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CMPT 102 Introduction to Scientific Computer Programming

CMPT 102 Introduction to Scientific Computer Programming. Strings. Simple and Composite Variables. We have studied simple variables A simple variable describes a single value A simple variable has an identifier

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CMPT 102 Introduction to Scientific Computer Programming

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  1. CMPT 102Introduction to Scientific Computer Programming Strings

  2. Simple and Composite Variables • We have studied simple variables • A simple variable describes a single value • A simple variable has an identifier • A simple variable has a type that describes the properties of the value of the variable, the permissible operations for the variable, and the representation of the variable in computer memory • We can also have composite variables • These variables describe a group of values • Arrays: all values in the group have the same type • Structures: different values in the group can have different types

  3. Composite Variables • composite variables describe a group of values • 1 dimensional arrays or variables of a particular type (all entries must have the same type) • multi dimensional arrays or variables of a particular type (all entries must have the same type) • Structures containing groups of variables of different types • Strings are another special type that builds on arrays • An array of characters • A set of special operations appropriate for text

  4. One-Dimensional (1-D) Arrays • An array is an indexed data structure • All variables stored in an array are of the same data type • An element of an array is accessed using the array name and an index or subscript • The name of the array is the address of the first element and the subscript is the offset • In C, the subscripts always start with 0 and increment by 1

  5. Example String Declaration in C list[0] char list[10]; • allocates memory for 10 characters. • Ten adjacent locations in memory are allocated • Remember C does not perform any bounds checking on arrays list[1] list[2] list[3] list[4] list[5] list[6] list[7] list[8] list[9]

  6. Initializing 1-D Arrays • Strings are not the same as 1-D character arrays • You can specify individual values for each character in a 1-D character array /* put one character in each element of the array*/ char list[8] = {‘h’,’e’,’l’,’l’, ‘o’}; After initialization memory looks like list[1] ‘e’ list[2] ‘l’ list[3] ‘l’ list[4] ‘o’ list[5] ? list[6] ? list[7] ? list[8] ? list[0] ‘h’

  7. Difference: String vs 1D character array • Method of initialization • A string always in a null termination character (\0) • This tells all the functions in the string library where the string ends • Use of the null termination character allows strings of different length to be stored in a character array of a single length

  8. Strings of different lengths • Strings of different lengths can be stored in a character array • The maximum number of character in the string is the number of characters in the array minus one • Blanks can be included in the string • Blanks count as characters char list[8] = {“hello”}; char list1[8] = {“hi jane”}; list[1] ‘e’ list[2] ‘l’ list[3] ‘l’ list[4] ‘o’ list[5] ‘\0’ list[6] ? list[7] ? list[8] ? list[0] ‘h’ list[1] ‘i’ list[2] ‘ ’ list[3] ‘j’ list[4] ‘a’ list[5] ‘n’ list[6] ‘e’ list[7] ‘\0’ list[8] ? list[0] ‘h’

  9. Avoid a common problem (1) • C does not perform any bounds checking on arrays • This means that you can accidentally change the values of other variables by changing a value you refer to as an element of the array, which is not actually part of the array • For a string variable this is particularly easy. • You must remember that character array mystring[20] holds a string of no more than 19 characters • “hello my friend” has 15+1 characters • “joe” has 3+1 characters • REMEMBER THE NULL TERMINATION CHARACTER

  10. Avoid a common problem (2) int count = 3; char myArray[5] = {“hello”}; After the first declaration memory looks like After the second declaration statement above REMEMBER: Leave room for the \0 myArray[1] ? myArray[2] ? myArray[3] ? myArray[4] ? count 3 myArray[0] ? myArray[1] ‘e’ myArray[2] ‘l’ myArray[3] ‘l’ myArray[4] ‘o’ count ‘\0” myArray[0] ‘h’

  11. mySt[1] ? mySt[1] ‘y’ mySt[2] ? mySt[2] ‘ ‘ mySt[3] ? mySt[3] ‘n’ mySt[4] ? mySt[4] ‘a’ Count[0] ‘m’ Count[0] 1 mySt[0] ? mySt[0] ‘m’ Avoid a common problem (3) int count [4]= {1,2,3,5}; char mySt[5] = {“my name”}; After the first declaration memory looks like After the second declaration statement above mySt has no terminating \0, string library breaks Array count has been corrupted and now contains the integer equivalent of “e” Count[2] 3 Count[3] 5 Count[1] 2 Count[2] 3 Count[3] 5 Count[1] ‘\0’

  12. Avoid a common problem (4) • C does not perform any bounds checking on arrays • By initializing or changing the contents of a string with a string that is longer than will fit into the character array associated with the string it is possible to change the value of a completely different variable and to break the string library for the string being initialized • It is imperative that you be very careful to avoid using strings longer than the allocated space

