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<html><body>
<BR />8 Basics of Programming
<BR />
<BR />Licensed under the Creative Commons Attribution licence (CC-BY),
<BR />either version 4.0 or at your option, any later version. See:
<BR />https://creativecommons.org/licenses/by/4.0/
<BR />
<BR />Author:Usmar A. Padow (amigojapan), contact: usmpadow@gmail.com
<BR />
<BR />Written in 2017.
<BR />
<BR />
<BR />0. Sequentiality
<BR />1. Variables
<BR />2. Statements
<BR />3. Conditionals/blocks
<BR />4. Functions/Types
<BR />5. Input/output
<BR />6. Loops
<BR />7. Arrays/lists
<BR />
<BR />before we start you can use <a href=https://repl.it/languages/python>this link to run python code</a> and <a href=https://repl.it/languages/c>this one for C</a>
<BR />
<BR />0. Sequentiality/Following instructions in order
<BR />the first thing we need to learn in order to become a programmer is that instructions always follow a regular order, usually from top
to down (and sometimes from left to right)
for example, if I write:
<pre><code>
BrushMyTeeth();
CombMyHair();
GoToWork();
</code></pre>
it will never got or work before I brush my teeth and comb my hair...
A great program to learn instruction order is lightbot (which does it from left to right and then top to down, but the idea is still there)
what would happen if I wrote?:
<pre><code>
BrushMyTeeth();
GoToWork();
CombMyHair();
</code></pre>
well, the answer is that I would first brush my teeth, but I would get to work unkempt, which would cause trouble, that is why the order of
the instructions in a program is so important… also because "computers are stupid”(recommend
watching <a href=https://www.youtube.com/watch?v=EKWGGDXe5MA>this video</a> ) which is another thing to keep in mind, with the
previous script I just wrote, the computer will not intelligently know i should comb my hair before going to work.. computers are just
not capable of such decision making at this level of programming.
If you make a large program, and shuffle its instructions around, the program will surely not do what it was originally intended to do.
By the way, in C this would also work:
BrushMyTeeth();CombMyHair();GoToWork();
because C separates instructions by the semicolons
<BR />
<BR />1.Variables
<BR />a variable is a place in the computer memory where some kind of value sometimes a string(some letters),sometimes a number is stored.
like the value 5 or the string “hello”. a variable can have a simple name like just the letter x or any combination of letters like
last_name or social_security_number.
if we have a variable called social_security_number we could assign it the value 123458789 as a sample social security number.
variables as their name implied can vary, which seems to be different to the mathematical view on variables, in math if you
say x=5 then x is always equal to 5, but in programming we can say x=3; and then x=5; and the value would first be set to 3 and
then change to 5.
in the case of a string, the value between the double quotes will be assigned to the variable, for example if we had last_name we could
assign “Albert Einstein” to it easily as follows: last_name= “Albert Einstein”. this is in python, but in C you would need to use
strcpy(last_name, “Albert Einstein”); but beware, because in C if last_name does nto have the size to hold “Albert Einstein” undefined
things can happen, I will go more into depth of string on the chapter about arrays because strings in C are nothing but arrays of
characters
Some languages have different requirements for variable names, for example, some languages require that they begin with $ such as $total
<BR />
<BR />2. Statements
<BR />this may be the most difficult part to explain of programming,
almost everything you see in a computer program is a statement, statements are commonly ended with semicolons, but can also appear as
keywords such as while() or the thing that goes inside the while(), the condition is also a statement init of itself
Lets talk about assignment statements.
for example, lets take the following program
<pre><code>
x=3;
x=5;
x=x+1;
</code></pre>
what will be the result of such a program…
well, the first statement, x=3; will set x to 3, then the next statement x=5; will (maybe counter-intuitively to those who know math)
set x to 5, then the final statement x=x+1; we need to break down into its parts.
we have the x on the left called an “lvalue” which is the same as the first x=3;, where the x is what stores the value, then we have x+1,
the value of x at this point of the program is 5 because it comes after x=5; so when we do x+1 it is the same as doing 5+1, and then
finally we assign that to the lvalue, so at the end of the program, x will contain 6 which is 5+1.
*(insert m-programmer program that steps on this program step by step)
other kinds of statements we will look at later in the book are things like function calls, return statements, conditional statements
and boolean statements.
I would like to touch on boolean statements before we go on.
boolean values.
a boolean value is a value that can either contain a true or false value.
a boolean statement is something that evaluates(is calculated) to either a true or false value
for example x=5>3; the x is our usual lvalue which will store the result of the following operation. the operation is 5>3 , and
since 5 is more than 3, this will evaluate to true. in the C programming language and many other languages any value that is not 0 is
true, and true is usually 1, so 5>3 will become the value 1 which will make x equal to 1 which means x is true now.
if we did this instead, x=3>5 then the x would end up being 0 in C. other languages like Javascript have actual boolean values
which are not necessarily a number at all, they just contain true or false, so the variables worn contain a 1 or a 0, instead of
that they will actually contain true or false.
