© Janice Regan, CMPT 128, Sept CMPT 128: Introduction to Computing Science for Engineering Students C++ Basic Input and output

© Janice Regan, CMPT 128, C++ Basic Input and Output (I/O)  To print to your computer’s screen (console) or the read data typed into your keyboard you will need to use objects from the C++ iostream library.  To access the objects in the iostream library you must use a #include pre-processor directive to include their declarations:  #include  This directive tells C++ to use appropriate library so we can use the I/O objects cin, cout, cerr, endl needed for simple input and output

C++ output by example  Consider the code sampleIO.cpp posted with this set of notes.  Lets discuss the examples given in the code © Janice Regan, CMPT 128,

Integers, Doubles and Floats  Each variable or constant in C or C++ must have a type (e. g. int double or float)  Whole numbers with no fractional parts are integers, type int.  Floating point numbers, numbers with fractional points (even when the fractional part is 0) are type float or type double © Janice Regan, CMPT 128,

float vs double  Historically, in both C and C++, type double used more bits to represent each number  More bits allowed variables of type double  To be larger than variables of type float  To be smaller than variables of type float  Be more accurate than variables of type float  The reasons these statements are true will be explained later. © Janice Regan, CMPT 128,

5 C++ Spaces inside “ “ cout << fixed << setprecision(2) << "The balance is $" << balance << endl; cout << “the interest is “ << percent << “ % ” ; cout << precision(5) << percent/100 << endl; PRINTS The balance is $78.50 The interest is % Thee is no space between $ and the final quote in code, there is a space between % and the final “ in the code. Therefore, in the output $ is immediately followed by the value (no space) but % is followed by a space.

C++ endl  Consider the output window (the window in which the output from your program, and the input from your keyboard is displayed.)  When your code executes the instruction cout << endl; The cursor in the output window is moved to the beginning of the next line © Janice Regan, CMPT 128,

7 C++: Moving to a new output line  C++ style: endl  C++ can also use C style: "\n"  escape sequence for the char "newline“ cout << "Hello World" << endl; cout << "Hello World\n";  In C++ both the statements print string "Hello World" to display, then move to next line

© Janice Regan, CMPT 128, C++ Output Window Output  What can you print to your computer’s output window using the insertion operator <<? string name=“John”; const double scale_factor = 2.2; cout << scale_factor // Prints 2.2, the value of constant scale_factor cout << name ; // Prints John, the name stored in variable name cout << “This is a string literal”; // Prints This is a string literal Each cout above prints the value of one variable or constant

© Janice Regan, CMPT 128, C++ Output window Output  You can print the values of more than one variable or constant in a single cout statement cout << “the number of games was ” << numGames << endl << “our team won “ << numWin << “ games” << endl;  Given that the variables have the values  numGames = 23 and numWin = 15  This single cout statement prints the number of games was 23 our team won 15 games to the output window

© Janice Regan, CMPT 128, C++ 3 Equivalent Outputs cout << “the number of games was ” << numGames endl << “ out team won “ << numWin << “ games” << endl; // One cout statement printing many values in succession cout << “the number of games was ” << numGames << endl; cout << “ out team won “ << numWin << “ games” << endl; // Two cout statements, each printing half the values cout << “the number of games was ” ; cout << numGames; cout << endl; cout << “ out team won “ ; cout << numWin ; cout << “ games” ; cout << endl; // each value printed using a single cout statement

Number of digits after decimal  There are two ways in C++ to specify how many digits to print after the decimal point  One method is shown in your text Uses library only Uses “magic formula” to set number of digits after the decimal point  The other method uses manipulators Uses and libraries Syntax is simpler to remember © Janice Regan, CMPT 128,

Explain the “magic formula” cout.setf(ios::fixed); //Tells C++ to display output in fixed format xxx.yyy cout.setf(ios::showpoint); //Tells C++ to always print the decimal point cout.precision(3); //Tells C++ to print 3 digits after the decimal point  Sets your program so all floating point numbers printed after these statements will print in fixed format with 3 digits after the decimal point  Adding another cout.precision(N) statement later in your code will cause all floating point numbers after that statement to print with N digits after the decimal point … © Janice Regan, CMPT 128,

