Lecture #5: Data types

Bob Pitha

CSE 5a

bpitha@cs.ucsd.edu

http://www-cse.ucsd.edu/classes/fa99/cse5a

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 may have types

–   some computers handle integers differently from real numbers

•      Performance

–   old computers did everything with integers

–   calculations involving real numbers were slow

•      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

–   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

–   we won’t worry about it in this class

•      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

what is ASCII?


 


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

–   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

Text Box: /* * FtoC1 – 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

Text Box: #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

Text Box: #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

Text Box: #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

Text Box: #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

Text Box: #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

Text Box: #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

Text Box: #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

Text Box: #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

Text Box: #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