<BR />
<BR />3. Conditionals/block of code
<BR />a conditional statement is a statement which give usually two different flow controls depending on wether the statement it is
evaluation is true or false, for example:
<pre><code>
x=5>3;
if(x) {
//this block of code which will run
}
</code></pre>
when explaining conditionals I am forced to explain block of code as well…
basically a block of code is an area of code surrounded by curly braces { }
a block of code can contain many statements or just one statement or even no statements.
but before I go deep into blocks of code, lets look at the previous example. x became true because of the first statement in the
program, if(x) checks to see if x is true or false, if it is true it will run the following statement which is usually a block of code.
If I switch it around to:
<pre><code>
x=3>5;
if(x) {
//this block of code won't run
}
</code></pre>
then the block of code won’t run. because x is false.
we can make these into shorter programs as follows:
<pre><code>
if(5>3) {
//this block of code which will run
}
if(3>5) {
//this block of code won't run
}
</code></pre>
the if statement becomes even more useful when we introduce the “else” statement, for example in the following program:
<pre><code>
x=5>3;
if(x) {
//this block of code which will run
} else {
//this block of code won't run
}
</code></pre>
now lets get into blocks of code.
what if I wrote the following program?
<pre><code>
x=5>3;
if(x) {
//this block of code which will run
y=10;
} else {
//this block of code won't run
y=20
}
</code></pre>
what would y be at the end of the program execution?
try to solve this yourself.
the answer is: 10 because the block of code that run made y into 10
then again, lets say the program said this:
<pre><code>
x=3>5;
if(x) {
//this block of code won't run
y=20
y=y+1;
} else {
//this block of code which will run
y=10;
y=y+2;
}
</code></pre>
what would y contain at the end of the program?
try to solve this.
the answer is it would contain 12.
why is this?
the reason is that in the first statement of the program x becomes false
then the fi statement tests that to be false, and jumpy to the else part of the program, where y is set to 10 and then 2 is added to y
*(show a comparison between scratch blocks and a program made in C like i did to andre)
*(show the am I able to vote in Japan flowchart)
<BR />
<BR />4. Procedures/functions
<BR />now functions need to be broken down into 2 separate parts, the easy part which is a function call and the hard part which is a
function declaration.
in the be inning go this paper I used three function calls to say what I was telling the computer to do:
<pre><code>
BrushMyTeeth();
CombMyHair();
GoToWork();
</code></pre>
I think I will use C as an example of how function declarations should be written, because it is “explicitly typed” which means you
need to write down what kind of data you get and take from it… in many other languages like Javascript and Python just don’t use that
system at all, but if you are ever in the situation of needing to learn an explicit typed language, it is better if you know
it beforehand...
OK, this is an example of a function definition:
<pre><code>
float calculate_tax(float tax_rate, float amount) {
return amount * tax_rate;
}
</code></pre>
this is a function call, let me explain every part of the function call,
the gift “float” is the “return type” or what kind of information the function will return, the reason why I used float instead of
int (integer), is because floating point variables will allow me to represent the result of such an operation, the tax value of some
money will usually have a decimal part and the fractional part of a number, in other words, a number like 3.99 , needs to be a float
because of the .99 part of the number… and altho the tax rate here in Japan in currently 0.8% finally amount, again, since amount can
be 3.99 it again needs to be a float number.
by the way tax_rare and amount are what we call “parameters to the function” they are the information the function takes in.
then we see a code block. and the first instruction in the code block is the return statement… return takes the following value,
evals is (calculates it) and finally gives that back to the function call.
now lets take a look at some sample function calls:
float candy_value=2.00;
float candy_value_plus_tax;
candy_value_plus_tax = calculate_tax(0.8, candy_value);//this is a function call
The first line in C will setup the value of the candy, in this case 2.00 in a float variable.
The second line sets up the variable which will eventually hold the value of the candy after tax.
then comes the function call, first the parameters are sent tot he function so 8.0 and 2.00 are sent to the previously defined function...
taking their place as tax_rate now being equal to 8.0 and amount now being equal to 2.00.
then we get return amount * tax_rate. so int this specific function call, it will be like this:
return 2.00*0.8;
first we solve the vale of 2.00*0.8 which is 1.6 then the return statement hands that back to the function call, and
sets candy_value_plus_tax to 1.6
<BR />
<BR />5.Input/output
<BR />ok, first to explain output you need to understand that computers used to be connected to something called a teletype, which was
like a typewriter and a printer, and instead of using a screen they used to print on paper for output...
