help50 make buggy1
cs50, lec2
Monday, August 31, 2020
linking - combining 0s and 1s from our source code and the various libraries
links the various 0s and 1s into an overall program in machine code
assembling - taking assembly code and converting to 0s and 1s
this portion is also done by the clang compiler
compiling - although this is often used as a generalized and simplified term, it actually has a more precise definition
compiling is taking your source code (after being pre-processed) and converting it into assembly code
for example:
...
main: # @main
.cfi_startproc
# BB#0:
pushq %rbp
.Ltmp0:
.cfi_def_cfa_offset 16
.Ltmp1:
.cfi_offset %rbp, -16
movq %rsp, %rbp
Ltmp2:
.cfi_def_cfa_register %rbp
subq $16, %rsp
xor1 %eax, %eax
mov1 %eax, %edi
movabsq $.L.str, %rsi
movb $0, %al
callq get_string
movabsq $.L.str, %rdi
movq %rax, -8(%rbp)
movq -8(%rbp), %rsi
movb %0, %al
callq printf
...
cs50, lec2
for example:
...
main: # @main
.cfi_startproc
# BB#0:
pushq %rbp
.Ltmp0:
.cfi_def_cfa_offset 16
.Ltmp1:
.cfi_offset %rbp, -16
movq %rsp, %rbp
Ltmp2:
.cfi_def_cfa_register %rbp
subq $16, %rsp
xor1 %eax, %eax
mov1 %eax, %edi
movabsq $.L.str, %rsi
movb $0, %al
callq get_string
movabsq $.L.str, %rdi
movq %rax, -8(%rbp)
movq -8(%rbp), %rsi
movb %0, %al
callq printf
...
cs50, lec2
pre-processing
in this step, the pre-processor goes through your code, and copy pastes relevant code from .h header files into your code
for example:
#include <cs50.h>
#include <stdio.h>
int main(void)
{
string s = get_string("what is your name\n");
printf("hello, %s\n", s);
}
becomes
....
string get_string(string prompt);
void printf(string format, .....);
....
int main(void)
{
string s = get_string("what is your name\n");
printf("hello, %s\n", s);
}
.................the actual code pasted in is simplified here, but this is enough to illustrate the concept......
cs50, lec2
for example:
#include <cs50.h>
#include <stdio.h>
int main(void)
{
string s = get_string("what is your name\n");
printf("hello, %s\n", s);
}
becomes
....
string get_string(string prompt);
void printf(string format, .....);
....
int main(void)
{
string s = get_string("what is your name\n");
printf("hello, %s\n", s);
}
.................the actual code pasted in is simplified here, but this is enough to illustrate the concept......
cs50, lec2
The term "compile" is often used as an "over-simplification" of 4 actions:
pre-processing
compiling
assembling
linking
.......these 4 steps are invoked when we run clang or make
cs50, lec2
compiling
assembling
linking
.......these 4 steps are invoked when we run clang or make
cs50, lec2
get a string and print a string (lec2 reviewing lec1)
#include <cs50.h>
#include <stdio.h>
int main(void)
{
string s = get_string("what is your name?\n");
printf("hi, %s\n", s);
}
cs50, lec2 (reviewing lec1)
#include <stdio.h>
int main(void)
{
string s = get_string("what is your name?\n");
printf("hi, %s\n", s);
}
cs50, lec2 (reviewing lec1)
Sunday, August 30, 2020
say - command (program) that comes with some systems that speaks out what you type
for example:
say this is cs50
(this works on mac)
say this is cs50
(this works on mac)
figlet - command (program) that comes with some systems which produces ASCII art
for example:
figlet this is cs50
figlet this is cs50
floating point imprecision & integer overflow
both exist because computers only use a finite amount of bits to store numbers
positive.c
#include <cs50.h>
#include <stdio.h>
int get_positive_int(void);
int main(void)
{
int i = get_positive_int();
printf("The positive integer you entered is: %i\n", i);
}
get_positive_int(void)
{
int n;
do
{
n = get_int("Enter a positive integer:\n");
}while(n<1)
return n;
}
#include <stdio.h>
int get_positive_int(void);
int main(void)
{
int i = get_positive_int();
printf("The positive integer you entered is: %i\n", i);
}
get_positive_int(void)
{
int n;
do
{
n = get_int("Enter a positive integer:\n");
