Chapter 2 C Basics 1 Overview n Variables

Chapter 2 C++ Basics 1

Overview n Variables, Constants and Assignments (2. 1) n Input and Output (2. 2) n Data Types and Expressions (2. 3) n Simple Flow of Control (2. 4) n Program Style (2. 5) 2

2. 1 Variables and Assignments n Variables are like small blackboards We can write, change or erase a number on them n C++ variables are names for memory locations n We can write or change a value in them n We cannot erase the memory location n Variables names are called identifiers n First character must be n n Remaining characters must be n n n letter underscore character letters numbers underscore character Use meaningful identifiers that indicate the purpose of the variable Keywords cannot be used as identifiers Display 2. 1 3

Declaring Variables (Part 1) n Tells the compiler the type of data to store Examples: int number_of_bars; double one_weight, total_weight; n int is an abbreviation for integer. n n could store 3, 102, 3211, -456, etc. number_of_bars is of type integer n double represents numbers with a fractional component n n could store 1. 34, 4. 0, -345. 6, etc. one_weight and total_weight are both of type double 4

Declaring Variables (Part 2) n When declared? Always, immediately prior to use n Either at the beginning or before use int main() { … int sum; sum = score 1 + score 2; … return 0; } int main() { int sum; … sum = score 1 + score 2; … return 0; } n Declaration syntax: n Type_name Variable_1 , Variable_2, . . . ; 5

Constants n const is the keyword to declare a constant n Contrary to variables constants can never change throughout a program n Change all occurrences simply by changing the value of the constant n Example: const int WINDOW_COUNT = 10; declares a constant named WINDOW_COUNT n Its value cannot be changed by the program like a variable n It is common to name constants with all capitals n Number constants have no mnemonic value but allow us to name number constants so they have meaning n otherwise they are difficult to find and change throughout a program when needed 6

Assignment Statements n An assignment statement changes the value of a variable total_weight = one_weight + number_of_bars; n n n Assignment statements end with a semi-colon The single variable to be changed is always on the left of the assignment operator ‘=‘ On the right of the assignment operator can be n Constants --age = 21; n n Variables --my_cost = your_cost; Expressions --- circumference=diameter*3. 14159; ATTENTION! The ‘=‘ operator in C++ is not an equal sign!!! 7

Initializing Variables n Declaring a variable does not give it a value n Giving a variable its first value is initializing the variable n Variables are initialized in assignment statements double mpg; // declare the variable mpg = 26. 3; // initialize the variable n Declaration and initialization can be combined using two methods n Method 1 double mpg = 26. 3, area = 0. 0 , volume; n Method 2 double mpg(26. 3), area(0. 0), volume; 8

Section 2. 1 Conclusion n Can you n Declare and initialize two integers variables to zero? The variables are named feet and inches. n Declare and initialize two variables, one int and one double? Both should be initialized to the appropriate form of 5. n Give good variable names for identifiers to store n the speed of an automobile? n an hourly pay rate? n the highest score on an exam? 9

2. 2 Input and Output n A data stream is a sequence of data n Typically in the form of characters or numbers n An input stream is data for the program to use n Typically originates n at the keyboard n at a file n An output stream is the program’s output n Destination is typically n the monitor n a file 10

Output using cout n cout is an output stream sending data to the monitor n The insertion operator "<<" inserts data into cout Example: cout << number_of_bars << " candy barsn"; n Here arithmetic is performed in the cout statement cout << "Total cost is $" << (price + tax); n Quoted strings are enclosed in double quotes ("Walter") n Don’t use two single quotes (') n A blank space can also be inserted with n n cout << " " ; or in the quotes, i. e. cout << number_of_bars << " candy barsn"; 11

Include Directives n Include Directives add library files to our programs n To make the definitions of the cin and cout available to the program: #include <iostream> n Using Directives include a collection of defined names n To make the names cin and cout available to our program: using namespace std; 12

Escape Sequences n Escape sequences tell the compiler to treat characters in a special way n '' is the escape character n To create a newline in output use n – cout << "n"; or the newer alternative cout << endl; n Other escape sequences: t -- a tab \ -- a backslash character " -- a quote character 13

