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C++ is a superset of the C programming language. Any C code is also C++ code, but some C++ code is not C. Much of what you learn in this course could also be used in a C program. There are lots of new features and capabilities in C++. We will learn some of them (but far from all of them) in this course.
The C++ Language is made up of
These programmer-defined names:
Each data item has a type and a name chosen by the programmer. The type determines the range of possible values it can hold as well as the operations that can be used on it. For example, you can add a number to a numeric data type, but you cannot add a number to someone's name. (What would "Joe" + 1 mean?)
To make a variable that your program can use, you must declare it.
Declaration Examples:
int x; //gives type and name; no value yet int x, y; //declares 2 ints; no values yet int population = 16000; //declares & sets the initial value
So you could picture the results of the last two lines as:
x
|
y
|
population
|
Note: It is critically important that variables have values before they are used in a program. In the examples above, x and y have no values even though they have been declared. More accurately, they do have values but those values could be anything between -2 billion and +2 billion. Before you use x or y you would have to give them values.
These data types are also commonly referred to as real variables (following mathematics usage) when it is not important which one we mean and we don't want to always have to say "float or double".
Declaration Examples:
float pi = 3.1416; //declares, names, sets init value
double x = 3.5, //note comma here
y = 1245.6543; //can use > 6 digits
OR
double x = 3.5;
double y = 1245.6543;
float big = 5.28e3; // exponential notation: 5280
Declaration examples:
char ch; char choice = 'q';
Declaration Examples:
string s; string MyName = "Amy";
The arithmetic operators are:
Note: there is no exponentiation operator.
Special Note on Division:
Recall 3rd grade division:
3 R1
_______ 4 is divisor
4| 13 13 is dividend
12 3 is quotient
1 1 is remainder
In C++, a division with 2 int operands has an int resulting value:
13/4 --> 3 // int quotient 13%4 --> 1 // int remainder
But with 1 or 2 float/double operands, the resulting value is a float or double:
So: 13.0 / 4 --> 3.25
13 / 4.0 --> 3.25
Be aware. Forgetting this can easily cause an error.
Arithmetic Expressions are formed by operands and operators. Operands are the values used, operators are the things done to the numbers. Parts of arithmetic expressions are often grouped by () for clarity or to affect the meaning of the expression:
//declare and initialize some variables
int x = 11,
y = 2;
double realnum = 2.0;
| expression | value | notes |
| x + y | 13 | |
| x * y | 22 | |
| x * y + x | 33 | |
| x - y | 9 | |
| -x + y | -9 | unary negation: "minus x" |
| x / y | 5 | int since both ops are int |
| x % y | 1 | rem when x divided by y |
| x / realnum | 5.5 | one op is real so result is real |
Note: The expressions above by themselves are not valid C++ statements - they would be part of a statement. But each expression (if it is in a valid C++ statement) has a value.
Also - the spaces written between operands and operators are not required by C++, but do improve legibility.
More complex expressions are evaluated by C++ using Rules of Precedence (like algebra)
1. sub-expressions in ()
2. unary -
3. * and / - left to right
4. + and - - left to right
In the examples below, the red numbers show what operation is done as C++ evaluates and expression:
3 + 5 * 4 / 2 = 3 + 20 / 2 = 3 + 10 = 13
But () can be used to change this (or to make expression more readable)
(3 + 5) * 4 / 2 = 8 * 4 / 2 = 32 / 2 = 16
The symbol "=" is a command. It means: "evaluate the expression on the right and store this value in the variable on the left"
In general:
someVariable = some expression;
Examples:
x = y * 3; x = x + y;
Notice that there is always exactly one variable on the left to receive the value of the expression on the right.
This is NOT like algebra; it is NOT an equation.
Algebra: x = 1 (T or F for a given x) C++: x = 1; // store 1 into x Algebra: x = x + 1 (false for all x) C++: x = x + 1; //find x+1; result in x
More Assignment Examples
//declare variables double dAns, x, y; int iAns, i, j; //assign values via numeric literals i = 5; j = 8; x = 4.0; y = 1.5; //assign values from complex expressions i = i + 2; // 7 dAns = x / y; // 2.666.. iAns = j/2*2; // 8/2*2 = 4*2 = 8 iAns = 8/5; // 1 iAns = 8 % 5; // 3 iAns = 5 / 8; // 0 iAns = 5 % 8; // 5 i = x/y; // 2
The last needs an explanation. the value of the expression x/y is 2.666.. but i is an int variable and so cannot hold the .666 part. C++ simply drops it (truncates it) and so only the whole number part of the value is stored into i.
Notes:
1. All variables in expressions on the right must have defined values or you get random results. This is an example of what I meant before when I said variables must have values before they are used.
int x, y; // declared; no init values x = y + 1; // ??? in x
int x; // declared; no init value int y = 2; // declared; given a value x = y + 1; // x has value 3. // It is Ok if x has no value before this
Inadvertent use of un-initialized variables is one of the most common causes of program errors. Learn to be very careful about this.
