C programming session 07

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  • Begin the session by explaining the objectives of the session.
  • Pointers contain addresses of variables of a specific type. Hence, the pointer and the variable to which it is pointing should be of the same type. The number of bytes used by the pointer to hold an address does not depend on the type of the variable the variable it is pointing to. The number of bytes used by a pointer is generally 4 bytes, or 2 bytes in some implementations. Type casting may be used to initialize a pointer with the address of a variable of a different type. For example, the following code is valid : char *ptr ; int i = 4 ; ptr = (char *) (&i) ; though this is seldom done. Pointers to type void can contain addresses of any data. It does not make sense to do any arithmetic with these pointers.
  • Use this slide to test the student’s understanding on declaring pointers.
  • Preceding a pointer with ampersand will yield the address of the pointer itself, and can be stored in a pointer to a pointer. Consider the following code: char *ptr, **pptr, chr; ptr = &chr; /* address of a character */ pptr = &ptr; /* address of a pointer */
  • Use this slide to test the student’s understanding on declaring and initializing pointers.
  • Generally, only the operators + and – are used with pointers. This results in the address in the pointer getting altered by a value equal to the size of the data type it is associated with. The following declaration is for a pointer to an array of 10 elements: char (*ptr) [10] ; /* note, it is not an array of pointers */ If the pointer ptr is initialized and contains the address 100 , ptr++ results in the address in ptr being scaled by 10. ptr will now contain 110.
  • Use this slide to test the student’s understanding on pointers arithmetic.
  • Use this slide to test the student’s understanding on one-dimensional arrays and pointers.
  • Use this slide to test the student’s understanding on 2-D arrays and pointers.
  • Use this and the next slide to summarize the session.
  • C programming session 07

    1. 1. Programming in CObjectives In this session, you will learn to: Declare and manipulate pointers Use pointers to manipulate character arrays Ver. 1.0 Slide 1 of 29
    2. 2. Programming in CDeclaring and Manipulating Pointers Every stored data item occupies one or more contiguous memory cells. Every cell in the memory has a unique address. Any reference to a variable, declared in memory, accesses the variable through the address of memory location. Pointers are variables, which contain the addresses of other variables (of any data type) in memory. Ver. 1.0 Slide 2 of 29
    3. 3. Programming in CDeclaring Pointers A pointer variable must be declared before use in a program. A pointer variable is declared preceded by an asterisk. The declaration: int *ptr; /* ptr is pointing to an int */ Indicates that ptr is a pointer to an integer variable. An uninitialized pointer may potentially point to any area of the memory and can cause a program to crash. A pointer can be initialized as follows: ptr= &x; In the preceding initialization, the pointer ptr is pointing to x. Ver. 1.0 Slide 3 of 29
    4. 4. Programming in CPractice: 4.1 1. In the following declaration: float *ptr_to_float; The pointer variable ptr_to_float is pointing to a variable of type ____________. 2. Is the following declaration valid? *ptr_to_something; 3. State whether True or False: An integer is declared In the following declaration: int *ptr_to_int; 4. Is the following declaration valid? int some_int, *ptr_to_int; Ver. 1.0 Slide 4 of 29
    5. 5. Programming in CPractice: 4.1 (Contd.) Solution: 1. float 2. No. When a pointer variable is being declared, the type of variable to which it is pointing to (int, float, or char) should also be indicated. 3. False. A pointer to an integer is being declared and not an integer. 4. Yes. It is okay to club declaration of a certain type along with pointers to the same type. Ver. 1.0 Slide 5 of 29
    6. 6. Programming in CManipulating Pointers Pointers can be manipulated like variables. The unary operator * gives value of the variable a pointer is pointing to. The unary operator * is also termed as the indirection operator. The indirection operator can be used only on pointers. Ver. 1.0 Slide 6 of 29
    7. 7. Programming in CPractice: 4.2 1. The symbol _________ is used to obtain the address of a variable while the symbol__________ is used to obtain the value of the variable to which a pointer is pointing to. 2. With the following declarations: int x, y, *ptr; Which of the following are meaningful assignments? a. x = y; b. y=*ptr; c. x = ptr; d. x = &.ptr; e. ptr = &x; f. x = &y; Ver. 1.0 Slide 7 of 29
