Computer Programming (2)

administrivia

Web Page - http://www.cs.tau.ac.il/~efif/courses/ComputerProgramming

a second example

• here's another example C program
  • it's not much more sophisticated
  • it also doesn't do much of any practical use

/* This program prints a list of integers, from 0 to 9 */

#include <stdio.h>

int main (void)
{
   int i;
   for (i = 0; i < 10; i = i + 1)
      printf ("i is %d\n", i);

   return 0;
}

new concepts

• variables
• loops

variables

• how we manipulate data in our program
• name associated with some memory
• has a specific data type
• in this program:
  • a variable named i is used to hold an integer value
  • as the program runs, i's value goes from 0 to 9

/* This program prints a list of integers, from 0 to 9 */

#include <stdio.h>

int main (void)
{
   int i;
   for (i = 0; i < 10; i = i + 1)
      printf ("i is %d\n", i);

   return 0;
}

loops

• a way of repeating the same instructions over and over
• C provides some flexible ways of deciding how many times to loop, or when to exit a loop
• so let's look at this example again…

second example

/* This program prints a list of integers, from 0 to 9 */

#include <stdio.h>

int main (void)
{
   int i;
   for (i = 0; i < 10; i = i + 1)
      printf ("i is %d\n", i);
      
   return 0;
}

• here's that include directive again
  • like before, this allows printf to work
  • we'll just do it now, without worrying about why

second example

/* This program prints a list of integers, from 0 to 9 */

#include <stdio.h>

int main (void)
{
   int i;
   for (i = 0; i < 10; i = i + 1)
      printf ("i is %d\n", i);

   return 0;
}

• and like before, this line introduces our main function.
  • remember that main is where the program starts running

second example

/* This program prints a list of integers, from 0 to 9 */

#include <stdio.h>

int main (void)
{
   int i;
   for (i = 0; i < 10; i = i + 1)
      printf ("i is %d\n", i);

   return 0;
}

• these braces enclose blocks of statements
  • a function is an example of a block

second example

/* This program prints a list of integers, from 0 to 9 */

#include <stdio.h>

int main (void)
{
   int i;
   for (i = 0; i < 10; i = i + 1)
      printf ("i is %d\n", i);

   return 0;
}

• this line introduces a variable
• it's called a declaration
• it tells the compiler that we'll be using a variable
  • named i
  • it holds an integer

second example

/* This program prints a list of integers, from 0 to 9 */

#include <stdio.h>

int main (void)
{
   int i;
   for (i = 0; i < 10; i = i + 1)
      printf ("i is %d\n", i);

   return 0;
}

• here is a loop
  • it will execute its contents 10 times
  • each time, i will have a different value
    • from 0 to 9

second example

/* This program prints a list of integers, from 0 to 9 */

#include <stdio.h>

int main (void)
{
   int i;
   for (i = 0; i < 10; i = i + 1)
      printf ("i is %d\n", i);

   return 0;
}

• this statement is the "contents" of the loop
• so it gets executed 10 times

  i is 0
  i is 1
  i is 2
  etc.
  

we could have written it like this…

/* This program prints a list of integers, from 0 to 9 */

#include <stdio.h>

int main (void)
{
   printf ("i is 0\n");
   printf ("i is 1\n");
   printf ("i is 2\n");
   printf ("i is 3\n");
   printf ("i is 4\n");
   printf ("i is 5\n");
   printf ("i is 6\n");
   printf ("i is 7\n");
   printf ("i is 8\n");
   printf ("i is 9\n");

   return 0;
}

ok, what's going on here?

• we've just seen 2 simple programs
• these are C programs
  • not machine language
    • the C compiler is used to turn them into something the machine understands
• the C compiler is itself a program
  • it reads the C program you write
  • it writes out (hopefully) a program the computer can run

remember what a program is

• the real program is just numbers
• a C program is a sequence of C instructions
  • we can define variables to hold our data
  • we write C statements to do things with our data
  • we can group our statements into functions

so what's a compiler?

