Control flow

Learning goals

• apply control flow syntax to implement a program

control flow

the order in which individual statements are executed

Syntax

if (question1)
  action1;
else if (question2)
  action2;
else
  action3;

Question examples:

• is the time 1?

  • time == 1

• is the temperature lower or equal 20?

  • temperature <= 20

Action examples:

• set the speed to 10

  • speed = 10

Example

In words:

• assume everything is turned off, i.e., 0

• if temperature is greater than 30 then activate air_conditioner

• else if temperature greater than 25 activate ventilator

• else turn everything off.

In code:

if (temperature > 30)
  air_conditioner = 1;
else if (temperature > 25)
  ventilator = 1;
else {  // ️⚠️ Use parantheses if you have multiple actions 
  air_conditioner = 0;
  ventilator = 0;
}

Terms

if-else-if-else chain and actions (e.g., a = 0) are statements.

statement

a syntactic unit of a programming language that expresses some action to be carried out

C statements usually end with a semicolon. Example exception: if we have a compound statement

compound statement

a statement that contains sequences of statements

Questions must be expressions.

expression

syntactic entity in a programming language that may be evaluated to determine its value

Other expression examples:

• 42

• a + b

• temperature > 30

Not expressions but statements:

• temperature = 10;

• if (temperature > 30) printf("Hot");

Activity 2

Which of the below are statements?

1. air_conditioner = 1;

2. if (...) else ...

3. a > b

4. 5

5. success = a > b;

Process (1 min):

1. think individually

2. answer the poll in the virtual classroom

We have to adhere to this syntax, otherwise we get a syntax error.

syntax error

a mismatch in the syntax

Can you spot the syntax error in the following example?

if temperature > 30
  air_conditioner = 1;

Lunar lander control

Activity 3

Write code for landing using the following flowchart:

Copy the following template into your IDE and work on it:

#include <stdio.h>

int Control(int altitude) {
  int thruster = 0;

  // YOUR CODE HERE

  return thruster;
}

void Test(int altitude) {
  int thruster = Control(altitude);
  int behaviorCorrect = (altitude > 100 && thruster == 0) ||
                        (altitude <= 100 && altitude > 0 && thruster == 1) ||
                        (altitude <= 0 && thruster == 0);
  char *behaviorCorrectIcon = behaviorCorrect ? "✅" : "❌";
  printf("For altitude %3d, your thruster is %d |%s|\n", altitude, thruster,
         behaviorCorrectIcon);
}

int main(void) {
  Test(150);
  Test(100);
  Test(50);
  Test(0);
  Test(-1);
}

Steps:

1. Work ~2 min individually. The program automatically tests your code.

2. Compare your result with your neighbor and help them.

Confused 😕 and want some clues? Then click here.

Use the following independent lines:

else
else if (altitude > 0)
if (altitude > 0)
thruster = 1;
thruster = 0;
thruster = 0;