    8. 8. Programming in CPractice: 4.2 (Contd.) 3. Consider the following sequence of statements and complete the partially-filled table: int x, y, *ptrl, *ptr2; x = 65; y = 89; ptr1 = &x; /*ptrl points to x */ ptr2 = &y/; /* ptr2 points to y */ x = *ptr1; /* statement A*) ptr1 = ptr2: /* statement B */ x = *ptr1; /* statement C*/ After statement &x x &y y ptr1 ptr2 A 1006 1018 B C Ver. 1.0 Slide 8 of 29
    9. 9. Programming in CPractice: 4.2 (Contd.) 4. What is the output of the following sequence of statements: int x, y, temp,*ptrl, *ptr2; /* declare */ x = 23; y = 37; ptrl = &x; /* ptrl points to x */ ptr2 = &y; /* ptr2 points to y */ temp = *ptrl; *ptr1 = *ptr2; *ptr2 = temp; printf(“x is %d while y is %d”, x, y); Ver. 1.0 Slide 9 of 29
    10. 10. Programming in CPractice: 4.2 (Contd.) Solution: Microsoft Office Word 97 - 2003 Document Ver. 1.0 Slide 10 of 29
    11. 11. Programming in CPointer Arithmetic Pointer Arithmetic: Arithmetic operations can be performed on pointers. Therefore, it is essential to declare a pointer as pointing to a certain datatype so that when the pointer is incremented or decremented, it moves by the appropriate number of bytes. Consider the following statement: ptr++; It does not necessarily mean that ptr now points to the next memory location. The memory location it will point to will depend upon the datatype to which the pointer points. – May be initialized when declared if done outside main(). – Consider the following example: #include <stdio.h> char movie[]= “Jurassic Park”; main() { char *ptr; Ver. 1.0 Slide 11 of 29
    12. 12. Programming in CPointer Arithmetic (Contd.) Consider the following example: #include <stdio.h> char movie[]= “Jurassic Park”; main() { char *ptr; ptr=movie; printf(“%s”, movie); /* output: Jurassic Park */ printf(“%s”,ptr); /* output: Jurassic Park */ ptr++; printf(“%s”,movie); /* output: Jurassic Park */ printf(“%s",prr); /* output: urassic Park */ ptr++; printf(“%s”,movie); /* output; Jurassic Park */ printf(“%s”,ptr); /* output: rassic Park */ /* Note that the incrementing of the pointer ptr does not in any way affect the pointer movie */ } Ver. 1.0 Slide 12 of 29
    13. 13. Programming in CPractice: 4.3 1. Consider the following code snippet: #include <stdio.h> int one_d[] = {l,2,3}; main(){ int *ptr; ptr = one_d; ptr +=3; /* statement A*/ printf(“%dn”, *ptr); /*statement B */ } – After statement A is executed, the new address of ptr will be ____ bytes more than the old address. – State whether True or False: The statement B will print 3. Ver. 1.0 Slide 13 of 29
    14. 14. Programming in CPractice: 4.3 (Contd.) Solution: – 12 ( Size of integer = 4*3) – False. Note that ptr is now pointing past the one-d array. So, whatever is stored (junk or some value) at this address is printed out. Again, note the dangers of arbitrary assignments to pointer variables. Ver. 1.0 Slide 14 of 29
    15. 15. Programming in CUsing Pointers to Manipulate Character Arrays Array name contains the address of the first element of the array. A pointer is a variable, which can store the address of another variable. It can be said that an array name is a pointer. Therefore, a pointer can be used to manipulate an array. Ver. 1.0 Slide 15 of 29
    16. 16. Programming in COne-Dimensional Arrays and Pointers One-Dimensional Arrays and Pointers: Consider the following example: #include <stdio.h> char str[13]={“Jiggerypokry”}; char strl[]={ “Magic”}; main() { char *ptr; printf(“We are playing around with %s", str); /* Output: We are playing around with Jiggerypokry*/ ptr=str ; /* ptr now points to whatever str is pointing to */ printf(“We are playing around with %s" ,ptr); /* Output: We are playing around with Jiggerypokry */ } Ver. 1.0 Slide 16 of 29
    17. 17. Programming in COne-Dimensional Arrays and Pointers (Contd.) In the preceding example the statement: ptr=str; Gives the impression that the two pointers are equal. However, there is a very subtle difference between str and ptr. str is a static pointer, which means that the address contained in str cannot be changed. While ptr is a dynamic pointer. The address in ptr can be changed. Ver. 1.0 Slide 17 of 29
    18. 18. Programming in CPractice: 4.4 1. Given the declaration: char some_string [10]; some_string points to _________. 2. State whether True or False: In the following declaration, the pointer err_msg contains a valid address: char *err_msg = “Some error message”; 3. State whether True or False: Consider the following declaration: char *err_msg = “Some error message”; It is more flexible than the following declaration: char err_msg[19]=”Some error message”; Ver. 1.0 Slide 18 of 29
    19. 19. Programming in CPractice: 4.4 (Contd.) Solution: 1. some_string [0] 2. True 3. True. Note that one does not have to count the size of the error message in the first declaration. Ver. 1.0 Slide 19 of 29
