midterm review 2014
TRANSCRIPT
CSE240 – Intro to
Programming LanguagesMidterm Review
Overview of Midterm
Chapter 01 (Common Aspects of Programming Languages)
Chapter 02 (Imperative Paradigm)
Tips and Hints:
Review Slides, HW, Quizzes
Do the extra exercises on website: https://sites.google.com/a/asu.edu/cse240/
Some questions will try to trick you on the exam
This presentation is only an overview, study a lot more after this
The Structure of Programs
Lexical: the building blocks of the programming language
Variable name, symbol, {…};
Error example: starting a variable name with a number
Ex. int 0num; char 1letter;
Syntactic: describes how to put lexical units together
if…then…else… switch, for… Do structures
Error example:
Missing a semicolon(;) at the end of a statement
Missed parenthesis or bracket to match another one
The Structure of Programs
Contextual: defines semantics before dynamic instruction
Variable type and initialization, type checking
Error example: expression lacks type consistency
string str = “abc”;
int I = 1 + str; (syntactically correct, but contextually incorrect)
Error example: use of a variable that was never declared
Error example: calling an object’s method that does not exist
Compiler can check for Lexical, Syntactic and Contextual errors!
The Structure of Programs
Semantic: describes the meaning of the program
Semantic errors are also called “logic errors”
The code below compiles
if(head = NULL) {
head = newEntry;
}
Vs.
if(head == NULL) {
head = newEntry;
}
Semantic errors cannot be found by the compiler!
Syntactic Analysis: Backus-Naur Form
Terminal vs Non-terminal symbols:
Terminal: can be seen in your code / actual program
Ex. % ; “ * 0, 1, …9
Non-Terminal: symbols representing the grammar of a language
Ex. <digit>, <object>, <statement>
BNF Example
<Phone Number> ::= (<Area Code>) <Prefix> - <Suffix> |
<Area Code> - <Prefix> - <Suffix> |
911 | 511
<Area Code> ::= 602 | 480 | 623
<digits> ::= 0 | 1 | 2 |… | 9
<start digits> ::= 1 | 2 | … | 9
<Prefix> ::= <start digits><digits><digits>
<Suffix> ::= <start digits><digits><digits><digits>
Based on this BNF,
Valid numbers: 602-123-4567, (480)327-9999, 911
Invalid numbers: 923-123-4567, (480)027-0999, (480) – 123 - 4567
Syntactic Analysis: Syntax Graphs Defines syntax, NOT logic (COMMON MISTAKE)
This is why the syntax graph for a while/for loop does not need an arrow going
back
if (condition) statements ;
else statements ;
if (a < 0) return -a;
if (a < 0) return -a; else return a;
Syntactic Analysis: Syntax Graph
switch (expr) { case value: statements ;
default: statements;
}
switch (ch) {case '+': x = a + b; break;case '-': x = a - b; break;case '*': x = a * b; break;case '': x = a / b; x = round(x); break; default: printf("invalid operator"); break;
}
For-loop example (in class exercise solution) available in the course website
https://sites.google.com/a/asu.edu/cse240/
Macros vs Functions
Macros
Done in pre-processing phase of compilation
Eliminate function call overhead, or cost (better performance)
Why?
Can introduce side-effects if not done correctly
A macro simply replaces the text with what is defined. For example:
#define MAC(a) a + 3 * 5
int j = MAC(2)
will become
int j = 2 + 3 * 5
Macros Example
For x = 3 and y = 4, get the output for the following
randomMacro(x++, --y);
randomFunction(x++, --y);
WRITE IT OUT. It helps during the test to avoid mistakes
Answer: Function should return 3 and the macro should return 4
What happens to the x and y variables?
#define randomMacro(x, y) ((x <= y) ? x : y)
int randomFunction(int x, int y){
return ((x <= y) ? x : y);
}
Type Equivalence
Type checking: ensuring the operands of an operator are of legal type
Coercion (implicit):
float num = 1.23 + 4;
Implicity converts the integer into a float type
Casting (explicit):
int num = (int) num + 5;
Strongly typed vs Weakly typed
Arrays and Strings
Strings are arrays of characters in C
Analysis:
char name[] = “Bob”;
4 elements: ‘B’ ‘o’ ‘b’ ‘\0’
The NULL character
char name[3] = {‘B’, ‘o’, ‘b’};
The null character does not exist
Printf will have side effects when trying to print a string (try it!)
