Pointer Perplexity?!?!?

1. Pointer Perplexity?!?!?

2. Why? • A function can only return one value using the return statement • Thus, we use pointers to “return” more than one value to the calling function • We also use pointers to “pass” arrays, particularly strings of characters

3. How? • By calling a function with the address of the data, rather than the value of data, the called function can modify the data in memory • This is called “pass by reference” as opposed to “pass by value” • The & operator in front of a variable name provides the location of the variable in memory, not the value of the variable.

4. Example #1 • The example below shows a function call in which the function can modify three items: one through the return value and the other two through memory • var1, var2, and var3 are variables in main() • &var1 is a pointer to the location of var1 in memory, i.e., a memory address var3 = my_func (&var1, &var2); // pass the addresses of var1 and var2

5. Example #1 (cont) • The arguments must be identified as pointers using * in the argument list • my_func can access the data using the pointers • read the data tmp_var = *var1_addr; • modify the data tmp_var = tmp_var + [...] • write new data into the memory location *var1_addr = tmp_var;

6. Example #1 (cont) intmy_func (int *var1_addr, int *var2_addr) { int var1_data; // declare local variable var1_data = *var1_addr; // read data at address var1_addr [...] // modify the data *var1_addr = var1_data; // write the new data into memory [...] return (var3_data); } • Key Points: • var1_addr is a local variable in my_func of type “integer pointer” • *var1_addr allows access to that address location for reading or writing • &var1_addr is the location of the pointer!!!(not something we ever use)