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Chapter 7 - Strings and Arrays WHAT IS A STRING? A string is a group of characters, usually letters of the alphabet. In order to format your printout in such a way that it looks nice, has meaningful titles and names, and is esthetically pleasing to you and the people using the output of your program, you need the ability to output text data. Actually you have already been using strings, because the second program in this tutorial, way back in Chapter 2, output a message that was handled internally as a string. A complete definition is a series of "char" type data terminated by a NULL character, which is a zero. When C is going to use a string of data in some way, either to compare it with another, output it, copy it to another string, or whatever, the functions are set up to do what they are called to do until a NULL, which is a zero, is detected. WHAT IS AN ARRAY? An array is a series of homogeneous pieces of data that are all identical in type, but the type can be quite complex as we will see when we get to the chapter of this tutorial discussing structures. A string is simply a special case of an array. The best way to see these principles is by use of an example, so load the program CHRSTRG.C and display it on your monitor. The first thing new is the line that defines a "char" type of data entity. The square brackets define an array subscript in C, and in the case of the data definition statement, the 5 in the brackets defines 5 data fields of type "char" all defined as the variable "name". In the C language, all subscripts start at 0 and increase by 1 each step up to the maximum which in this case is 4. We therefore have 5 "char" type variables named, "name[0]", "name[1]", "name[2]", "name[3]", and "name[4]". You must keep in mind that in C, the subscripts actually go from 0 to one less than the number defined in the definition statement. HOW DO WE USE THE STRING? The variable "name" is therefore a string which can hold up to 5 characters, but since we need room for the NULL character, there are actually only four useful characters. To load something useful into the string, we have 5 statements, each of which assigns one alphabetical character to one of the string characters. Finally, the last place in the string is filled with the numeral 0 as the end indicator Page 46 Chapter 7 - Strings and Arrays and the string is complete. (A "define" would allow us to use "NULL" instead of a zero, and this would add greatly to the clarity of the program. It would be very obvious that this was a NULL and not simply a zero for some other purpose.) Now that we have the string, we will simply print it out with some other string data in the output statement. The %s is the output definition to output a string and the system will output characters starting with the first one in "name" until it comes to the NULL character, and it will quit. Notice that in the "printf" statement, only the variable name "name" needs to be given, with no subscript since we are interested in starting at the beginning. (There is actually another reason that only the variable name is given without brackets. The discussion of that topic will be given in the next chapter.) OUTPUTTING PART OF A STRING The next "printf" illustrates that we can output any single character of the string by using the "%c" and naming the particular character of "name" we want by including the subscript. The last "printf" illustrates how we can output part of the string by stating the starting point by using a subscript. The & specifies the address of "name[1]". We will study this in the next chapter but I thought you would benefit from a little glimpse ahead. This example may make you feel that strings are rather cumbersome to use since you have to set up each character one at a time. That is an incorrect conclusion because strings are very easy to use as we will see in the next example program. Compile and run this program. SOME STRING SUBROUTINES Load the example program STRINGS.C for an example of some ways to use strings. First we define four strings. Next we come to a new function that you will find very useful, the "strcpy" function, or string copy. It copies from one string to another until it comes to the NULL character. Remember that the NULL is actually a "0" and is added to the character string by the system. It is easy to remember which one gets copied to which if you think of them like an assignment statement. Thus if you were to say, for example, "x = 23;", the data is copied from the right entity to the left one. In the "strcpy" function, the data is also copied from the right entity to the left, so that after execution of the first statement, name1 will contain the Page 47 Chapter 7 - Strings and Arrays string "Rosalinda", but without the double quotes, they are the compiler's way of knowing that you are defining a string. Likewise, "Zeke" is copied into "name2" by the second statement, then the "title" is copied. The title and both names are then printed out. Note that it is not necessary for the defined string to be exactly the same size as the string it will be called upon to store, only that it is at least as long as the string plus one more character for the NULL. ALPHABETICAL SORTING OF STRINGS The next function we will look at is the "strcmp" or the string compare function. It will return a 1 if the first string is larger than the second, zero if they are the same length and have the same characters, and -1 if the first string is smaller than the second. One of the strings, depending on the result of the compare is copied into the variable "mixed", and the largest name alphabetically is printed out. It should come as no surprise to you that "Zeke" wins because it is alphabetically larger, length doesn't matter, only the alphabet. It might be wise to mention that the result would also depend on whether the letters were upper or lower case. There are functions available with your C compiler to change the case of a string to all upper or all lower case if you desire. These will be used in an example program later in this tutorial. COMBINING STRINGS The last four statements have another new feature, the "strcat", or string concatenation function. This function simply adds the characters from one string onto the end of another string taking care to adjust the NULL so everything is still all right. In this case, "name1" is copied into "mixed", then two blanks are concatenated to "mixed", and finally "name2" is concatenated to the combination. The result is printed out with both names in the one variable "mixed". Strings are not difficult and are extremely useful. You should spend some time getting familiar with them before proceeding on to the next topic. Compile and run this program and observe the results for compliance with this definition. Page 