Extend the “magic formula” cout.unsetf(ios::fixed); //Tells C++ to stop displaying in fixed format xxx.yyy cout.setf(ios::scientific); //Tell C++ to start displaying in scientific notation xxx.yyy Ezz  You can switch from fixed point notation to scientific notation (or from scientific notation to fixed point notation)  First you must unset the flag telling C++ to print using floating point. Then you must set the flag to tell C++ to print using scientific notation. (Or unset scientific and set fixed)  Results will not be predictable if you set both flags simultaneously © Janice Regan, CMPT 128,

What else can you specify?  The programmer can explicitly specify how C++ should format the numbers output by their programs using the library  In particular you can specify  The number of digits printed  The number of digits printed after the decimal point  width of field (how many spaces to leave for a value)  Fixed point or scientific notation E2 © Janice Regan, CMPT 128,

© Janice Regan, CMPT 128, C++ Manipulators  Used to control how output is formatted  Require user to include library  fixed  scientific  setw()  setprecision()  left  right © Janice Regan, CMPT 128,

© Janice Regan, CMPT 128, Manipulators: fixed + setprecision()  fixed, prints number as fixed point, xx.yyy  setprecision(2) indicates 2 digits after the decimal point  Continues to use precision 2 and fixed until told to change in another cout command cout << "$" << fixed << setprecision(2) << 10.3 << " "<< "$" << << endl;  Prints $10.30 $20.51 © Janice Regan, CMPT 128,

© Janice Regan, CMPT 128, Manipulators: scientific + setprecision()  scientific and setprecision() manipulators:  setprecision(4) indicates 4 digits after the decimal point cout << “population is " << scientific << setprecision(4) << " is " << << mypop << endl;  If mypop has value cout statement prints: population is e+005 © Janice Regan, CMPT 128, scientific prints number in scientific notation, xxx.yy Ezz

Changing Format: Example cout << fixed << setprecision(2) << "the interest is " << << "% or $”<< dollars << endl; cout << scientific << setprecision(4) << "The amount is " << << endl;  Format changes after scientific is used. The code above print the following when dollars has value 33.12: the interest is 12.34% or $33.12 The amount is e+002 © Janice Regan, CMPT 128,

© Janice Regan, CMPT 128, Manipulator: setw()  setw()  Sets the width in characters of the output field  By default output will be right justified in the output field  If the output has the same number of characters as the number or spaces available within the field it will fill the field  If the output has fewer characters as the number or spaces available within the field it will by default be right justified within the field  Note: setw() affects only NEXT value output  Must include setw() manipulator before each item output © Janice Regan, CMPT 128,

© Janice Regan, CMPT 128, Manipulator Example: setw()  setw() manipulator: cout << “xxxxxxxxxxxxxxxxxxx”; cout << endl<<"Start" << setw(5) << 10 << setw(4) << 20 << setw(6) << 30;  Prints:  xxxxxxxxxxxxxxxxxxxx Start © Janice Regan, CMPT 128,

© Janice Regan, CMPT 128, Manipulator Example: left, right  left and right manipulators: cout << fixed << left << setw(20) << interest << " %" l; cout << endl << setw(20) << balance << endl; cout << setw(20) << right << ;  Prints: xxxxxxxxxxxxxxxxxxxxxx % © Janice Regan, CMPT 128,

© Janice Regan, CMPT 128, Input Using cin  Differences:  " >> " (extraction operator) points opposite Think of it as "pointing toward where the data goes"  Object name "cin" used instead of "cout"  No literals allowed for cin Must input "to a variable"  cin >> num;  Waits on-screen for keyboard entry  Value entered at keyboard is "assigned" to num

