CSCI 101 Fundamentals of Computer Programming Lab 5 Pointers Fall 2005 Introduction As of now the functions learnt have a severe restriction in the sense that it can only return one value Thus if there is a need to have a function that returns multiple values one would have to make multiple functions This can be annoying and cumbersome Thus the introduction of this week s material pointers Pointers are a new datatype that is somewhat an extension of the current known datatypes Using pointers functions can now indirectly change the values of multiple variables as long as they have access to the pointers pointing to them A pointer in essence usually contains the address of the variable it is pointing to Initialization of pointers is best shown in the example below int var1 10 int pointer1 pointer1 var1 The first line of code above simply makes a new variable called var1 and initializes it to 10 The second line makes a new pointer called pointer1 The compiler recognizes that it is a pointer from the sign in front of the pointer name Hence this is the reason why variable names CANNOT start with a This pointer s value is then set to the address of var1 using the sign on the third line Access to the var1 variable can now be done indirectly using the pointer For example pointer1 5 would change the value of var1 to 5 A good way for you to better understand pointers is to make a simple program that simply initializes the values of the variables and pointers and print them out Try to answer the questions below to see if you fully understand pointers a b c d Is the value of pointer1 the same as pointer1 If the value of pointer1 is changed to 100 what is the value of var1 Why would the value of pointer1 need to be set to var1 What would happen if pointer1 is set to var1 Pointers are generally simple concepts and extremely useful when used correctly However because it is a little indirect and has severe repercussions if used wrongly it has generated a sense of fear around it But as long as the foundations are set right it should be a piece of cake Example An example of a program that uses pointers is below It changes the values of two variables without directly accessing them in the function changeVal It firsts initializes two variables x and y to 0 and makes two pointers that points to them respectively The function changeVal then changes the values that the pointers are pointing to thus indirectly changing the values of x and y You can download this at http scf usc edu csci101 labs lab6pointer c include stdio h void changeVal int int int int int main int x 0 y 0 int xp yp xp x yp y printf Value printf Value changeVal xp printf Value printf Value of x before changeVal d n x of y before changeVal d n y yp 10 100 of x after changeVal d n x of y after changeVal d n y return 0 void changeVal int xp int yp int valx int valy xp valx yp valy Exercise For your first pointer exercise write a program to do the following Parts a and b are to be done in two separate functions where you need to pass by reference Parts c d and e are to be done in only ONE function where you will call the three subfunctions written from your previous lab Reuse part f from your previous lab in this exercise a Prompt the user for his her name b Prompt the user to enter two coordinates in the Cartesian coordinate system x1 y1 x2 y2 c Calculate and outputs the distance of the two points given the two points d Calculates and outputs the angle it makes with the horizontal e Calculates and outputs the slope of the line f Print out your outputs in the main function Show your completed program to your TA LA to receive lab credit