2. You can do multiple assignments on one line:
x = y = z = 3; //all get value 3
3. Naming Variables: use meaningful names that explain themselves to reader of the program. (You, me, the maintainer of the program after you go to your next job...)
No Yes s score cs class_size ClassSize classSize cav class_average ClassAvg classAvg num student_count NumStudents numStudents stnum student_id StudentID studentID
#include <iostream> // a.
#include <iomanip>
using namespace std; // b.
int main() // c.
{
int num1, num2, sum; // d.
num1 = 10; // e.
num2 = 15;
sum = num1 + num2; // f.
return 0; // g.
}
Notes:
C++ standard libraries that must be included at the beginning of C++ programs
So the standard (std) objects can be used from the included libraries
program name is always main with int (which stands for integer) before it
num1, num2 and sum are the data items (which we often call variables)
num1 and num2 get initial values from instructions via assignment (the = is the assignment command)
sum gets its value from an arithmetic expression
return an integer: main "promised" to provide an integer "answer". In this program (and usually) the "answer" doesn't matter. But we have to keep the promise, so we just return 0. We will discuss this further later.
on any line, typing // turns the rest of the line into a comment. A comment is not "seen" by the computer - it is not part of the program. It is used by human readers of the program to explain something in the program.
there are no program control or modularization statements
this program has no input or output
notice the semicolons - where they are and aren't
We will use cout for output. cout's task is to display information on the screen.
The simplest form of cout is:
cout << "Hello";
You can output (for display) several values, including the values calculated and stored into variables, by stringing them together connected by "<<":
cout << "The average is: " << avg; // avg is a variable
cout << "The temperature is: "
<< temp
<< " and the rainfall today was: "
<< rainAmt;
Note that I have put each item to display on a separate line for clarity. This is both legal and is usually desirable if you have more than 2 values.
There are a couple of special techniques to position cursor before or after you display characters, using an escape sequence. (A \ followed by a char. For example, \n is the newline escape sequence.) Note that this is a backslash.
cout << "\nhello"; //moves to next line, shows hello cout << "hello\n"; //shows hello, then goes to new line cout << "hel\nlo"; //shows hel, then lo on next line
Note: to display a single \, you must code two: \\
cout << "the \\ char is special";
This shows: "the \ char is special"
Also, there is an alternate way to move the cursor to the beginning of a new line: the special value "endl":
cout << endl << "Some stuff" << endl; cout << "\nSome stuff\n";
Both will first move the cursor to a new line, then display "Some stuff" and then move the cursor to the next line.
Note that if you don't include endl or \n, all your output will be on one line:
cout << "The average is: " << avg; // avg is a variable
cout << "The temperature is: "
<< temp
<< " and the rainfall today was: "
<< rainAmt;
This will display something like this:
The average is 77.5The temperature is: 78 and the rainfall today was: 1.32362
We will use cin for input. cin's task is to accept user input and store it into variables.
Note that
the C++ language itself has no defined I/O functions or commands; just a standard library that can accomplish I/O. We could use other libraries, including those from the C language or those developed by others.
most often when we want user input, we will need to do two things:
So typically we will do something like this:
int i; // a. ... cout << "Enter the temperature: "; // b. cin >> i; // c.
Notes:
we declare an int to hold the value to be typed in
we tell (prompt) the user to enter something
we get the value that is entered by the user
when a cout occurs in a program, the characters are displayed immediately on the screen
when cin occurs in a program, the program waits for user to type in a value and press <Enter>.
the << and >> are meant to suggest the direction the data is going:
for numeric input with cin, if the user types a non-number like "w" or "23e" then results are unpredictable. (For a double or float, "2e5" is ok - it's the same as 200000)
the variable receiving the value from cin must be declared (of course) but need not have an initial value. Any previous value in that variable is lost.
details about formatting numeric output (number of decimal places, etc.) will be covered later.
cin "understands" the different data types we have covered. So you can do cin with int, float, double, char, and string. But you must ensure that the data item getting the value is compatible with what the user is asked to enter. You can't store "jim" into an int, for example. You can store the value 4 into a double, however.
1. Compile error - invalid C++ statements; so your code cannot be translated to machine language. Examples:
In example 1 you must use int, not integer. In example 2 the ; at the end of the line is missing
2. Link error: one of the functions used in the program is not found when it's time to run the program. In the program above, there are no such functions, but we could encounter this problem later.
3. Run-time error - program does not run to completion. Example:
4. Logic error - program runs to completion, but the results are wrong. Example:
Fixing 3 and 4 is called debugging.
You must learn to understand that the computer did what you said, not necessarily what you meant.