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Fundamental Data Type. Chapter 12. Number. Don't use "magic numbers", use "defined" constants int Array[100] vs. int size = 100;   int Array[size] for i = 0 to 99 do ... vs. for i = 0 to MAX_ENTRIES-1 do ... Use hard-coded 0s and 1s if you need to for i = 0 to CONSTANT do ...

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Number
Number

  • Don't use "magic numbers", use "defined" constants

    • int Array[100] vs. int size = 100;   int Array[size]

    • for i = 0 to 99 do ... vs. for i = 0 to MAX_ENTRIES-1 do ...

  • Use hard-coded 0s and 1s if you need to

    • for i = 0 to CONSTANT do ...

    • total = total + 1

  • Anticipate divide-by-zero errors

  • Make type conversions obvious

    • y = x + (float) I

  • Avoid mixed-type comparisons

    • If x is a floating-point number and i is an integer, the following test is almost guaranteed not to work.

    • if ( i = x ) then ...

  • Heed your compiler's warnings


Integers
Integers

  • Check for integer division

    • 7/10 does equal 0.7?

  • Check for integer overflow

    • overflow is always silent

    • 16 bit --> (-32768, + 32767)   or unsigned is 65536

    • 250 * 300=75,000.

    • But if the maximum integer is 65,535, the answer you'll get is probably 9464

      • because of integer overflow (75,000 - 65,536 = 9464).

  • Check for overflow in intermediate results


Why?

stored = value % (limit + 1) or 65536 % (65535 + 1) = 0

1000 0000 0000 0000

0111 1111 1111 1111 (2’s compliment)

+ 1

1000 0000 0000 0000


Floating point numbers
Floating-Point Numbers

  • Around problem:

    • With a 32-bit floating-point variable,

    • 1,000,000.00 + 0.1 =1,000,000.00

      • because 32 bits don't give you enough significant digits to encompass the range between 1,000,000 and 0.1.

    • Likewise, 5,000,000.02 - 5,000,000.01 is probably 0.0.

  • Solutions?

    • Avoid additions and subtractions on numbers that have greatly different magnitudes

    • If you have to add a sequence of numbers that contains huge differences like this, sort the numbers first, and then add them starting with the smallest values.

    • if you need to sum an infinite series, start with the smallest term—essentially, sum the terms backwards.

    • This doesn't eliminate round-off problems, but it minimizes them.




Characters and strings
Characters and Strings

  • Avoid magic characters and strings

  • Use named constants and CONSTANT + 1 for string allocations

    • eg. char Name[16]; //bad #define NAME_LENGTH 15 char Name[NAME_LENGTH + 1]  (add 1 for '\0' character)

  • always initialize strings to something

    • if nothing else fits, create a NULL string

    • char String[50] = ""

  • use calloc rather than malloc

    • calloc also initializes the acquired memory to 0x00

    • in C++, always use new for class objects so that constructors are involve


Characters and strings cont
Characters and Strings –cont’

  • use strncpy instead of strcpy

  • use strncmp instead of strcmp

    • both of the above commands have an extra parameter that specifies the maximum length of the arguments


Boolean variables
Boolean Variables

  • use them as self-documenting codeeg. if(SearchDone)eg. if(InputEnd)

  • use them to split complicated conditional expressions into simpler termseg. InputEnd = feof( ) || ferror( ) ...;             ....            if (InputEnd || ..... ||.....) ...


Enumerated types
Enumerated Types

  • Enumerated types are a powerful alternative to shopworn schemes

    • “1 stands for red, 2 stands for green, 3 stands for blue…."

    • This ability suggests several guidelines for using enumerated types:

  • Use enumerated types for readability

    • if chosenColor = 1you can write more readable expressions like

    • if chosenColor = Color_Red


Enumerated types cont
Enumerated Types –cont’

  • Use enumerated types for reliability

    • With named constants, the compiler has no way of knowing that the only legal values are Color_Red, Color_Green, and Color_Blue.

    • The compiler won't object to statements like color = Country_England or country = Output_Printer.

    • If you use an enumerated type, declaring a variable as Color, the compiler will allow the variable to be assigned only the values Color_Red, Color_Green, and Color_Blue

  • Use enumerated types for modifiability

    • If you discover a flaw in your "1 stands for red, 2 stands for green, 3 stands for blue" scheme,

      • you have to go through your code and change all the 1s, 2s, 3s, and so on.

    • If you use an enumerated type,

      • you can continue adding elements to the list just by putting them into the type definition and recompiling.


Enumerated types cont1
Enumerated Types –cont’

  • Use enumerated types as an alternative to boolean variables

  • For example, suppose you have a routine

    • Return true = successfully performed its task

    • Return false = otherwise.

    • Later you might find that you really have two kinds of False.

      • False 1= the task failed and the effects are limited to the routine itself;

      • False 2= the task failed and caused a fatal error that will need to be propagated to the rest of the program.

