Welcome to Programming and Data Structures

When you look at a program that contains several functions, it is tempting to read it in order from top to bottom. But that is not the flow of execution, or the order the program actually runs.

Programs always begin at the first statement of main, regardless of where it is in the source file. Statements are executed one at a time, in order, until you reach a function invocation, which you can think of as a detour. Instead of going to the next statement, you jump to the first line of the invoked function, execute all the statements there, and then come back and pick up exactly where you left off.

Watching this process play out in Codelens is perhaps the best way to understand what is happening. Don’t worry too much about what this code does (or what void means), pay attention to how the flow of execution works as we encounter function calls:

That flow is a little convoluted. In the middle of main, we jump off to printMessageTwice. While in that function, we twice jump off to execute printMessage. Then, once we are back in main, we call invokes printMessage directly, which causes yet another detour.

Fortunately, C++ is good at keeping track of which functions are running. So when printMessage or printMessageTwice completes, it picks up where we left off in the calling function.

#include <iostream>
using namespace std;

void doThing() {
    cout << "A" << endl;
}

void doOther() {
    cout << "B" << endl;
}

void doIt() {
    cout << "C" << endl;
}

void doThat() {
    cout << "D" << endl;
}

void doMultiple() {
    doThat();
    doOther();
    cout << "E" << endl;
    doIt();
}

int main() {
    cout << "F" << endl;
    doMultiple();
    doThing();
}

F
---
D
---
B
---
E
---
C
---
A

void hi() {
  cout << "hiii !"<< endl;
}

void printGreeting(){
  hi();
  cout << "how are you doing today. "<< endl;
  hi();
}

int main () {
  hi();
  printGreeting();
  hi();
}