}while(n<1)
return n;
}
it is convention to have main() up top before other function definitions
hence the need for function prototypes ("one liners")
(C is dumb in that it does not scan further down to look for function definitions)
(C is dumb in that it does not scan further down to look for function definitions)
function prototypes : C is dumb
#include <cs50.h>
#include <stdio.h>
void cough(int n);
int main(void)
{
cough(3);
}
void cough(int n)
{
for (int i; i<n; i++)
{
printf("cough\n");
}
}
#include <stdio.h>
void cough(int n);
int main(void)
{
cough(3);
}
void cough(int n)
{
for (int i; i<n; i++)
{
printf("cough\n");
}
}
defining your own functions: abstraction
#include <cs50.h>
#include <stdio.h>
int main(void)
{
void cough(void)
{
printf("cough\n");
}
for (int i=0; i<3; i++)
{
cough();
}
}
#include <stdio.h>
int main(void)
{
void cough(void)
{
printf("cough\n");
}
for (int i=0; i<3; i++)
{
cough();
}
}
agree.c
#include <cs50.h>
#include <stdio.h>
int main(void)
{
char c = get_char("do you agree?\n");
if (c== 'y' || c=='Y')
{
printf("agreed\n");
}
else if (c=='n' || c=='N')
{
printf("do not agree\n");
}
}
#include <stdio.h>
int main(void)
{
char c = get_char("do you agree?\n");
if (c== 'y' || c=='Y')
{
printf("agreed\n");
}
else if (c=='n' || c=='N')
{
printf("do not agree\n");
}
}
parity.c
#include <cs50.h>
#include <stdio.h>
int main(void)
{
int n = get_int("please enter an integer:\n");
if(n % 2 == 0)
{
printf("the number you entered is an even number\n");
}
else
{
printf("the number you entered is odd.\n");
}
}
#include <stdio.h>
int main(void)
{
int n = get_int("please enter an integer:\n");
if(n % 2 == 0)
{
printf("the number you entered is an even number\n");
}
else
{
printf("the number you entered is odd.\n");
}
}
Saturday, August 29, 2020
float.c : get a floating point number from the user
#include <cs50.h>
#include <stdio.h>
int main(void)
{
float price = get_float("what is the price?\n");
printf("the price plus tax is %f\n", price*1.0625);
}
limit the output to 2 digits after the decimal point:
#include <cs50.h>
#include <stdio.h>
int main(void)
{
float price = get_float("what is the price?\n");
printf("the price plus tax is %.2f\n", price*1.0625);
}
[Source: harvard, cs50]
#include <stdio.h>
int main(void)
{
float price = get_float("what is the price?\n");
printf("the price plus tax is %f\n", price*1.0625);
}
limit the output to 2 digits after the decimal point:
#include <cs50.h>
#include <stdio.h>
int main(void)
{
float price = get_float("what is the price?\n");
printf("the price plus tax is %.2f\n", price*1.0625);
}
[Source: harvard, cs50]
int.c : get an integer from user
#include <cs50.h>
#include <stdio.h>
int main(void)
{
int age = get_int("What's our age?\n");
int days = age*365;
printf("You are at least %i days old\n", days);
}
better design:
#include <cs50.h>
#include <stdio.h>
int main(void)
{
int age = get_int("What's our age?\n");
printf("You are at least %i days old\n", age*365);
}
you can even do this: (but in terms of design, this may reach an inflection point at which the code becomes too hard to read)
#include <cs50.h>
#include <stdio.h>
int main(void)
{
printf("You are at least %i days old\n", get_int("What's your age?\n")*365);
}
[Source: harvard, cs50]
#include <stdio.h>
int main(void)
{
int age = get_int("What's our age?\n");
int days = age*365;
printf("You are at least %i days old\n", days);
}
better design:
#include <cs50.h>
#include <stdio.h>
int main(void)
{
int age = get_int("What's our age?\n");
printf("You are at least %i days old\n", age*365);
}
you can even do this: (but in terms of design, this may reach an inflection point at which the code becomes too hard to read)
#include <cs50.h>
#include <stdio.h>
int main(void)
{
printf("You are at least %i days old\n", get_int("What's your age?\n")*365);
}
[Source: harvard, cs50]
get_string, get_char, get_int, get_long, get_float, get_double, ...