    • an enumerated type with the values Status_Success, Status_Warning, and Status_FatalError would be more useful than a boolean with the values true and false.

    • This scheme can easily be expanded to handle additional distinctions in the kinds of success or failure.


Enumerated types cont2
Enumerated Types –cont’

  • Check for invalid values

  • When you test an enumerated type in an if or case statement, check for invalid values.

  • Use the else clause in a case statement to trap invalid values:


Enumerated types cont3
Enumerated Types –cont’

  • If your language doesn't have enumerated types, You can simulate them with global variables or classes.

  • For example, you could use these declarations in Java:



Arrays
Arrays

  • bounds check

  • use arrays sequentially rather than randomly

    • random accesses in arrays are similar to random gotos in a program

    • such accesses tend to be undisciplined, error prone, and hard to prove correct.

    • consider using container classes

  • watch subscript order on multidimensional arrays

    • Array[i][j] vs Array[j][i] //order!

    • Using meaningful name for i and j


Container classes example
container classes example

        class SafeArray{                private:enum (size = 100);int Array[size];                    .....                public:int & operator [ ] (int n);                    {                        if (n<size)                            return Array[n];                        else                            .... do something dramatic...                    }            }


Pointer c
Pointer C++

  • Pointer usage is one of the most error-prone areas of modern programming

  • modern languages like Java, C#, and Visual Basic don't provide a pointer data type.

  • Using pointers is inherently complicated, and using them correctly requires that you have an excellent understanding of your compiler's memory-management scheme.

  • Many common security problems, especially buffer overruns, can be traced back to erroneous use of pointers


Pointer c1
Pointer C++

  • Paradigm for Understanding Pointers (two parts: )

    • a location in memory

    • a knowledge of how to interpret the contents of that location

  • Location in Memory

    • the location in memory is an address

    • expressed in hexadecimal notation

    • an address on a 32-bit processor would be a 32-bit value,

      • 0x0001EA40.

    • the pointer itself contains only this address

    • If you were to look at the memory in that location, it would be just a collection of bits. It has to be interpreted to be meaningful.


Pointer c2
Pointer C++

  • Knowledge of How to Interpret the Contents

    • interpret the contents of a location in memory is provided by the base type of the pointer.

    • If a pointer points to an integer

      • the compiler interprets the memory location given by the pointer as an integer.

      • you can have an integer pointer, a string pointer, and a floating-point pointer all pointing at the same memory location.

        • only one of the pointers interprets the contents at that location correctly.



Tips on pointers
Tips on Pointers

  • Declare and define pointers at the same time


Tips on pointers cont
Tips on Pointers –cont’

  • Use extra pointer variables for clarity

    • It's hard enough to figure out what someone is doing with a linked list without having to figure out

      • why one genericLink variable is used over and over again or

      • what pointer->next->last->next is pointing at.


Tips on pointers cont1
Tips on Pointers –cont’

  • Draw a picture

    • Code descriptions of pointers can get confusing. It usually helps to draw a picture.


Tips on pointers cont2
Tips on Pointers –cont’

  • Set pointers to null after deleting or freeing them

    • A common type of pointer error is the "dangling pointer," use of a pointer that has been delete'd or free'd.

    • By setting pointers to null after freeing them, you don't change the fact that you can read data pointed to by a dangling pointer.

    • But you do ensure that writing data to a dangling pointer produces an error.


Tips on pointers cont3
Tips on Pointers –cont’

  • Simplify complicated pointer expressions

    • complicated pointer expressions are hard to read

    • if your code contains expressions like p->q->r->s.data

    • think about the person who has to read the expression


C pointer pointers
C++-Pointer Pointers

  • The difference between pointers and references

    • in C++, both pointers (*) and the references (&) refer indirectly to an object.

    • to the uninitiated the only difference appears to be a purely cosmetic distinction between referring to fields as object->field vs. object.field.

    • the most significant differences are that

      • a reference must always refer to an object,

      • a pointer can point to null

    • what a reference refers to can't be changed after the reference is initialized.


C pointer pointers1
C++-Pointer Pointers

  • Use pointers for "pass by reference" parameters and use const references for "pass by value" parameters

    • C++ defaults to passing arguments to routines by value rather than by reference.

    • When you pass an object to a routine by value, C++ creates a copy of the object, and when the object is passed back to the calling routine, a copy is created again.

    • For large objects, that copying can eat up time and other resources.

    • Consequently, when passing objects to a routine, you usually want to avoid copying the object, which means you want to pass it by reference rather than by value.


C pointer pointers cont
C++-Pointer Pointers – cont’

  • Sometimes, however, you would like to have

    • the semantics of a pass by reference

      • the passed object should not be altered

    • the implementation of a pass by value

      • passing the actual object rather than a copy


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