- Scope of a variable is the part of the program where the variable is accessible. Like C/C, in Java, all identifiers are lexically (or statically) scoped, i.e.scope of a variable can determined at compile time and independent of function call stack.
- For the input of specific types of variables in the C programming language, you’ll find that the scanf function comes in handy. It’s not a general-purpose input function, and it has some limitations, but it’s great for testing code or grabbing values. In a way, you could argue that scanf is the input version of.
- Hello: Is it possible to declare a variable with multiple data types. For example in my program I would like to declare my variable as int or char depending on the situation the variable.
- I have already created a file called ( student.txt) and i put the following information inside the file Will Smith 99 Sarah Johnson 100 Tim Howard 70.
A char variable in C++ is designed to hold an ASCII character, an int an integer number, and a double a floating-point number. Similarly, a pointer variable is designed to hold a memory address. You declare a pointer variable by adding an asterisk (*) to the end of the type of the object that the pointer points at, as in the following example:
A pointer variable that has not otherwise been initialized contains an unknown value. Using the ampersand (&) operator, you can initialize a pointer variable with the address of a variable of the same type:
In this snippet, the variable cSomeChar has some address. For argument’s sake, say that C++ assigned it the address 0x1000. (C++ also initialized that location with the character ‘a’.) The variable pChar also has a location of its own, perhaps 0x1004. The value of the expression &cSomeChar is 0x1000, and its type is char* (read “pointer to char”).
String variables are declared by using the string type, however as strings aren't actually 'primitive' types in C (and are instead defined by the standard library of stuff that comes bundled with C), you are required to #include string to use this data-type. An example of string declaration and initialization to a string constant is as.
So the assignment on the third line of the snippet example stores the value 0x1000 in the variable pChar.
Take a minute to really understand the relationship between the figure and the three lines of C++ code in the snippet. The first declaration says, “go out and find a 1-byte location in memory, assign it the name cSomeChar, and initialize it to ‘a’.” In this example, C++ picked the location 0x1000.
The next line says, “go out and find a location large enough to hold the address of a char variable and assign it the name pChar.” In this example, C++ assigned pChar to the location 0x1004.
The third line says, “assign the address of cSomeChar (0x1000) to the variable pChar.” The figure represents the state of the program after these three statements.
“So what?” you say. Here comes the really cool part, as demonstrated in the following expression:
This line says, “store a ‘b’ at the char location pointed at by pChar.” This is demonstrated in the following figure. To execute this expression, C++ first retrieves the value stored in pChar (that would be 0x1000). It then stores the character ‘b’ at that location.
The * when used as a binary operator means “multiply”; when used as a unary operator, * means “find the thing pointed at by.” Similarly & has a meaning as a binary operator, but as a unary operator, it means “take the address of.”
So what’s so exciting about that? After all, you could achieve the same effect by simply assigning a ‘b’ to cSomeChar directly:
Why go through the intermediate step of retrieving its address in memory? Because there are several problems that can be solved only with pointers.
Variables are an extremely core concept to most object orientated programming languages. I like to visualize a variable much like a box. We can put things in the box, we can take things out of the box, and at any point we can see what is inside the box. Each box also has a name to which we can refer to it by, and in C++, each box can only hold a certain type of data.
When we create variables we call this the variable declaration, and then when we set them for the first time, we call this the initialization. To declare a variable in C++, we write the function. To declare a basic integer variable called 'age', we could write the following:
From this point we can then refer to the variable by its name, so in this case, we can just write 'age' whenever we want to refer to the variable. To initialise the variable we can write its name, followed by the equals sign, followed by the value we want to set the variable to (followed by a semicolon). The value we set it to can be a constant (a value that doesn't change), or another variable of the same type. An operator is a symbol which has a certain meaning in the programming language, in this case, the equals operator, represented by the
=
symbol, is an operator which sets whatever is on the left of the operator to whatever is on the right.The constant value we set the variable to depends on the to 5 with something like the following:
Completely wipe ipad 3utools download. We can actually combine the variable declaration and initialization into one more-compact line, like the following:
The 'age' variable now contains the number '5', and we can refer to this '5' by writing 'age' anywhere in our program. We can also change the value of the variable at any point by using the equals operator as we did for the first initialization:
Although this seems purely for convenience at the moment (as we could just write '5', '3', or '21' in place of 'age'), trust me when I say that these become extremely useful and powerful when you start dealing with dynamic logic and user input (the latter of which we'll be covering later in this tutorial).
Just to give an example of accessing the contents of variables by using their names, we could create a new variable called 'age_two' which is set to the value of 'age', and then we can also try outputting one or both of these variables:
To be clear, all this code should be going into the basic program structure which we learnt how to create in the last tutorial. So we want our 'iostream' include for
cout
, cin
, and some other stuff, we want the std
namespace, and we want the majority of our code to be going in our 'main' function. So our full code to demonstrate variables so far, which you can compile and run at any point to test the functionality, is as follows:Some number variables can handle positive and negative numbers, whereas 'unsigned' number variables can only handle positive numbers, although because of this restriction, can hold larger numbers. You can write the
signed
or unsigned
keywords before the and 'short' - numbers with a decimal place in. Floats are accurate to around 6 or 7 digits and are declared using the float
type. Float constants can be defined by simply writing a number with a decimal point followed by the 'f' notation. An example of a simple float declaration and initialization to a float constant is as follows:Care must be taken, however, with float (and other decimal) operations, as rounding and precision problems to do with how the numbers are stored can trip you up (we don't have infinite memory for recurring decimals like
1/3
for example) -- I recommend reading this article for more information on this if you're interested.Doubles
The 'double' or 'e'. Character variables are declared by using the
char
type, and character constants are defined by using single quotes (apostrophes) around the character. An example of character declaration and initialization to a character constant is as follows:Strings
How To Declare String Variable In Dev C 2017
The lastve talked about string variables in relation to
cout
before, and as such you should know that string constants are defined by using double quotes. String variables are declared by using the string
type, however as strings aren't actually 'primitive' types in C++ (and are instead defined by the standard library of stuff that comes bundled with C++), you are required to #include <string>
to use thist strings aren't massively useful, but this is just because we don't really know how to utilize all the functionality of different data-types yet - for example, we don't know how to perform simple mathematics on number types, or how to check the value of booleans to change the logic of the program. All will be revealed in future tutorials.