Formatting Real Numbers n Real numbers (type double) produce a variety of outputs double price = 78. 5; cout << "The price is $" << price << endl; n n The output could be any of these: The price is $78. 500000 The price is $7. 850000 e 01 The most unlikely output is: The price is $78. 50 14

Showing Decimal Places n cout includes tools to specify the output of type double n To specify fixed point notation setf(ios: : fixed) n To specify that the decimal point will always be shown n setf(ios: : showpoint) n To specify that two decimal places will always be shown n precision(2) n n Example: cout. setf(ios: : fixed); cout. setf(ios: : showpoint); cout. precision(2); cout << "The price is " << price << endl; 15

Reading Data From cin n n cin is an input stream bringing data from the keyboard The extraction operator (>>) removes data to be used Multiple data items are separated by spaces Data is not read until the enter key is pressed n Allows user to make corrections n Example: n n cin >> v 1 >> v 2 >> v 3; Requires three space separated values User might type 34 45 12 <enter key> 16

Designing Input and Output n Prompt the user for input that is desired n cout statements provide instructions cout << "Enter your age: "; cin >> age; n Notice the absence of a new line before using cin n Echo the input by displaying what was read n Gives the user a chance to verify data cout << age << " was entered. " << endl; 17

Section 2. 2 Conclusion n Can you n Write an input statement to place a value in the variable the_number? n Create a named constant of type double? n Determine if a program can modify the value of a constant? n Write the output statement to prompt for the value to store in the_number? n Write an output statement that produces a newline? n Format output of rational numbers to show 4 decimal places? 18

2. 3 Data Types and Expressions n 2 and 2. 0 are not the same number n n A whole number such as 2 is of type int A real number such as 2. 0 is of type double n Numbers of type int are stored as exact values n Numbers of type double may be stored as approximate values (due to limitations on number of significant digits that can be represented) n Type double can be written in two ways n Simple form must include a decimal point n n n Examples: 34. 1 23. 0034 1. 0 89. 9 Floating Point Notation (Scientific Notation) n n Examples: 3. 41 e 1 means 34. 1 3. 67 e 17 means 36700000000. 0 5. 89 e-6 means 0. 00000589 Exponent cannot contain a decimal point 19

Other Number Types n Various number types have different memory requirements More precision requires more bytes of memory n Very large numbers require more bytes of memory n Very small numbers require more bytes of memory n Display 2. 2 20

Integer types n long or long int (often 4 bytes) n Equivalent forms to declare very large integers long big_total; long int big_total; n short or short int (often 2 bytes) n Equivalent forms to declare smaller integers short small_total; short int small_total; 21

Floating point types n long double (often 10 bytes) n Declares floating point numbers with up to 19 significant digits long double big_number; n float (often 4 bytes) n Declares floating point numbers with up to 7 significant digits float not_so_big_number; 22

Type char n char - Short for character n Can be any single character from the keyboard n To declare a variable of type char: char letter; To declare a constant of type char enclose the character in single quotes char letter = 'a'; n Strings of characters, even if only one character are enclosed in double quotes n "a" is a string of characters containing one character n 'a' is a value of type character 23

Reading Character Data n cin skips blanks and line breaks looking for data n The following reads two characters but skips any space that might be between char symbol 1, symbol 2; cin >> symbol 1 >> symbol 2; n User normally separate data items by spaces J D n Results are the same if the data is not separated by spaces JD Display 2. 3 24

Type bool n bool is a new addition to C++ n Short for boolean n Boolean values are either true or false n To declare a variable of type bool: bool old_enough; 25

