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CSE1301Computer Programming

Lecture 6:Components of a C Program

(Part 2)

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Topics

• Type

• Variables

• Keywords and Identifiers

• Assignments

• Constant Variables

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Type

• The kind of a value is its “type”

• Not all values are of the same kind– For example: can’t logically evaluate 7 + “cat”

ValueVariable

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Type

• Built-in types: char, int, float• Type modifiers: long, short, const• User-defined types (arrays and records)

• What about “strings”?– Strings are arrays of char (discussed later)

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Character Representation

• Characters are stored as a small integer• Each character has a unique integer

equivalent specified by its position in the ASCII table (pronounced “ask-key”)

– American Standard Code for Information Interchange

– see Appendix D of D&D

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Character Representation

• The ASCII values range from 0 to 127– value 0: special character ’\0’

(a.k.a. NUL character)– value 127: special character <DEL>– other special characters: ’\n’ ’\t’ ’\’’ ’\\’ etc.

– various “extended” sets from 128 to 255

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Variable

• Is a logical name for a container (an actual piece of computer memory for values)

• Have a type associated with them, which tells the computer how to interpret the bits

• Must be declared before use:

[modifiers] <type name> <variable name> ;

[modifiers] <type name> <variable name> = <init value> ;

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Variable Declaration: Examples

int myID;

myID

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Variable Declaration: Examples

int myID;

char my_initial = ’J’;

Single “forward quotes” or apostrophe (’) rather than

“back quotes” (‘)

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Variable Declaration: Examples

int myID;

char my_initial = ’J’; 01001010my_initial

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Variable Declaration: Examples

int myID;

char my_initial = ’J’;

char my_initial = 74 ;

01001010my_initial

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Variable Declaration: Examples

float commission = 0.05;

short int myHeight = 165; /* cm */

long int mySalary = 100000000000000000000;

long float chanceOfADate = 3e-500;

double chance_of_a_2nd_date = 1.5e-500;

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float commission = 0.05;

short int myHeight = 165; /* cm */

long int mySalary = 100000000000000000000;

long float chanceOfADate = 3e-500;

double chance_of_a_2nd_date = 1.5e-500;

Variable Declaration: Examples

“Keywords”

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Keyword

• ...has a special meaning in C

• ...is “case-sensitive”

• ...cannot be used as variable names

• Examples:int, char, long, main, float, double, const, while, for, if, else, return, break, case, switch, default, typedef, struct, etc. (see D&D 2/e Appendix A or D&D 3/e page 545, Figure 15.4)

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Variable Declaration: Examples

float commission = 0.05;

short int myHeight = 165; /* cm */

long int mySalary = 100000000000000000000;

long float chanceOfADate = 3e-500;

double chance_of_a_2nd_date = 1.5e-500;

“Identifiers”

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Identifier

• ...is a series of characters consisting of letters, digits and underscores ( _)

• ...cannot begin with a digit

• ...must not be a keyword

• ...is “case-sensitive”

• Examples:sUmoFA, x1, y2, _my_ID_, Main (careful!)

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Assignment

• Puts a specified value into a specified variable

• Assignment operator: =

<variable name> = <expression> ;

not to be confused with ==

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Assignment: Examplesfloat x = 2.5 ;

char ch ;

int number ;

ch = ’\n’ ;

number = 4 + 5 ;

/* current value of number is 9. */

number = number * 2;

/* current value of number is now 18. */

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Assignment

• Value must have a type assignable to the variable

• Value may be automatically converted to fit the new container

• Example:– various.c

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#include <stdio.h>

/* Do various assignment statements */

int main(){ int integer;

char character;

float floatingPoint;

integer = 33;

character = 33;

floatingPoint = 33;

integer = 'A';

character = 'A';

floatingPoint = 'A';

integer = 33.33;

character = 33.33;

floatingPoint = 33.33;

integer = floatingPoint;

floatingPoint = integer;

return 0;

}

various.c

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Constant Variables

• ...are variables that don’t vary

• ...may not be assigned to.

• ...must be initialized

const [modifiers] <type> <constant name> = <init value>;

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Constant Variables: Examples

const int myID = 192;

myID = 666; /* Error! */

const int passMark = 80;

short char pAsSgRaDe = ’P’;

const float e = 3.1415926; /* oops */

const double golden_ratio = 1.61803398874989;

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Converts an angle from degrees to radians

output “Enter angle in degrees”input angleInDegrees

angleInRadians = / 180 * angleInDegrees

output angleInRadians

#include <stdio.h>

/* Converts an angle in degrees to radians. */

const float PI = 3.1415926;

int main(){ float angleInDegrees; float angleInRadians;

printf("Enter angle in degrees: "); scanf("%f", &angleInDegrees);

angleInRadians = PI / 180 * angleInDegrees;

printf("%f\n", angleInRadians); return 0;}

Example: Constants

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Example: Constants

“Global” (constant) variable

“Local” variables

more on this later...

#include <stdio.h>

/* Converts an angle in degrees to radians. */

const float PI = 3.1415926;

int main(){ float angleInDegrees; float angleInRadians;

printf("Enter angle in degrees: "); scanf("%f", &angleInDegrees);

angleInRadians = PI / 180 * angleInDegrees;

printf("%f\n", angleInRadians); return 0;}

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Readings

This Lecture:

• D&D 2/e: Sections 2.1 to 2.10

• D&D 3/e: Sections 2.1 to 2.5

Next Lecture: Booleans

• D&D 2/e: – Sections 2.12 to 2.13, 3.2 and 4.13 to 4.15

• D&D 3/e: Sections 2.6, 3.5, 4.10 and 4.11