48 Chapter 7 - Strings and Arrays AN ARRAY OF INTEGERS Load the file INTARRAY.C and display it on your monitor for an example of an array of integers. Notice that the array is defined in much the same way we defined an array of char in order to do the string manipulations in the last section. We have 12 integer variables to work with not counting the one named "index". The names of the variables are "values[0]", "values[1]", ... , and "values[11]". Next we have a loop to assign nonsense, but well defined, data to each of the 12 variables, then print all 12 out. You should have no trouble following this program, but be sure you understand it. Compile and run it to see if it does what you expect it to do. AN ARRAY OF FLOATING POINT DATA Load and display the program named BIGARRAY.C for an example of a program with an array of "float" type data. This program has an extra feature to illustrate how strings can be initialized. The first line of the program illustrates to you how to initialize a string of characters. Notice that the square brackets are empty leaving it up to the compiler to count the characters and allocate enough space for our string including the terminating NULL. Another string is initialized in the body of the program but it must be declared "static" here. This prevents it from being allocated as an "automatic" variable and allows it to retain the string once the program is started. There is nothing else new here, the variables are assigned nonsense data and the results of all the nonsense are printed out along with a header. This program should also be easy for you to follow, so study it until you are sure of what it is doing before going on to the next topic. GETTING DATA BACK FROM A FUNCTION Back in chapter 5 when we studied functions, I hinted to you that there was a way to get data back from a function by using an array, and that is true. Load the program PASSBACK.C for an example of doing that. In this program, we define an array of 20 variables named "matrix", then assign some nonsense data to the variables, and print out the first five. Then we call the function "dosome" taking along the entire array by putting the name of the array in the parentheses. The function "dosome" has a name in its parentheses also but it prefers to call the array "list". The function needs to be told that it is really getting an array passed to it and that the array is of type "int". The following Page 49 Chapter 7 - Strings and Arrays line, prior to the bracket which starts the program, does that by defining "list" as an integer type variable and including the square brackets to indicate an array. It is not necessary to tell the function how many elements are in the array, but you could if you so desired. Generally a function works with an array until some end-of-data marker is found, such as a NULL for a string, or some other previously defined data or pattern. Many times, another piece of data is passed to the function with a count of how many elements to work with. In our present illustration, we will use a fixed number of elements to keep it simple. So far nothing is different from the previous functions we have called except that we have passed more data points to the function this time than we ever have before, having passed 20 integer values. We print out the first 5 again to see if they did indeed get passed here. Then we add ten to each of the elements and print out the new values. Finally we return to the main program and print out the same 5 data points. We find that we have indeed modified the data in the function, and when we returned to the main program, we brought the changes back. Compile and run this program to verify this conclusion. ARRAYS PASS DATA BOTH WAYS We stated during our study of functions that when we passed data to a function, the system made a copy to use in the function which was thrown away when we returned. This is not the case with arrays. The actual array is passed to the function and the function can modify it any way it wishes to. The result of the modifications will be available back in the calling program. This may seem strange to you that arrays are handled differently from single point data, but they are. It really does make sense, but you will have to wait until we get to pointers to understand it. A HINT AT A FUTURE LESSON Another way of getting data back from a function to the calling program is by using pointers which we will cover in the next chapter. When we get there we will find that an array is in reality a pointer to a list of values. Don't let that worry you now, it will make sense when we get there. In the meantime concentrate on arrays and understand the basics of them because when we get to the study of structures we will be able to define some pretty elaborate arrays. Page 50 Chapter 7 - Strings and Arrays MULTIPLY DIMENSIONED ARRAYS Load and display the file named MULTIARY.C for an example of a program with doubly dimensioned arrays. The variable "big" is an 8 by 8 array that contains 8 times 8 or 64 elements total. The first element is "big[0][0]", and the last is "big[7][7]". Another array named "large" is also defined which is not square to illustrate that the array need not be square. Both are filled up with data, one representing a multiplication table and the other being formed into an addition table. To illustrate that individual elements can be modified at will, one of the elements of "big" is assigned the value from one of the elements of "huge" after being multiplied by 22. Next "big[2][2]" is assigned the arbitrary value of 5, and this value is used for the subscripts of the next assignment statement. The third assignment statement is in reality "big[5][5] = 177" because each of the subscripts contain the value 5. This is only done to illustrate that any valid expression can be used for a subscript. It must only meet two conditions, it must be an integer (although a "char" will work just as well), and it must be within the range of the subscript it is being used for. The entire matrix variable "big" is printed out in a square form so you can check the values to see if they did get set the way you expected them to. PROGRAMMING EXERCISES 1. Write a program with three short strings, about 6 characters each, and use "strcpy" to copy "one", "two", and "three" into them. Concatenate the three strings into one string and print the result out 10 times. 2. Define two integer arrays, each 10 elements long, called "array1" and "array2". Using a loop, put some kind of nonsense data in each and add them term for term into another 10 element array named "arrays". Finally, print all results in a table with an index number. 1 2 + 10 = 12 2 4 + 20 = 24 3 6 + 30 = 36 etc. Hint; The print statement will be similar to; printf("%4d %4d + %4d = %4d\n",index,array1[index], array2[index],arrays[index]); Page 51