Lecture 5 - the assert() Macro, Random access File I/O, typedef, Structures

Question 1

the assert() macro

Answer 1

• asserting that something must be true
• if condition is false, program aborts and tells you where it occurred
• MUST #include <assert.h></assert.h>

Question 2

when to use assert()

Answer 2

• if AND ONLY if you have to
• use alternative error detections if you can
• can be used for debugging
• check code expectation and error conditions you can’t handle

Question 3

how to turn off assertion check when compiling

Answer 3

• gcc -DNDEBUG=TRUE -o my_prog my_prog.c
• all assertion checks will be skipped

Question 4

random file access

Answer 4

• we can use an open file pointer to say the following things
• where are we at in the file (what’s our offset)?
• go back to the beginning/end/specific position in the file

Question 5

ftell()

Answer 5

• int ftell(FILE *file_pointer);
• used to find out where we are in the file
• return value is either current offset from beginning of file or -1 if error occurs

Question 6

fseek()

Answer 6

• used to “go somewhere” in the file
• int fseek(FILE *fp, long int offset, int whence);
• fseek() returns 0 on success and -1 in the event of an error

Question 7

fseek() from whence values (three things)

Answer 7

• SEEK_SET: new offset will be relative to beginning of the file
• SEEK_CUR: new offset will be relative to the current position of the file
• SEEK_END: new offset will be relative to the end of the file
• if offset is 0, will be exact position of whence
• offset can be negative to go backwards

Question 8

to determine how long file is

Answer 8

fseek(file_pointer, 0, SEEK_END);
long x = ftell(file_pointer);
fseek(file_pointer, 0, SEEK_SET);

Question 9

how to check if we hit EOF

Answer 9

fgets(buffer, 200, file_pointer);
if (ftell(file_pointer) == len) {
…. }

Question 10

uses for random file access

Answer 10

• accessing a file as a database
• extracting parts of a file
• modifying a few bytes in the file without writing the whole thing

Question 11

typedef

Answer 11

• allows you to create your own data types as long as they aren’t already C keywords
• ex. typedef double array[3];

Question 12

when to use typedef

Answer 12

• when you have a variable type that is used a lot and has a really long description
• has a parameter type you might change later
• is a structure

Question 13

typedef syntax

Answer 13

• appears to be backwards
• pretend you’re defining a variable, then make it a type
• ex. typedef unsigned int uint5[5];

Question 14

purpose for structures

Answer 14

• large programs usually have many data
• instead of crating a separate variable for each one, it is helpful to group them together to better organize
• a structure is the thing that allows you to use one name to refer to many variables

Question 15

declaration of a structure

Answer 15

struct my_data {
int age;
float height;
};
- note the semicolon!!
- storage definition and initialization
struct my_data my_var = {19, 5.3}

Question 16

declaration and definition of a struct together

Answer 16

struct my_data {
int age;
float height;
} my_var = {19, 5.3};

Question 17

accessing elements of a struct

Answer 17

• once a structure variable has been defined, you can access it internal elements with the dot operator
• ex. my_var.height = 6.1;
• ex. x = my_var.age;

Question 18

properties of structures

Answer 18

• anything can be defined inside a struct EXCEPT functions
• you can put arrays, structs, in structs in any order
• no limit to num of elements in a struct but each have unique name
• structs can be passed to functions and returned from functions
• can assign one struct from another

Question 19

where to put declarations of a structure

Answer 19

• structure variables can be defined inside or outside of a function (but shouldn’t)
• declaration should be OUTSIDE of a function