fgets etc.

String functions - just a  few important ones

1. strcmp
2. strlen
3. strcpy
4. strcat

strcmp

Subtitle

#include <stdio.h>

int main() {
    char *input = malloc(100);
    scanf("%s", input);
    printf("Strcmp returned %d\n", strcmp(input, "Some"));
    return 0;
}

strcpy

#include <stdio.h>

int main() {
    char *input = malloc(100);
    char *copyinput = malloc(100);
    scanf("%s", input);
    strcpy(copyinput, input);
    printf("%p: %s\t%p: %s\n", input, input, copyinput, copyinput);
    // 0x93de008: ZOllers	0x93de070: ZOllers
    return 0;
}

strcat

#include <stdio.h>

int main() {
    char *string = malloc(200);
    char *suffix = malloc(100);
    strcpy(string, "Aneesh is ");
    scanf("%s", suffix);
    strcat(string, suffix);
    printf("%s\n", string);
    return 0;
}

That's a lot about strings ... lets so something fun with em!

Let's create our version of strcpy  -using pointers

Strings in functions

1. Passing a string.
2. Returning strings.

FunFact: Char * and Char arrays are not same.

Pointers trickery!

#include <stdio.h>

int main(void)
{
   int a[5] = { 1, 2, 3, 4, 5 };
   int *ptr = (int*)(&a + 1);

   printf("%d %d\n", *(a + 1), *(ptr - 1));

   return 0;
}

Pointers and Structures

The stuff we have covered so far was just the beginning ...

Structures

on the board please...

Memory allocation

Malloc(sizeof(structure))

.

.

.

free(structure.array)

free(structure)

A stack data structure

#define STACK_MAX 100


struct Stack {
    int     data[STACK_MAX];
    int     size;
};
typedef struct Stack Stack;


void Stack_Init(Stack *S)
{
    S->size = 0;
}

int Stack_Top(Stack *S)
{
    if (S->size == 0) {
        fprintf(stderr, "Error: stack empty\n");
        return -1;
    } 

    return S->data[S->size-1];
}

void Stack_Push(Stack *S, int d)
{
    if (S->size < STACK_MAX)
        S->data[S->size++] = d;
    else
        fprintf(stderr, "Error: stack full\n");
}

void Stack_Pop(Stack *S)
{
    if (S->size == 0)
        fprintf(stderr, "Error: stack empty\n");
    else
        S->size--;
}

A stack data structure

#define STACK_MAX 100


struct Stack {
    int     data[STACK_MAX];
    int     size;
};
typedef struct Stack Stack;


void Stack_Init(Stack *S)
{
    S->size = 0;
}

int Stack_Top(Stack *S)
{
    if (S->size == 0) {
        fprintf(stderr, "Error: stack empty\n");
        return -1;
    } 

    return S->data[S->size-1];
}

void Stack_Push(Stack *S, int d)
{
    if (S->size < STACK_MAX)
        S->data[S->size++] = d;
    else
        fprintf(stderr, "Error: stack full\n");
}

void Stack_Pop(Stack *S)
{
    if (S->size == 0)
        fprintf(stderr, "Error: stack empty\n");
    else
        S->size--;
}

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