1. ENEE150 Discussion 07
Section 0101
Adam Wang

2. Overview
More on project 2
Structs
Malloc
Quiz

3. Reading in the title and Author
One big loop that runs until you’re done with the file
In the loop, write your code that parses the title, author, time, categories, instructions, and ingredients
When the loop starts again that same code should work for the next recipe

4. Method 1 (From last week)
We have read in everything into raw_recipes[MAX_SIZE]
We need to find the character right after “Recipe: ”
Use a counter that will move across the entire array
int i = 0
Reference the character with raw_recipes[i]

5. Method 1 We’ll keep moving it until we see the ‘:’ character
Then we’ll move until we hit the first non-space character
raw_recipes[i] != ‘ ’
Alternatively, just bump i up by 6
Then every char until the ‘\n’ we copy into the titles array
titles[index] = raw_recipes[i]
Don’t forget to append the ‘\0’ char at the end to make it a string
titles[index] = ‘\0’

6. Method 2 (maybe more intuitive)
We’ll use our string methods to find the title
Create a new pointer, set it to raw_recipes
Use strstr to find “Recipe: “
Bump the pointer up by 13
Now you can copy it in char by char like method 1 (this time you’re using pointer arithmetic though)

7. Method 2 You could alternatively use strncpy to copy in the title
Use strstr again to find the next “\n” char
Use strncpy to copy it into the title array
Source would be p_start
Destination is the title array
Number of chars is p_end – p_start
Append the ‘\0’ to title[p_end – p_start]

8. Structs struct books { char title[50]; char author[50];
char subject[100];
int book_id; };
struct books s1 = {“a”, “b”, “c”, 20};
Everything is stored right next to each other (more or less)
s1 = s2 is VALID for structs!
s1 == s2 is NOT VALID for structs

9. Typedef
typedef struct data { float score1; int score2; int score3; int score4; } data_el; data_el example = {2.5,7,8,9}; // just makes typing it out easier example.score4++;

10. Struct pointers
typedef struct my_structure { char name[20]; int number; int rank; } my_struct; my_struct var = {“bob”, 5, 10}; my_struct *ptr = &var; printf(“Name: %s”, ptr->name); // same as (*ptr).name

11. Malloc Dynamically allocating memory for new variables
Memory gets taken from the heap
Regular variables are made on the runtime stack
+ Main advantage is now we don’t have to waste memory
- Taking from the heap is a low slower than stack
Computer has to figure out where to find the memory (this is called fragmentation)
Heap is considered global memory whereas variables on the stack are restricted to the function they’re in

12. Malloc int *ptr = (int *)malloc(sizeof(int)); heap
Why do we use sizeof instead of 4 bytes?
Not all computers will use 4 bytes for a size of an integer
Malloc normally returns a void pointer; this cannot be dereferenced, but can be casted to anything
heap
ptr
0xEF
0xA4
0x37
0x9D
0xAE536…

13. Free
Free(ptr);
Computer knows this is memory is no longer needed
Pointer that points to freed memory is called a dangling pointer
C does not have automatic garbage collection like other languages

14. Arrays Arrays are basically pointers to the first element
int *array = (int *) malloc (10 * sizeof(int)); or
int *array = (int *) calloc (10 * sizeof(int));

15. Very important concepts to know
Stack vs heap
When would you use either
Tradeoffs
How does computer visualize memory
Void pointers, NULL pointers, dangling pointers, when they are dangerous
Malloc; how it works
Memory leaks: occurs when user is not freeing memory
Tradeoffs in usage

16. Quiz wget ece.umd.edu/~aw97/quizzes/quiz2.c
Write a function count_x that takes in a string and returns the number of x’s
count_x("abcxyz") > 1
count_x("20458x4820x;?$#") > 2
count_x("abc") > 0
You MUST use pointer arithmetic, no array indexing
Submit under assignment 101:
submit 2017 fall enee quiz2.c