© Janice Regan, CMPT 128, Prompting for Input:  When using console output and keyboard input always "prompt" user for input cout > numOfDragons;  No "\n" in cout means that the prompt "waits" on same line for keyboard input as follows: Enter number of dragons : Waits here for input

© Janice Regan, CMPT 128, Prompting for Input:  When using console output and keyboard input always "prompt" user for input cout > numOfDragons;  "\n" in cout means that the prompt "waits" on next line for keyboard input as follows: Enter number of dragons : Waits here for input

Variable types and input  Be careful to give the correct type of data when responding to the prompt in a program.  Items from the keyboard will be converted but this may still not give the results you expect © Janice Regan, CMPT 128,

Input the correct type of data  Think of all the information you type in as a continuous stream of characters  If you are reading an integer and you read a decimal point you will stop reading at the decimal point because the decimal point is not a part of an integer  The next time you read you will begin with the decimal point left over from the last input © Janice Regan, CMPT 128,

Incorrect input examples int one; double two; Int three; cout << "enter an integer"; cin >> one; cout << "enter a double "; cin >> two; cout << "enter an integer"; cin >> three; cout << one << “ “ << two << “ " << three; © Janice Regan, CMPT 128, enter an integer4 enter a double 2.4 enter an integer enter an integer5.6 enter a double enter an integer enter an integer12 enter a double 44 enter an integer

Incorrect input examples int one; int two; int three; cout << "enter an integer "; cin >> one; cout << "enter integer 2"; cin >> two; cout<<"enter integer 3"; cin>>three; cout << "XX" << endl << one << " "<< two <<three; © Janice Regan, CMPT 128, enter an integer 4.44 enter integer 2enter integer 3XX enter an integer hello enter integer 2enter integer 3XX

© Janice Regan, CMPT 128, Sept CMPT 128: Introduction to Computing Science for Engineering Students C Basic Input and output Differences from C++

© Janice Regan, CMPT 128, C++ Basic Input and Output (I/O)  To print to your computer’s screen (output window) or the read data typed into your keyboard you will need to use objects from the C stdio library.  To access the objects in the stdio library you must use a #include pre-processor directive to include their declarations:  #include  This directive tells C to use appropriate library so we can use fprintf, fscanf etc

C++ output by example  Consider the code sampleIO.c posted with this set of notes.  Lets discuss the examples given in the code  Each read or print used a C input or output conversion. The input or output conversion used depends on the type of the variable we are trying to print © Janice Regan, CMPT 128,

C input / output conversions © Janice Regan, CMPT 128,

Always check your conversion  A common problem is using a conversion specifier that does no match the type of your variable  This usually causes the value to be printed incorrectly, even if the value itself is correct © Janice Regan, CMPT 128,

© Janice Regan, CMPT 128, First example: output /* Print the sum. */ printf(“%f \n", sum);  The %f is the C conversion, all other characters (including blanks, commas, etc.) between the " " will be printed directly to the output line  The %f will be replaced by the value of the variable sum when the program prints the output line

Setting the size of the field  The size of the field is the number of spaces available in which the value can be printed.  The size of the field is specified by an integer between the % and the conversion specifier  %12d /* an integer filling up to 12 characters */  %14lf /* a double filling up to 14 characters */  %17f /* a float filling up to 17 characters */ © Janice Regan, CMPT 128,

© Janice Regan, CMPT 128, What is printed? printf ("%6d ", sum); /* sum is an int */  If the value of sum is /*the number is preceded by 3 spaces*/  If the value of sum is -98, /*no preceding blank spaces */  If the value of the sum is -57,639, /*overflows field, no blanks */

Setting the precision  Suppose we have a floating point number and we want it to print with a specified number of digits after the decimal point  The default for C is 6 digits after the decimal point  If we want more or less than 6 digits we must indicate this in our C conversion %12.4f %9.2lf %18.4e © Janice Regan, CMPT 128,