  13. Arrays of strings • Declare your array of Strings #define NUMNAMES 20 #define MAXNAMELEN 32 char names[NUMNAMES][MAXNAMELEN] • Declare and Initialize your array #define NUMMONTHS 12 #define MONTHNAMESIZE 10 char month[NUMMONTHS][MONTHNAMESIZE] = { “January”, “February”, “March”, “April”, “May”, “June”, “July”, “August”, “September”, “October”, “November”, December” };

  14. ? ‘\0’ ? ? ? ? ‘a’ ‘a’ ‘y’ ‘n’ ‘u’ ‘\0’? ‘a’ ? ? ‘r’ ‘y’ ? ‘M’ ‘J’ ‘y’ ? ? ‘\0’ ? ? ‘u’ ‘e’ ‘b’ ‘n’ ‘r’ ‘e’ ‘\0’ ‘u’ ? ‘a’ ? ‘r’ ‘J’ ‘F’ ? ? ? ? ? ? ‘u’ ‘a’ ‘l’ ‘r’ ‘c’ ‘y’ ‘\0’ ‘h’ ? ‘\0’ ? ? ‘M’ ‘J’ ? ? ? ? ? ? ‘p’ ‘u’ ‘r’ ‘g’ ‘u’ ‘i’ ‘l’ ‘s’ ‘t’ ‘\0’ ? ‘0’ ‘A’ ‘A’ Initializing and array of strings Char month[12][10] = { “January”, “February”, “March”, “April”, “May”, “June”, “July”, “August”, “September”, “October”, “November”, December” }; ‘\0’

  15. Initializing a string • You can initialize a simple variable either in the declaration statement, or using separate assignment statements following the declaration statements. • You can also initialize the values of the string following using assignment statements following the declaration statements • When initializing a string remember to be sure that you do not put more characters in the string than there is space for

  16. Initializing or using 1-D arrays char *list[10]; int i; /*Initialize each element of array list to “mystart” */ for(i=0; i<10; i++) { strcpy(list[i], “mystart”); }

  17. Putting data into a 1-D Array • Another common way of assigning values to strings or arrays of strings is to read data values from a file directly into the string or array of strings • Each value read from the file is assigned to a single string(for example names[6]) • A single row stored in the ith row in the array of strings names is referred to as names[i], • Note that checks to determine the file was opened correctly and that data was read correctly have been omitted from the example, they should not be omitted from your code

  18. Array Input from a data file • #define NUMPEOPLE 30 • #define NAMELEN 32 • char names[NUMPEOPLE][NAMELEN]; • char title[30]; • int ages[NUMPEOPLE]; • int k; • FILE *registrants; • registrants = fopen(“registrants”, “r”); • scanf( “%s”, title); • printf(“%s\n”, title); • for(k=0; k<NUMPEOPLE; k++) • { • fscanf(registrants, “%s %d”, names[k], &ages[k]); • printf(“%33s, %3d\n”, names[k], ages[k]); • }

  19. Notes on array input • The string is read or written using %s • When you read or write a string your read or write all characters in that string • The final character in the string is determined by the location of the null termination character \0 • When reading a string using scanf or fscanf there is no & before the name of the string • The name of the string is a reference (address) of the first element of the string

  20. Strings as function parameters • Arrays, or parts of arrays, can be passed as arguments to functions. • An element of an string can be used as a simple character variable parameter • It can be passed by value or by reference • An entire string can be used as a parameter of a function • It can only be passed by reference using the name of the string (the name of the string is a reference to the location in memory of the first character in the string)

  21. Strings as a data type • Remember a data type has a group of objects (things) that can be combined in different ways using the operands for that data type. • The operands for strings are not those used for other data types (like +, -, = …) • All operations on strings are performed using functions from the string library (other than reading and writing) • To include the string library in your program include <string.h>

  22. Assigning strings • You have seen that = can be used when assigning values to strings in a declaration • = cannot be used to assign a string literal to a string: The following is not valid mystring = “testinput”; • To copy one string to another the string library functions strcpy is usually used strcpy(mystring, “testinput”); strcpy(myCopiedString, myOriginalString);

  23. Assigning strings • To copy one string to another the string library functions strcpy is usually used • To copy part of a string, (a substring) or to assure you do not copy more characters into a string that it can hold you can also use strncpy strncpy(mystring, “testinput”, 5); • Note that strncpy copies the first 5 characters of “testinput” only (testi) and does not add a \0 to the end of the copied string • strcpy copies the entire string (even it it is longer than the available space!) including the \0