the cs50 library provides these functions
these are all functions that will prompt the human user for certain values
these are all functions that will prompt the human user for certain values
Data types in C: bool, char, double, float, int, long, string
bool (true or false)
char (a single character, not two or more)
int (an integer; it has a certain size; you can only count up to a certain size number with int, typically
4 billion)
float (floating point number, which is a fancy way of saying a real number [something that has a
decimal point])
double (just a real number that can have even more digits after the decimal point)
long (uses more bits so can count higher than int; companies like Facebook and Google have more
data than 4 billion, so they may need these types of numbers)
string (one or more characters inside double quotes " " )
...
.
.
.
[source: harvard, cs50, david malan]
char (a single character, not two or more)
int (an integer; it has a certain size; you can only count up to a certain size number with int, typically
4 billion)
float (floating point number, which is a fancy way of saying a real number [something that has a
decimal point])
double (just a real number that can have even more digits after the decimal point)
long (uses more bits so can count higher than int; companies like Facebook and Google have more
data than 4 billion, so they may need these types of numbers)
string (one or more characters inside double quotes " " )
...
.
.
.
[source: harvard, cs50, david malan]
Repeat 50 times: for ( int i=0 ; i < 50 ; i++ ) { }
for (int i=0; i<50; i++)
{
printf("hello, world\n");
}
....... the syntax of the "for loop" is a little more funky (than the while loop), but the for loop is more succinct while achieving the exact same thing
[Source: Harvard, CS50]
{
printf("hello, world\n");
}
....... the syntax of the "for loop" is a little more funky (than the while loop), but the for loop is more succinct while achieving the exact same thing
[Source: Harvard, CS50]
Thursday, August 27, 2020
if, else if, else
if (x<y)
{
printf("x is less than y\n");
}
else if (x>y)
{
printf("x is greater than y\n");
}
else
{
printf("x is equal to y\n");
}
{
printf("x is less than y\n");
}
else if (x>y)
{
printf("x is greater than y\n");
}
else
{
printf("x is equal to y\n");
}
if, else if, else if
if (x<y)
{
printf("x is less than y \n");
}
else if (x>y)
{
printf("x is greater than y \n");
}
else if (x==y)
{
printf("x is equal to y \n");
}
{
printf("x is less than y \n");
}
else if (x>y)
{
printf("x is greater than y \n");
}
else if (x==y)
{
printf("x is equal to y \n");
}
C: no semicolons after condition statements (semicolons after some kind of "action")
Source: Harvard , CS50
make - included in Linux, MacOS, and Windows these days
the make program is now included with these operating systems
make string
when you type this command, the program "make" will look for a file called "string.c" to compile into the executable "string"
if you type the command "make hello" , the program make will look for a file called "hello.c" and compile it into the executable "hello"
[Source: Harvard, CS50]
if you type the command "make hello" , the program make will look for a file called "hello.c" and compile it into the executable "hello"
[Source: Harvard, CS50]
clang -o string string.c -lcs50
in addition to " #include <cs50.h> "
you also have to "-lcs50"
where "l" stands for "link"
[Source: Harvard, CS50]
you also have to "-lcs50"
where "l" stands for "link"
[Source: Harvard, CS50]
Monday, August 24, 2020
String.c
#include <stdio.h>
#include <cs50.h>
int main(void)
{
String name = get_string("Please enter your name:");
printf("Hello, %s\n", name);
}
Source: Harvard, CS50
#include <cs50.h>
int main(void)
{
String name = get_string("Please enter your name:");
printf("Hello, %s\n", name);
}
Source: Harvard, CS50
clang compile error message: hello.c:5:26 ......... this tells you to look on line 5 character 26
it can't tell you exactly what's wrong, but can tell you approximately where it is happening.....start googling
Sunday, August 23, 2020
"rm" = "remove"
for example:
rm a.out
.........the system will then prompt:
remove regular file a.out?