© Janice Regan, CMPT 128, What is printed? printf ("%12.2f ", sum); /* sum is a float */  If the value of sum is /*the number is preceded by 6 spaces*/  If the value of sum is /*number is preceded by 3 spaces */  If the value of sum is 4.58 * /*no preceding blank spaces */  If the value of the sum is 2.2* /*overflows field, no blanks */

© Janice Regan, CMPT 128, What is printed? printf ("%12.3 E", sum); /* sum is a float */  If the value of sum is E-006 /* 3 digits after the decimal point, */  If the value of sum is E+005 /* 5 digits for exponent */  If the value of sum is 4.58 * E+009 /* 1 digit for decimal point */  If the value of the sum is 2.2* E+012 /* total 11 digits */

Justification  By default the number is right justified inside the specified field.  You can cause the number to be left justified by adding a – before the integer indicating the size of the field printf ("%12.2fend\n", sum); /* sum is a float */ printf ("%-12.2fend\n", sum); /* sum is a float */  If the value of sum is end © Janice Regan, CMPT 128,

Displaying signs  By default the sign of a number is displayed only when that sign is negative. When the sign is positive no space is reserved for the sign.  You can cause the signs to be shown all the time (both positive and negative) by adding a + before the number indicating the width of the field. © Janice Regan, CMPT 128,

Displaying Signs: Example printf (“%8.3fMMM%8.2fEEE\n", sump, sumn); printf (“%+8.3MMM%+8.2fEEE\n", sump, sumn);  Assume the value of sump is and the value of sumn is MMM EEE MMM EEE © Janice Regan, CMPT 128,

© Janice Regan, CMPT 128, C conversions: Things to remember  ALWAYS use the correct C conversion  When you specify a minimum field width, remember that the sign, the decimal point, and the E (or e) for the exponent all count as digits.  If you specify a minimum field length smaller than the width of the variable being printed, your output will overflow the specified field (continue to the right of the specified field)  Do not specify a precision (.nn) for integers. Integers do not have digits after the decimal

© Janice Regan, CMPT 128, printf(): Things to remember  When there is no space between conversions, there are no spaces between the printed outputs.  Anything you wish to appear in the output that is not the value of a variable must appear in the format statement. This includes spaces  The number of variables in the variable list must match the number of C conversions in the format statement

© Janice Regan, CMPT 128, Special characters in printf statements  There are other strings of characters, called escape sequences, you can place within your format statement to produce particular results  \n newline: move to the next output line  \b backspace: move back one space before printing the next character  \t horizontal tab  \v vertical tab  \\ print a \\? Print a ?  \" print a “\’ print a ‘ % print a %  Note that a format statement should not contain the character produced by typing

First example: C input /* Read the value of temperature. */ scanf(“%f \n", &temperature);  The %f is the C conversion, C will read only characters that can be part of a float variable (we are assuming temperature is a float variable)  The characters typed into the keyboard will replace the value of the variable temperature when the program reads the variable © Janice Regan, CMPT 128,

IMPORTANT  In C ALWAYS use the correction C conversion specifier. If you do not you will probably put the wrong value in your variable  In C always prepend a & to the name of the variable you are reading in the scanf statement © Janice Regan, CMPT 128,

Reading a double  N = scanf(“%12lf”, &myDoubleVariable);  C will read until  It has read 12 characters that could be part of a double variable (digits, decimal point, sign) Leading white space (space, tab, newline) is ignored. It does not count toward the 12 characters.  It encounters a character that could not be part of a double variable (the character that cannot be part of the double variable will be the first character read when the next variable is read) or white space (space, tab, newline … ) © Janice Regan, CMPT 128,

Reading using scanf()  N = scanf(“%12lf %8d”, &myVar1, &my2);  If both myVar1 and my2 are successfully read N will be 2  If myVar1 can be read and my2 cannot be read N will be 1  Suppose the input was  myVar1 would be 12.0  My2 could not be read (first character is. which cannot be part of an integer) © Janice Regan, CMPT 128,