    20. 20. Programming in CTwo-Dimensional Arrays and Pointers Two-dimensional arrays can be used to manipulate multiple strings at a time. String manipulation can also be done by using the array of pointers, as shown in the following example: char *things[6]; /* declaring an array of 6 pointers to char */ things[0]=”Raindrops on roses”; things[1]=”And Whiskers on kettles”; things[2]=”Bright copper kettles”; things[3]=”And warm woolen mittens”; things[4]=”Brown paper packages tied up with strings”; things[5]=”These are a few of my favorite things”; Ver. 1.0 Slide 20 of 29
    21. 21. Programming in CTwo-Dimensional Arrays and Pointers (Contd.) The third line of the song can be printed by the following statement: printf(“%s”, things[2]); /*Output: Bright copper kettles */ Ver. 1.0 Slide 21 of 29
    22. 22. Programming in CPractice: 4.5 1. State whether True or False: While declaring two-dimensional character arrays using pointers, yon do not have to go through the tedium of counting the number of characters in the longest string. 2. Given the following error messages: Alls well File not found No read permission for file Insufficient memory No write permission for file Write a program to print all the error messages on screen, using pointers to array. Ver. 1.0 Slide 22 of 29
    23. 23. Programming in CPractice: 4.5 (Contd.) Solution: 1. True. New strings can be typed straight away within the {}. As in: char *err_msg_msg[]= { “Alls well”, “File not found”, “No read permission for file”, “Insufficient memory”, “No write permission for file” }; The number of strings will define the size of the array. Ver. 1.0 Slide 23 of 29
    24. 24. Programming in CPractice: 4.5 (Contd.) 2. The program is: # include<stdio.h> # define ERRORS 5 char *err_msg[]= { /*Note the missing index*/ “Alls well”, “File not found”, “No read permission for file”, “Insufficient memory”, “No write permission for file” }; main() { int err_no; for ( err_no = 0; err_no < ERRORS; err_no++ ) { printf ( “nError message %d is : %sn”, err_no + 1, err_msg[err_no]); } } Ver. 1.0 Slide 24 of 29
    25. 25. Programming in CTwo-Dimensional Arrays and Pointers (Contd.) Consider the following two-d array declaration: int num[3][4]= { {3, 6, 9, 12}, {15, 25, 30, 35}, {66, 77, 88, 99} }; This statement actually declares an array of 3 pointers (constant) num[0], num[l], and num[2] each containing the address of the first element of three single-dimensional arrays. The name of the array, num, contains the address of the first element of the array of pointers (the address of num[0]). Here, *num is equal to num[0] because num[0] points to num[0][0]. *(*num) will give the value 3. *num is equal to num[0]. Ver. 1.0 Slide 25 of 29
    26. 26. Programming in CString-Handling Functions Using Pointers Pointers can be used to write string-handling functions. Consider the following examples: /* function to calculate length of a string*/ #include <stdio.h> main() { char *ptr, str[20]; int size=0; printf(“nEnter string:”); gets(str); fflush(stdin); for(ptr=str ; *ptr != 0; ptr++) { size++; } printf(“String length is %d”, size); } Ver. 1.0 Slide 26 of 29
    27. 27. Programming in CUsing Pointers to Manipulate Character Arrays (Contd.) /*function to check for a palindrome */ # include <stdio.h> main() { char str[50],*ptr,*lptr; printf(“nEnter string :”); gets(str); fflush(stdin); for(lptr=str; *lptr !=’0; lptr++); /*reach the string terminator */ lptr--; /*position on the last character */ for(ptr=str; ptr<=lptr; lptr--,ptr++) { if(*ptr != *lptr) break;} if(ptr>lptr) printf(“%s is a palindrome” ); else printf(“%s is not a palindrome"); } Ver. 1.0 Slide 27 of 29
    28. 28. Programming in CSummary In this session, you learned that: A pointer is a variable, which contains the address of some other variable in memory. A pointer may point to a variable of any data type. A pointer can point to any portion of the memory. A pointer variable is declared as: datatype *<pointer variable name> A pointer variable is initialized as: pointer variable name> = &<variable name to which the pointer will point to> – The & returns the address of the variable. – The * before a pointer name gives the value of the variable to which it is pointing. Ver. 1.0 Slide 28 of 29
    29. 29. Programming in CSummary (Contd.) Pointers can also be subjected to arithmetic. Incrementing a pointer by 1 makes it point to a memory location given by the formula: New address = Old address + Scaling factor One-dimensional character arrays can be declared by declaring a pointer and initializing it. The name of a character array is actually a pointer to the first element of the array. Two-dimensional character arrays can also be declared by using pointers. Ver. 1.0 Slide 29 of 29

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