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Understanding Pointer Manipulation in C++: Tracing Function Execution

This document provides a step-by-step examination of a C++ program that illustrates the manipulation of pointers through the use of a function, `mysteryFunction`. The function takes a pointer to an integer, modifies its value by multiplying it by three, and demonstrates how the changes affect the original variable. The tracing process goes through an initial value of 7, how it's accessed and manipulated through pointers, and culminates in a final output of 21. This serves as a useful educational tool for students learning about pointers and function calls in C++.

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Understanding Pointer Manipulation in C++: Tracing Function Execution

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  1. Tracing through E01, question 9 – step 1 // p02.cc P. Conrad, for CISC181 07S // Exam question for E01 #include <iostream> using namespace std; void mysteryFunction(int *x); int main() { int k = 7; mysteryFunction(&k); cout << k << endl; return 0; } void mysteryFunction(int *x) { int result; result = (*x); result = result * 3; (*x) = result; } main

  2. Tracing through E01, question 9 – step 2 // p02.cc P. Conrad, for CISC181 07S // Exam question for E01 #include <iostream> using namespace std; void mysteryFunction(int *x); int main() { int k = 7; mysteryFunction(&k); cout << k << endl; return 0; } void mysteryFunction(int *x) { int result; result = (*x); result = result * 3; (*x) = result; } 7 k main

  3. Tracing through E01, question 9 – step 3 // p02.cc P. Conrad, for CISC181 07S // Exam question for E01 #include <iostream> using namespace std; void mysteryFunction(int *x); int main() { int k = 7; mysteryFunction(&k); cout << k << endl; return 0; } void mysteryFunction(int *x) { int result; result = (*x); result = result * 3; (*x) = result; } mysteryFunction() 7 k main()

  4. Tracing through E01, question 9 – step 4 // p02.cc P. Conrad, for CISC181 07S // Exam question for E01 #include <iostream> using namespace std; void mysteryFunction(int *x); int main() { int k = 7; mysteryFunction(&k); cout << k << endl; return 0; } void mysteryFunction(int *x) { int result; result = (*x); result = result * 3; (*x) = result; } The address of k is copied into xSince an address is a pointer,and a pointer is an address,x now points to k. x mysteryFunction() 7 k main()

  5. Tracing through E01, question 9 – step 5 // p02.cc P. Conrad, for CISC181 07S // Exam question for E01 #include <iostream> using namespace std; void mysteryFunction(int *x); int main() { int k = 7; mysteryFunction(&k); cout << k << endl; return 0; } void mysteryFunction(int *x) { int result; result = (*x); result = result * 3; (*x) = result; } result x mysteryFunction() 7 k main()

  6. Tracing through E01, question 9 – step 6 (*x) means dereference the pointer xto go to what it points to. That is, k. Since (*x) is really k, the value of k, which is 7, gets copied into result. // p02.cc P. Conrad, for CISC181 07S // Exam question for E01 #include <iostream> using namespace std; void mysteryFunction(int *x); int main() { int k = 7; mysteryFunction(&k); cout << k << endl; return 0; } void mysteryFunction(int *x) { int result; result = (*x); result = result * 3; (*x) = result; } result 7 x mysteryFunction() 7 k main()

  7. Tracing through E01, question 9 – step 7 result gets multiplied by 3,and 7 changes to 21 // p02.cc P. Conrad, for CISC181 07S // Exam question for E01 #include <iostream> using namespace std; void mysteryFunction(int *x); int main() { int k = 7; mysteryFunction(&k); cout << k << endl; return 0; } void mysteryFunction(int *x) { int result; result = (*x); result = result * 3; (*x) = result; } result 7 21 x mysteryFunction() 7 k main()

  8. Tracing through E01, question 9 – step 8 Now, (*x) is on the left hand side. (*x) dereferences the pointer x, follows the arrow to what is pointed to, which is k. So (*x) on the left hand side, really means that we are assigning a new value to k. // p02.cc P. Conrad, for CISC181 07S // Exam question for E01 #include <iostream> using namespace std; void mysteryFunction(int *x); int main() { int k = 7; mysteryFunction(&k); cout << k << endl; return 0; } void mysteryFunction(int *x) { int result; result = (*x); result = result * 3; (*x) = result; } result 21 x mysteryFunction() 7 21 k main()

  9. Tracing through E01, question 9 – step 9 When we reach the end of the function, the call frame, and all of its local variables are popped off of the stack, and they go away. // p02.cc P. Conrad, for CISC181 07S // Exam question for E01 #include <iostream> using namespace std; void mysteryFunction(int *x); int main() { int k = 7; mysteryFunction(&k); cout << k << endl; return 0; } void mysteryFunction(int *x) { int result; result = (*x); result = result * 3; (*x) = result; } result 21 x mysteryFunction() 21 k main()

  10. Tracing through E01, question 9 – step 10 We have returned from the function, and we print the answer, 21. // p02.cc P. Conrad, for CISC181 07S // Exam question for E01 #include <iostream> using namespace std; void mysteryFunction(int *x); int main() { int k = 7; mysteryFunction(&k); cout << k << endl; return 0; } void mysteryFunction(int *x) { int result; result = (*x); result = result * 3; (*x) = result; } 21 k main()

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