<BR />
<BR />now, here is the most simple program every programmer starts out with (in C)
<pre><code>
#include<stdio.h>
int main() {
printf("hello world");
return 0;
}
</code></pre>
or even more simple in python just
print “hello world”
the C version needs quite a bit to explain it, first of all, you are including the stdio.h header file in order to be able to call
the printf() function. then you have the main function definition, main is a special function in C which is called once per
execution of the program. main returns an integer, which at the end of the program is a 0, which basically
means “the program ended with no errors”
now lets look at printf(“hello world”); which is out output function.
the printf() function in it’s most simple form just takes a string (which in C is an array of characters(more on arrays later on)) and
prints them on the teletype or displays them on the screen nowadays.
this program would literally print hello world on a piece of paper in the old days, nowadays it prints it into a “virtual console” or
“terminal program”.
the python version does the same, but with obviously much more simplicity to it...
now lets try putting an integer into our string:
<pre><code>
#include<stdio.h>
int main() {
int a=42;
int b=36;
printf("value of a is %i and value of b is %i", a, b);
return 0;
}
</code></pre>
this printf has two placeholders called %i which %i means it is waiting for an integer value. the first parameter to
the printf statement is “value of a is %i and value of b is %i” which itself is a string, and contains two placeholders
for integers. then the second and third parameters which are a and b which contain the integers that will replace the
two %i’s in the string. then finally this is computed and printed of the screen.
in python this same program is quite a bit more simple, it would go as follows:
<pre><code>
a=42;
b=36;
</code></pre>
printf "value of a is " + str(a) + " and value of b is " + str(b)
in python the plus sign means “concatenation of string” in this context, which means python will take “value of a is “ and connect
a 42 to it then it will connect " and value of b is “ and connect 36
both the python and C programs will produce the same output, which is:
value of a is 42 and value of b is 36
note that str() is a function which takes a number and returns a string in python
<BR />
<BR />now lets try some input:
<pre><code>
#include<stdio.h>
int main() {
char name[100];
char lname[100];
printf("enter your name and last name:");
scanf("%s %s", &name, &lname);
printf("welcome to our program %s %s\n", name, lname);
return 0;
}
</code></pre>
this program will ask you for your name and last name, and then print out welcome to our program followed by your name and last name.
first we reserve 100 characters to hold name and 100 characters to hold lame, scanf is quite a com-plicated function, but lets just
say that in this case the two %s means it is expecting two words separated by a space and stores the first word in name and the
second word in lame. then printf has %s %s which are two placeholders for two separate strings, one for each variable that is
passed, namely name and lname. finally the string has a \n which in C means it will put a new line at the end of this print,
which means it will scroll down one more line
Variables that are setup inside a function belong to that function. Other functions can't see them. But
because we want scanf to send values back when we call it, we use & to give scanf a special value it can use to write to that value.
This value is called a pointer.
but pointers are one of the most difficult portions of C and are beyond the scope of this tutorial
almost the same program in python:
<pre><code>
print "enter your name:"
name=raw_input()
print "enter your last name:"
lname=raw_input()
print "welcome to our program " + name + " " + lname + "\n"
</code></pre>
<BR />
<BR />6. Loops
<BR />
while loop
a while loop is a loop that executed while a condition is true, and ends when a condition is not true.
an example of a while loop in C would be:
<BR />
<pre><code>
#include<stdio.h>
int main() {
int cont=1;
while(cont==1) {
printf("continue? 1)Yes 2)No:");
scanf("%i", &cont);
}
return 0;
}
</code></pre>
<BR />this program will loop over and over again until someone hits 2(or any other number other than 1.
<BR />
notice that the while() expression contains continue==1 instead of continue=1, this is so that the C programming
language can tell the difference between an equal sign for assignment and an double equal sign for comparison.
an equal for assignment like continue=1 would make continue equal to one, but a double equal sign is for comparison
will take continue which is set to 1 on the line int continue=1; and compare it to 1, if this is a match, the entire
expression continue==1 will be come 1 or true. and when the contents of the conditional inside while(conditional)
is true, the following block of code will be run. now lets say the program runs once and the user selects 2, then
it loops back onto while(continue==1) where continue is now 2, continue==1 is false, so in C it will become 0, and
the while loop will skip to the end of the block, ending execution of the loop and the program will end. if the
user chooses 1, then the contention will continue to be true, and the program will ask the user for his input
again, until the user no longer chooses 1
<BR />
<BR />the same program in python would be better written like this:
<pre><code>
cont="1"
while cont=="1":
print "continue? 1)Yes 2)No:"
cont=raw_input()
</code></pre>
<BR />note that in python we dont use curly braces, instead of that we use a tab, or any arbitrary number of spaces to indicate a new block
of code
this is designed to reduce the number of lines used when writing a program.