• a compiler is a program that converts a C program into machine language
  • remember, the computer can only run machine language
• exact definition

so let's look at the whole process

• write a program
  • text editor
• compile the program
  • C compiler
  • will do one of two things:
    • print error messages
    • produce an executable program
• run the program

let's talk about this variable concept…

• variables are central to almost any C program
• they are your program's data
  • the information your program works with
• if you want to store any data for use elsewhere in the program, you need to use a variable

variable declarations

• before you can use a variable, you must declare it.
  • a declaration introduces a variable name
  • associates it with a data type
  • sets some optional characteristics called qualifiers

variable names

• identifiers
  • used for all sorts of names (e.g. function names)
  • rules
    • letters, digits, underscore
      • i
      • CSE_5a
      • a_VERY_long_variable_name_that_isnt_very_useful
      • fahrenheit
    • first character cannot be a digit
      • 5a_cse is not legal
    • case sensitive
      • CSE_5a is different from cse_5a
  • some identifiers are already taken: keywords

data types

• Variables in C must have a data type
• integers and floating-point numbers
• the following are the basic C data types:
  • char
  • short int (or just "short")
  • int
  • long int (or just "long")
  • float
  • double
  • long double

how to know which to use?

• the C standard does not say how big each type is
  • "big" in this case meaning how many bytes
  • which basically means how big a range on numbers it can hold
• but the standard does make a few guarantees
• for now, you're probably safe sticking with
  • char for characters
  • int for integers
  • double for real numbers

why have data types?

• some languages don't
  • but they're the exception, not the rule
• the computer itself have types
  • computers handle integers differently from real numbers
• performance
  • old computers did everything with integers
  • operations involving real numbers take different time to calculate
• safety
  • the compiler can catch some errors

bits are fundamentally typeless

• the computer doesn't store the type, just bits
  • the same data can be interpreted differently depending on what type you tell the computer it is
• 01000000000000000000000000000000
  • hex: 0x40000000
  • integer: 1073741824
    • test it
  • floating-point: 2.0
  • string: "@"

integral types

• char is 1 byte (0-255)
• short is usually 2 bytes
  • but can be anything between char and int
• int is usually 4 bytes
  • whatever the computer's most efficient integer size is
  • guaranteed at least 2 bytes (-32768 to 32767)
• long is usually 4 or 8 bytes
  • can be anything equal to or bigger than an int
  • used when you know you need to deal with really big numbers
  • guaranteed at least 4 bytes (-2,147,483,648 to 2,147,483,647)

integral types continued

• short is rarely used
• int is most commonly used
  • what we'll use most often in this class

char – a special integral type

• used to store characters
  • letters
  • digits
  • keyboard symbols
  • some unprintable characters
• remember – in the computer, everything is numbers
  • even text

 text as numbers

• every character is assigned a numeric code
• there are different sets of codes
  • ASCII
    • most common
  • EBCDIC
    • dinosaur
  • probably others
• we will only consider ASCII in this class
  • but there's nothing intrinsic in C that specifies this

• American Standard Code for Information Interchange

more about character encoding

• most of the time, you don't care what the particular numbers are
• usually fit in a byte
  • 0 to 127 uses 7 bits
  • extended ASCII has 256 codes (8 bits)
• ASCII code 0 is important
  • this is the nul character
  • we'll see it again
• note contiguous sets of characters
• we'll see it again
  • all digits
  • all capital letters
  • all lower-case letters

int

• the most common integer type
• used when you have no specific reason to pick one of the others
  • use short when space is at a premium
  • use long when your numbers may be greater than 32,767
  • it's the most efficient integer type

double

• the most efficient floating-point type
• used when you have no specific reason to pick one of the others
  • float – when space is at a premium
  • long double – when you need very precise or very big numbers

let's look at an example

/*
 * Print a Fahrenheit-to-Celsius
 * conversion table
 */

#include <stdio.h>

int main (void)
{
   int fahr;
   int celsius;
   int lower = 0;
   int upper = 300;
   int step = 20;

   fahr = lower;
   while (fahr <= upper) {
      celsius = 5 * (fahr – 32) / 9;
      printf ("%d\t%d\n", fahr, celsius);
      fahr = fahr + step;
   }

   return 0;
}

what does this do?