Type Compatibilities n In general store values in variables of the same type n This is a type mismatch: int_variable; int_variable = 2. 99; n If your compiler allows this, int_variable will most likely contain the value 2, not 2. 99 26

int double n Variables of type double should not be assigned to variables of type int int_variable; double_variable; double_variable = 2. 00; int_variable = double_variable; n If allowed, int_variable contains 2, not 2. 00 n int values can normally be stored in variables of type double_variable; double_variable = 2; n double_variable will contain 2. 0 27

char int n The following actions are possible but generally not recommended! n It is possible to store char values in int variables int value = 'A'; value will contain an integer representing 'A' n It is possible to store int values in char variables char letter = 65; 28

bool int n The following actions are possible but generally not recommended! n Values of type bool can be assigned to int variables n True is stored as 1 n False is stored as 0 n Values of type int can be assigned to bool variables n Any non-zero integer is stored as true n Zero is stored as false 29

Arithmetic n Arithmetic is performed with operators n n + * / for addition for subtraction for multiplication for division n Example: total_weight number_of_bars; = one_weight * n Arithmetic operators can be used with any numeric type n n An operand is a number or variable used by the operator Result of an operator depends on the types of operands n n If both operands are int, the result is int If one or both operands are double, the result is double 30

Division of Integers n Be careful with the division operator! n int / int produces an integer result (true for variables or numeric constants) int dividend, divisor, quotient; dividend = 5; divisor = 3; quotient = dividend / divisor; n n The value of quotient is 1, not 1. 666…!!!! Integer division does not round the result, the fractional part is discarded! 31

Integer Remainders n % operator gives the remainder from integer division int dividend, divisor, remainder; dividend = 5; divisor = 3; remainder = dividend % divisor; The value of remainder is 2 Display 2. 4 32

Arithmetic Expressions n Use spacing to make expressions readable n Which is easier to read? x+y*z or x+y*z Display 2. 5 n Precedence rules for operators are the same as used in your algebra classes n Use parentheses to alter the order of operations x + y * z ( y is multiplied by z first) (x + y) * z ( x and y are added first) 33

Operator Shorthand n Some expressions occur so often that C++ contains to shorthand operators for them n All arithmetic operators can be used this way n += n n count = count + 2; becomes count += 2; bonus = bonus * 2; becomes bonus *= 2; *= n n /= n time = time / rush_factor; becomes time /= rush_factor; %= n remainder = remainder % (cnt 1+ cnt 2); becomes remainder %= (cnt 1 + cnt 2); n 34

2. 4 Simple Flow of Control n Flow of control n The order in which statements are executed n Branch n Lets program choose between two alternatives n if-else statement 35

Branch Example n To calculate hourly wages there are two choices n Regular time ( up to 40 hours) n n Overtime ( over 40 hours) n n gross_pay = rate * hours; gross_pay = rate * 40 + 1. 5 * rate * (hours - 40); The program must choose which of these expressions to use 36

Implementing the Branch n if-else statement is used in C++ to perform a branch if (hours > 40) gross_pay = rate * 40 + 1. 5 * rate * (hours - 40); else gross_pay = rate * hours; Display 2. 6 Display 2. 7 37

if-else Flow Control if (boolean expression) {true statement} else false statement n When the boolean expression is true n Only the true statements enclosed in { } are executed n The { } can be eliminated if there is a single statement n When the boolean expression is false n Only the true statements enclosed in { } are executed n The { } can be eliminated if there is a single statement REMEMBER: Boolean expressions are expressions that are either true or false n comparison operators are used to compare variables and/or numbers n See display for comparison operators Display 2. 8 38

AND n Boolean expressions can be combined into more complex expressions with n && -- The AND operator n True if both expressions are true n Syntax: (Comparison_1) && (Comparison_2) n Example: if ( (2 < x) && (x < 7) ) n True only if x is between 2 and 7 n Inside parentheses are optional but enhance meaning 39

OR n | | -- The OR operator (no space!) n True if either or both expressions are true n Syntax: (Comparison_1) | | (Comparison_2) n Example: if ( ( x = = 1) | | ( x = = y) ) n True if x contains 1 n True if x contains the same value as y n True if both comparisons are true 40

NOT n ! -- negates any boolean expression n !( x < y) n n True if x is NOT less than y !(x = = y) n True if x is NOT equal to y n ! Operator can make expressions difficult to understand…use only when appropriate 41