which brings to mind why I have been writing the programs with ought every line being ("indented", put some space) even in C, and
the answer is that even if C, even tho it is not a law like in python, it is still a better idea to indent than not to indent because when
we get
into blocks inside blocks, it can get very hard to tell which curly { ends which } so indentation gives us a visual way to recognize it
<BR />
<BR />do while loop
while this kind of loop is not as common as while loops, it is still useful and can be neater in certain situations:
lets take these two examples from wikipedia
<pre><code>
do {
do_work();
} while (condition);
</code></pre>
<BR />
<BR />is equivalent to
<BR />
<pre><code>
do_work();
while (condition) {
do_work();
}
</code></pre>
<BR />
<BR />so you can see how we can save ourselves from calling do_work(); twice
<BR />
for loop
<BR />for loop is the most prevalent kind of loop in my opinion.
a for loop had a portions where you initialize a variable, then a conditional statement, then a portion where you
can increment(or do other operations) with the variable you are using
and while while loops can do anything a for loop can do, it would take up more lines of code to do the same thing.
so a for loop is a nice shortcut for writing a loop that repeats a certain ammount of times.
<BR />
<pre><code>
#include<stdio.h>
int main() {
int sum = 0;
for(int counter = 1; counter < 10; ++counter) {
printf("counter is %i and sum is %i\n", counter, sum);
sum += counter;
}
}
</code></pre>
<BR />
notice that sum += counter is just a shortcut form to type sum = sum + counter
the counter is initialized to 1 then the condition for the loop to continue would be counter < 10 and ++counter is a shortcut for
saying counter=counter+1
<BR />
<BR />we could write the same program using a while loop, as follows:
<BR />
<pre><code>
#include<stdio.h>
int main() {
int sum = 0;
int counter = 1;
while(counter < 10) {
printf("counter is %i and sum is %i\n", counter, sum);
sum += counter;
++counter;
}
}
</code></pre>
<BR />
<BR />and in python it would be:
<BR />
<pre><code>
sum = 0
for counter in range(1,10):
print "counter is " + str(counter) + " and sum is " + str(sum)
sum += counter
</code></pre>
<BR />
<BR />the output of all of these programs is:
<BR />counter is 1 and sum is 0
<pre><code>
counter is 2 and sum is 1
counter is 3 and sum is 3
counter is 4 and sum is 6
counter is 5 and sum is 10
counter is 6 and sum is 15
counter is 7 and sum is 21
counter is 8 and sum is 28
counter is 9 and sum is 36
</code></pre>
<BR />
<BR />7. Arrays/lists
<BR />well, first lets take a look at a simple int array in C;
the easiest way to put information into an array in C is by "initializing it" I will give the following example:
int myarr[10] = [9, 8, 7, 6, 5, 4, 3, 2, 1, 0];
this makes an array of 10 integer elements who's zerowe'th element is can be accessed using myarray[0] and that will contain
the integer 9, it will behave exactly like an integer variable once you identify the element you want to access myarr[1]
will contain the integer 8 arr[2] the integer 7 and do on...
note, initialization can only be done when the array is created not later.
another very important thing to know in C is that we ALWAYS COUNT FROM 0, that is why it is myarray[0] and not myarray[1]
if I want to change the value of myarray[0] to 100, as I said, it behaves exactly like an integer. you just do myarray[0]=100;
from then on, tne contents of the array would look pretty much like this [100, 8, 7, 6, 5, 4, 3, 2, 1, 0]
*important* in C "strings" are really just arrays of characters. so giving a similar example to the last one:
<BR />char myarr[10] = "hello";
<BR />is the same as saying:
<BR />char myarr[10] = ['h','e','l','l','o','\0'];
let me explain this, same the zerowe'th element is myarray[0] and it contains 'h' , notice we MUST surrouns single character simbols
in single quotes, double quotes would make it a string and that is not what this array can hold on each slot. and the fifth element is '\0'
which is a special character that means "string terminator" or "this string ends here" this is because C is not smart enough to know where
the string ends and it needs help to find out where it ends even back in char myarr[10] = "hello"; this character is automatically appended
to the end of the string, it is just dont "magically"(so we dont notice it) if we make this following
string char mystr[10] = "hello\0world";
and then try to print that, you will notice that only hello prints, cause C thinks the string ends at hello.
another thing that is extremely important about arrays and strings is that you MUST always make sure there are enough elements to hold the
string you are assigning or copying
to it otherwise you will encounter "undefined behavior"(means we don't know exactly what will happen, but it is not good)
in python you just make a string by doing variablename="string" it is that simple.
</body></html>