Introduction to Programming Using C Basic Computations

Contents l l l 2 How do programs work? Out first C program Variables Data types Reading Evaluating expressions

How do Programs Work ? INPUT COMPUTATION OUTPUT 3 Program

How do Programs Work ? l The average program – – – 4 Reads some input Performs some calculations on the input Writes some output

Our First C Program The function main is always the first function called This is the start of a function called main() { printf(“Hello, world!n”); } 5 The braces show the start and end of the function The function body contains one or more statements. Printf prints a string or other data

Our First C Program Your program must have a main, but can contain other functions 6 A parameter list inside parentheses can be used to pass data to a function main() { printf(“Hello, world!n”); } The braces can be used to delimit the start and end of many things Strings are enclosed in double quotes Every statement is terminated by a semicolon n causes it to start printing on a new line

Program Structure l A program contains – – One or more functions Every program must have a function called main is the first function called A function contains l One or more statements A statement reads input, performs computations, or writes output – A statement is terminated by a semicolon – 7

Strings l l l A string is simply a collection of characters It is one way we can print text to a screen or printer Strings are enclosed in double quotes to mark the beginning and end – 8 “This is a string”

Control Characters l l Strings can contain special control characters These do not print, but have some effect on the printer or computer – – – 9 n a b f t \ newline sounds the beep or alarm backspace form feed tab a backslash character

Computer Memory l l l 10 Our first program simply printed a string which never changed Most program want to do something more exciting This means they want to read input which might change and produce output based on the input

Computer Memory l l After the program reads data, it performs calculations It needs to store – – l l 11 The data it read Temporary results These are stored in memory Memory is used like a scratchpad

Variables l l l Memory is a big place Modern computers have several billion locations in memory How do you find anything? – Use the address, like a house number l – Give the address a name l 12 Can be hard to remember Fred’s house is easier than 34487291 Elm St.

Variables l When we name a location in memory, it is called a variable addresses 0 1 2 3 4 5 6 A named location “X” 13 7 8 9 10 continues A long way …

Variables address 4 A variable can contain only one value 42 Variable name 14 X If another value is assigned, the original value is overwritten, like a whiteboard

Data Types l l l 15 Computers are dumb Not only do you have to name the memory locations, you have to state what type of data you will store there This makes sure that the computer knows how to treat the data stored in the variable

Data Types l l The simplest data types store numbers We make a major distinction between numbers which can have fractions and those which cannot – Integers l l – Floating point l l 16 Cannot have a fraction 2, 457, -87, 0, 1, 99999, -456 Can have a fraction 1. 5, -2. 98, 3. 14159, -11. 74

Data Types l The integer types we use are – – – l your average numbers less that 256 really big numbers The floating point types include – – 17 int short long double your average high-precision number float a less precise fractional number

Declaring Variables l l We must declare a variable before using it This tells the computer – – – l To create an int variable called age we would write – 18 To find some free memory for us to use What type will be stored in the memory What name to use for that memory int age;

Variable Names l Variable names must meet some rules – – 19 May contain letters (a-z, A-Z), digits (0 -9), and underscore (_) The first character must not be a digit Some compilers limit a variable name to 31 characters Cannot be the same as a keyword in the C language

Variable Names l Valid names – – – l Invalid names – – – 20 X First_number R 2 d 2 Day of week 1 st_number int (spaces not allowed) (can’t start with a digit) (language keyword)

Using a Variable l Let’s double the value of a variable and print the result main() { int number; number = 18; number = number * 2; printf(“The result is %dn”, number); } 21

Using a Variable l First we declare the variable number – l Then, we assign it a value – l number = 18; Then we replace the value with the result of a calculation – – 22 int number; number = number * 2; This multiplies the old value of number by 2 and makes tha the new value of number.

Using a Variable l Finally, we print the value, using a formatted print statement %d to format next parameter as a decimal number printf(“The result is %dn”, number); Value to be formatted by the format string %d 23

Reading Data l l If a program could only work with data you typed into the program while writing it, the program would not be very useful A better approach is to allow the program to read data from – – – 24 The keyboard A file The internet

Reading Data l l We start with reading data from the keyboard Reading data is the reverse of printing – – – 25 scanf is the reverse of printf It uses a format string to tell it what type of data to read It provides a list of variables into which to store the data being read

Reading Data l Now, let’s rewrite our program to double any number main() { int number; scanf(“%d”, &number); number = number * 2; printf(“The result is %dn”, number); } 26

Assignment Operator The equals sign “=“ is the assignment operator l It assigns the value on the right to the variable on the left number = number * 2; l – – 27 This calculates the right hand side It then assigns the result to the variable on the left hand side

Reading Data l The difference lies in using scanf … Variable where data should be stored scanf(“%d”, &number); 28 Format to read a decimal number Address operator which returns address of variable

Reading Data l When we reference a variable by its name – l l l 29 number We get the value stored there However, when we want to store a value at that location, we need the address, not the value Therefore, we use the address of operator, &, to get the address of a variable

Arithmetic Operators l l So far we have seen the multiply operator, * There are several arithmetic operators – – 30 + * / addition subtraction multiplication division

Expression Evaluation l Arithmetic expressions are evaluated – – 31 Left to right Things in parentheses are evaluated before things outside parentheses Mutiplication and division is performed first Addition and subtraction is performed second

Expression Evaluation Expression 2*3+4 10 2*(3+4) 14 2+3*4+1 15 (2+3)*(4+1) 25 (6 -4)*5/2 5 ((2+1)*2) / (3 – 1) 32 Result 3

Mixed Data Types l l All of out expressions have involved integers What happens when we combine an integer with a floating point number? The result is a floating point number Otherwise, we might lose the fraction – – – 33 2 * 3. 4 is 6. 8 Here we multiply integer 2 times floating point 3. 4 to yield another floating point, 6. 8 Writing a number without a fraction indicates it is an integer

Division l The division operator works differently depending on whether it is used with integers or floating point numbers – – – l 34 2. 0 / 3. 0 2/3 2 / 3. 0 1. 5 1 1. 5 The result is an integer and the fraction is discarded only if both numbers in the expression are integers

A Cash Register Example l l Now, let’s write a program to act as a simple cash register The program will – – – 35 Read a price for some product Read the number of product purchased Print the total, including taxes

$A Cash Register Example main() { int quantity; double unit_price, tax, total; printf(“Enter product$

A Cash Register Example main() { int quantity; double unit_price, tax, total; printf(“Enter product costn”); scanf(“%lf”, &unit_price); printf(“Enter quantity purchasedn”); scanf(“%d”, &quantity); total = quantity * unit_price; tax = total * 0. 14; total = total + tax; printf(“The amount due is %dn”, total); } 36