  24. mySt[1] ? mySt[1] ‘y’ mySt[2] ? mySt[2] ‘ ‘ mySt[3] ? mySt[3] ‘n’ mySt[4] ? mySt[4] ‘a’ Count[0] ‘m’ Count[0] 1 mySt[0] ? mySt[0] ‘m’ Avoid a common problem (3) int count [4]= {1,2,3,5}; char mySt[5] ; strcpy(mySt, “my name”); After the declarations memory looks like After the strcpy statement above mySt has no terminating \0 in its array Count[2] 3 Count[3] 5 Count[1] 2 Count[2] ‘\0’ Count[3] 5 Count[1] ‘e’

  25. Finding the length of a string • To find the number of characters actually stored in a string use the string library function strlen int len; char mystring[30]; strcpy(mystring, “testing”); len = strlen(mystring); /* len now has a value 7 */ • Strlen counts the number of characters in the string not including the terminating \0

  26. Concatenating Strings • Combining two strings into a single string • Use the string library functions strcat or strncat • strcat and strncat take one string and append it to the end of another string • The terminating \0 is removed from the end of the first string before the second string is added • The terminating \0 is replaced at the end of the second string • strcat and strncat can create a string too long to fit in the allocated string storage:

  27. Example: using strcat char name1[10] = “marie”; char name2[10] = “anne”; strcat( name2, name1); m a r i e \0 ? ? ? ? a n n e \0 ? ? ? ? ? m a r i e \0 ? ? ? ? a n n e m a r i e \0

  28. Concatenating Strings char mystring[20]=“start input: “; char mystring1[20] = “input1”; char mystring2[20] = “ and output” /* after the following strcat mystring1 contains */ /* “input1 and output” */ strcat(mystring1, mystring2); /* after the following strcat mystring1 contains */ /* “start input: input1 and output” */ /* this string overflows the array mystring */ strcat(mystring, mystring1);

  29. Example: using strcat char name1[10] = “marie”; char name2[10] = “anne”; strncat( name2, name1, 2); m a r i e \0 ? ? ? ? a n n e \0 ? ? ? ? ? m a r i e \0 ? ? ? ? a n n e m a \0 ? ? ?

  30. Concatenating Strings #define STRLEN 20 int len, added; char mystring[STRLEN]=“start input: “; char mystring1[STRLEN] = “input1 and output”; /* To prevent overflow find the number of */ /* characters that can be added to mystring */ /* added (6) = STRLEN (20) – len(13) – 1 */ len = strlen(mystring); added = STRLEN – len -1; /* after the following strcat mystring1 contains */ /* “start input: input1” */ strncat(mystring, mystring1, added);

  31. Comparing Strings • To compare 2 strings usually use the string library function strcmp strcmp(mystring1, mystring2) • strcmp returns an integer, • if mystring1 is alphabetically before mystring2 a negative number will be returned • If the strings are identical 0 will be returned • if mystring2 is alphabetically before mystring1 a positive number will be returned

  32. ASCII equivalents • Each alphabetic character, number, or other character (including whitespace characters) has an integer equivalent value • These integer values are used by strcmp to determine the alphabetical ordering. • All uppercase letters precede lower case letters • All numbers precede uppercase letter • A string st1 contains the first few characters of a longer string st2. st1 precedes st2 when compared

  33. Comparing Parts of Strings • To compare the first n characters of 2 strings use the string library function strncmp strncmp(mystring1, mystring2, n) • strncmp returns an integer, • if mystring1 is alphabetically before mystring2 a negative number will be returned • If the strings are identical 0 will be returned • if mystring2 is alphabetically before mystring1 a positive number will be returned

  34. Conversion to and from strings • Strings to numbers: sscanf • Works like fscanf bur read from a string • Numbers to strings: sprintf • Works just like fprintf but writes into a string char mystring[20]; int myvalue1= 23, myvalue2=46; sprintf( mystring, “%s:%2d, %2d”, “myvalues are”, myvalue1, myvalue2); /* now mystring contains */ /* myvalues are:23, 46 */

  35. Character analysis • You can also analyze a string (or a character array) one character at a time • The ctype library #include <ctype.h> includes functions for such analysis. Each of these functions returns an integer value. The value is nonzero if the condition checked is true (0 if it is false) isalpha(char mychar); /* is an alphanumeric char */ isdigit( char mychar); /* is a numeral */ ispunct(char mychar); /* is a non whitespace punctuation character */ isspace(char mychar); /* is a whitespace character */ tolower(char mychar); /* converts alphanumeric to lower case */ toupper(char mychar); /* converts alphanumeric to upper case */h

  36. The ctype and string Libraries • We have had an introduction to some of the functions in these libraries. • These libraries are much more flexible than this subset of functions indicates • You should be able to read the function descriptions for the other functions in the string library and then use those functions in your programs

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