-------> type "y" or "yes" to confirm deletion
Saturday, August 22, 2020
The * behind a file name denotes that that file is an "executable"
Which just means that that file contains machine code that can be run by the computer.
"clang -o hello hello.c"
"Options" or "command line arguments"
"-o" stands for output
Rename the output file to anything you want.
"New line character" is represented by "\n"
This is a special command or special character that tells the computer to have the cursor go to a new line.
"a.out" stands for "assembly output"
It is the machine code (binary) a computer can actually understand and run.
The dollar sign in the terminal window is just the "prompt" or "shell"
It just tells you to "type here".
A compiler is also a software program
A compiler takes source code and produces machine code (0's and 1's).
Friday, August 21, 2020
Software - abstraction - have functions instead of having everything mixed together
In code, abstract away functions instead of having long linear complex segments of code.
Software - Loop instead of Copy&Paste - better design
In code, your should not have to copy&paste any code, use loops instead - better design
Wednesday, August 19, 2020
Software - 2 threads at the same time
in scratch
2 "when green flag clicked" blocks
the 2 threads communicate with each other using things like variables
Software engineering - toggle - Technique often used
A technique often used in software engineering is :
when a key is pressed, check a variable and toggle it
Programming - Topics to be discussed
- functions
-conditions
-Boolean expressions
-loops
-variables
-threads
-events
...
[Source: Harvard, CS50]
Tuesday, August 18, 2020
Algorithms - time for solving problems
vertical axis: time to solve
horizontal axis: size of problem
[Source: Harvard, CS50]
Represent music in a computer
We can use three number values: note, duration, and volume
to represent each music sound
Sunday, August 16, 2020
RGB - System for representing colored dots
ASCII and unicode represents characters and emoji information.
The RGB system uses binary numbers to represent colored dots which in turn compose what is displayed on our computer screens.
[Source: Harvard CS50]
128514 - face with tears of joy (emoji)
Unicode number for "face with tears of joy" emoji is 128514.
Binary pattern: "111110011000000010"
[Source: Harvard CS50]
Unicode is a superset of ASCII
ASCII uses 8 bits to represent information,
Unicode uses 8 or 16 or 32 or 64 bits or more to represent information, therefore, Unicode can represent more types of characters and emoji.
Emoji are included in Unicode.
ASCII - American Standard Code for Information Interchange
.... A B C D E F G H I ....
.... 65 66 67 68 69 70 71 72 73 ....
[Source: Harvard Cs50 ]
Saturday, August 15, 2020
Humans often count in decimal
Humans often represent numbers using decimal notation,
with "dec" meaning "10"
0123456789
How to represent information - computer science
This is one of the first topics to discuss in computer science
Computers store and process information in binary at the most basic level
Friday, August 14, 2020
Waterfall Model
The waterfall model for software development was first proposed by Royce in 1970.
Agile software development methods are against some concepts proposed in the Waterfall Model.
Agile software development 1 - 2020aug14
On "requirements":
Bech (Extreme Programming):
Requirements gathering is in a phase that produces a static document, but an activity producing detail, just before it is needed, throughout development
Cohn (Scrum):
Scrum projects do not have an upfront analysis or design phase; all work occurs within the repeated cycle of sprints
Poppendieck (Lean):
And those things called requirements? They are really candidate solutions; separating requirements from implementation is just another form of handover
[source: edx course on Agile Software Development]
Tuesday, August 11, 2020
automate the process of compiling using clang with "make"
for example:
make hello
....make executes clang with various command-line arguments
execute the resulting executable:
make hello
specify executable file name for clang compiler
clang -o hello hello.c
-o is a "command-line argument", or a "flag" , or a "switch"
Commands in Unix are case sensitive
This means that a capital letter and a lower case letter are treated as different
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