Inequalities n Be careful translating inequalities to C++ n if x < y < z translates as if ( ( x < y ) && ( y < z ) ) NOT if ( x < y < z ) 42

Pitfall: Using = or == n ' = ' is the assignment operator n Used to assign values to variables n Example: x = 3; n '= = ' is the equality operator n Used to compare values n Example: if ( x == 3) n The compiler will accept this error: if (x = 3) but stores 3 in x instead of comparing x and 3 n Since the result is 3 (non-zero), the expression is true 43

Compound Statements n A compound statement is more than one statement enclosed in { } n Branches of if-else statements often need to execute more that one statement n Example: if (boolean expression) { true statements } else { n false statements } Display 2. 9 44

Branches Conclusion n Can you n Write an if-else statement that outputs the word High if the value of the variable score is greater than 100 and Low if the value of score is at most 100? The variables are of type int. n Write an if-else statement that outputs the word Warning provided that either the value of the variable temperature is greater than or equal to 100, or the of the variable pressure is greater than or equal to 200, or both. Otherwise, the if_else sttement outputs the word OK. The variables are of type int. 45

Simple Loops When an action must be repeated, a loop is used C++ includes several ways to create loops We start with the while-loop Example: while (count_down > 0) { cout << "Hello "; count_down -= 1; } n Output: Hello when count_down starts at 3 n n Display 2. 10 46

While Loop Operation n First, the boolean expression is evaluated n If false, the program skips to the line following the while loop n If true, the body of the loop is executed n During execution, some item from the boolean expression is changed n After executing the loop body, the boolean expression is checked again repeating the process until the expression becomes false n A while loop might not execute at all if the boolean expression is false on the first check 47

while Loop Syntax n while (boolean expression is true) { statements to repeat; } n n Semi-colons are used only to end the statements within the loop Display 2. 11 48

do-while loop n A variation of the while loop. n A do-while loop is always executed at least once n The body of the loop is first executed n The boolean expression is checked after the body has been executed n Syntax: do { Display 2. 12 Display 2. 13 statements to repeat; } while (boolean_expression); 49

Increment/Decrement n Unary operators require only one operand n + in front of a number such as +5 n - in front of a number such as -5 n ++ increment operator n Adds 1 to the value of a variable x ++; is equivalent to x = x + 1; n -decrement operator n Subtracts 1 from the value of a variable x --; is equivalent to x = x – 1; 50

Sample Program n Bank charge card balance of $50 n 2% per month interest n How many months without payments before your balance exceeds $100 n After 1 month: $50 + 2% of $50 = $51 n After 2 months: $51 + 2% of $51 = $52. 02 n After 3 months: $52. 02 + 2% of $52. 02 … Display 2. 14 51

Infinite Loops n Loops that never stop are infinite loops n The loop body should contain a line that will eventually cause the boolean expression to become false n Example: Print the odd numbers less than 12 x = 1; while (x != 12) { cout << x << endl; x = x + 2; } n Better to use this comparison: while ( x < 12) 52

Section 2. 4 Conclusion n Can you n Show the output of this code if x is of type int? x = 10; while ( x > 0) { cout << x << endl; x = x – 3; } n Show the output of the previous code using the comparison x < 0 instead of x > 0? 53

2. 5 Program Style n A program written with attention to style n is easier to read n easier to correct n easier to change n Use n Indentation n Comments 54

Program Style - Indenting n Items considered a group should look like a group n n Skip lines between logical groups of statements Indent statements within statements if (x = = 0) statement; n Braces {} create groups n Indent within braces to make the group clear n Braces placed on separate lines are easier to locate 55

Program Style - Comments n // is the symbol for a single line comment n Comments are explanatory notes for the programmer n All text on the line following // is ignored by the compiler n Example: //calculate regular wages gross_pay = rate * hours; n /* and */ enclose multiple line comments n Example: /* This is a comment that spans multiple lines without a comment symbol on the middle line */ Display 2. 15 56

Section 2. 5 Conclusion n Can you n Describe the benefits of comments? n Explain why indenting is important in a program? n Explain why blank lines are important in a program? 57