• this program will print out a temperature conversion table
  • prints a fahrenheit temperature and its celsius equivalent
  • for fahrenheit temperatures from 0 to 300 at 20 deg. intervals

let's look more closely

#include <stdio.h>

int main (void)
{
   int fahr;
   int celsius;
   int lower = 0;
   int upper = 300;
   int step = 20;

   fahr = lower;
   while (fahr <= upper) {
      celsius = 5 * (fahr – 32) / 9;
      printf ("%d\t%d\n", fahr, celsius);
      fahr = fahr + step;
   }

   return 0;
}

• remember this stuff goes on all our programs

let's look more closely

#include <stdio.h>

int main (void)
{
   int fahr;
   int celsius;
   int lower = 0;
   int upper = 300;
   int step = 20;

   fahr = lower;
   while (fahr <= upper) {
      celsius = 5 * (fahr – 32) / 9;
      printf ("%d\t%d\n", fahr, celsius);
      fahr = fahr + step;
   }

   return 0;
}

• here we declare five variables
  • they are all ints
  • we will talk about delcarations soon
  • basically, this creates five variables we can use later in the program

let's look more closely

#include <stdio.h>

int main (void)
{
   int fahr;
   int celsius;
   int lower = 0;
   int upper = 300;
   int step = 20;

   fahr = lower;
   while (fahr <= upper) {
      celsius = 5 * (fahr – 32) / 9;
      printf ("%d\t%d\n", fahr, celsius);
      fahr = fahr + step;
   }

   return 0;
}

• this is an assignment statement
  • this sets the value of a variable
  • in this case, it sets the value of fahr
  • to the value of lower, which is 0

let's look more closely

#include <stdio.h>

int main (void)
{
   int fahr;
   int celsius;
   int lower = 0;
   int upper = 300;
   int step = 20;

   fahr = lower;
   while (fahr <= upper) {
      celsius = 5 * (fahr – 32) / 9;
      printf ("%d\t%d\n", fahr, celsius);
      fahr = fahr + step;
   }

   return 0;
}

• this is a loop
  • we'll get to loops in a while
  • this will repeatedly execute the following 3 statements
    • as long as fahr is less than or equal to upper

let's look more closely

#include <stdio.h>

int main (void)
{
   int fahr;
   int celsius;
   int lower = 0;
   int upper = 300;
   int step = 20;

   fahr = lower;
   while (fahr <= upper) {
      celsius = 5 * (fahr – 32) / 9;
      printf ("%d\t%d\n", fahr, celsius);
      fahr = fahr + step;
   }

   return 0;
}

• this is another assignment statement
  • this time, we set the value of celsius
    • to an arithmetic expression
    • this calculates a celsius equivalent to the value of fahr

let's look more closely

#include <stdio.h>

int main (void)
{
   int fahr;
   int celsius;
   int lower = 0;
   int upper = 300;
   int step = 20;

   fahr = lower;
   while (fahr <= upper) {
      celsius = 5 * (fahr – 32) / 9;
      printf ("%d\t%d\n", fahr, celsius);
      fahr = fahr + step;
   }

   return 0;
}

• here's another use of printf to print things out
  • this is more complex than we've used before
  • it prints the values of two variables

let's look more closely

#include <stdio.h>

int main (void)
{
   int fahr;
   int celsius;
   int lower = 0;
   int upper = 300;
   int step = 20;

   fahr = lower;
   while (fahr <= upper) {
      celsius = 5 * (fahr – 32) / 9;
      printf ("%d\t%d\n", fahr, celsius);
      fahr = fahr + step;
   }

   return 0;
}

• another assignment
  • changes the value of fahr by adding the value of step to it

let's look more closely

#include <stdio.h>

int main (void)
{
   int fahr;
   int celsius;
   int lower = 0;
   int upper = 300;
   int step = 20;

   fahr = lower;
   while (fahr <= upper) {
      celsius = 5 * (fahr – 32) / 9;
      printf ("%d\t%d\n", fahr, celsius);
      fahr = fahr + step;
   }

   return 0;
} 

• these statements work together
  • the loop runs until fahr is greater than upper
  • the assignment increments fahr each time
  • the net effect is that fahr goes from lower to upper

let's look more closely

#include <stdio.h>

int main (void)
{
   int fahr;
   int celsius;
   int lower = 0;
   int upper = 300;
   int step = 20;

   fahr = lower;
   while (fahr <= upper) {
      celsius = 5 * (fahr – 32) / 9;
      printf ("%d\t%d\n", fahr, celsius);
      fahr = fahr + step;
   }

   return 0;
}   

• look at this program on your own
• try to understand why it works
• try to understand why it was written this way