Pointers:
char *name = “Bob”
Still 4 elements (including NULL character)
Pointers
& - Referencing – returns the address value of the variable it precedes
Called the R-value
* - Dereferencing – returns the value / name of the address it precedes
Called the L-value
Analogy: Mailbox/Addresses
Every mailbox has an address that can lead people to it (R-Value)
Mailboxes have envelopes/mail that can be placed inside of it(L-Value)
Pointers
#include <stdio.h>
int main() {
int i1 = 0, i2 = 10, *p1, *p2, **p3;
p1 = &i1;
p2 = &i2;
p3 = &p2;
**p3 = 20;
}
• &i1 = 5; &i1 is not a modifiable value
• 7000 = 5; again, 7000 is not modifiable
• *(7000) = 5; invalid: 7000 is not a pointer type
• *(&i1 ) = 5; Works, but redundant / bad code
• What is &p1?
• p3 = &p1;
Pointers
#include <stdio.h>
int main() {
int ma[2][4], *p;
p = ma;
*p = 5; // What is the value of ma?
*(p + 2) = 10; // What is the value of ma?
*(p + 5) = 20; // What is the value of ma?
return 0; ma =
} 5 10
20
Constants
Constants in C
Macro: Can substitute values for constant definitions at pre-processing
Faster execution, save a memory access
Const qualifier: may be a ‘variable’ that the program cannot modify
Slower, have to access memory
Modifying constants
Can be modified through pointers
Use the ampersand (&) function to find its address
Not good programming practice!
Structs, Unions
Structure
Collection of variables under a single name
Memory allocation as large as the variables inside combined/added together
Don’t forget about structure padding!
Union
Region of shared memory that can contain different types of values
Takes the largest required memory allocation from the largest variable
Linked List
#include <stdio.h>
#include <string.h>
#include <stdlib.h> // used by malloc
struct contact { // define a node holding a person's details
char name[30];
int phone;
char email[30];
struct contact *next; // pointer to contact structure
} *head = NULL; //head is a global pointer to the first entry
void branching(char c); // function forward declaration
int insertion();
struct contact *search();
void delete();
Linked List
int insertion() { // insert a new entry
struct contact *p;
p = (struct contact *) malloc(sizeof(struct contact));
if (p == 0) {
printf("out of memory\n"); return -1;
}
printf("Enter name, phone, email: \n");
scanf("%s", p->name);
scanf("%d", &p->phone);
scanf("%s", p->email);
p->next = head; // Where is the new node inserted?
head = p;
return 0;
}
Linked List
How to modify the insertion code so that the new node is inserted upon the
sorted?
Joe
1122334
next
John
1122556
head
Tom
1122667
p
temp
temp->next = p;
Zac
1122889
0
p->next = temp->next;
21
Linked List
else { while (temp->next != 0) {
if (stricmp(p->name, temp->next->name) <=0) {
p->next = temp->next;
temp->next = p;
return;
} else
temp = temp->next;
}
p->next = 0;
temp->next = p;
}
}
22
Joe
1122334
next
John
1122556
head
Tom
1122667
p
temp
temp->next = p;
Zac
1122889
0
p->next = temp->next;
Different Parameter Types
Call-by-value
Formal parameter is a local variable in function (copy)
foo(int m, int n)
Call-by-reference
Aka call by alias
Formal parameter is an alias name of the actual parameter
foo(int& m, int& n)
C++ only
Call-by-address
Aka Call by pointer
Address of the actual parameter is passed into a local variable in function
foo(int *m, int* n)
Advantages? Disadvantages?
Recursion
Four step approach for solving recursive problems
1. Formulate the size-n problem
2. Find the stopping condition and the corresponding return value
3. Formulate the size-m problem and find m. In may cases, m = n-1;
4. Construct the solution of size-n problem from size-m problem
Recursion
Example: Factorial
1. Size n problem:
int factorial(int n)
2. Stopping condition:
if (n == 0) return 1;
3. Formulate size m:
factorial(n-1)
4. Construct the solution to the size-n problem
n*factorial(n-1)
Recursion
Fibbonaci – try this example and build it
2)2()1(
11
00
)(
nnfibnfib
n
n
nfib
Recursion
Tail-recursion resembles a regular while loop
while-loop
12n
condition
Jump to
tail-
recursion
part 1
12n
recursive call
condition
loop-body
Good Luck!
Study further than these slides