There are some instructions to control your code, you can manage things thanks to the conditions and the loops.

Conditions

In this article we saw the boolean operators. A boolean expression always returns a value : 1 for true and 0 for false.

The if condition takes a boolean expression and execute the following code when it's true.

if (5 >= 2) {
    printf("Five is greater than two\n");
}

Five is greater than two

The condition was checked as valid, so the code was executed.

if (5 <= 2) {
    printf("Two is greater than five\n");
}

There will be no output because the condition is invalid, the code was not executed.

Also, we can create an else case.

if (false) {
    printf("This log will never be seen\n");
} else {
    printf("Hey !\n");
}

Hey !

As well as we can create else if cases.

if (0) {
    printf("1: This log will never be seen\n");
} else if (0) {
    printf("2: This log will never be seen\n");
} else if (1) {
    printf("3: Hey!\n");
} else {
    printf("4: This log will never be seen\n");
}

3: Hey !

Conditions and defensive programming

Alright, this is simple. There is a programming way that I would like to teach you. It's a part of the "defensive programming" principle.

To avoid having too many cases following an if expression, we return when the expression works.

#include <stdio.h>

int check(int a, int b) {
    if (a != b) {
        printf("a and b are different\n");
        return 1;
    }

    if (a > b) {
        printf("a is greater than b\n");
        return 1;
    }

    if (a < b) {
        printf("a is less than b\n");
        return 1;
    }

    printf("a and b are the same\n");
    return 0;
}

If you don't care about specific prints :

#include <stdio.h>

int check(int a, int b) {
    if (a != b) {
        printf("a and b are not the same\n");
        return 1;
    }

    printf("a and b are the same\n");
    return 0;
}

Loops

In programming, a loop is a way to repeat a code block while a boolean expression is true.

while (1) {
    printf("Pete and Repeat were on a boat. Pete fell out, who was left on the boat?\n");
    printf("Repeat...");  
}

Pete and Repeat were on a boat. Pete fell out, who was left on the boat?
Repeat...
Pete and Repeat were on a boat. Pete fell out, who was left on the boat?
Repeat...
...

My program won't stop ! How to stop it ?

For every running program, in your terminal you can press CTRL + c then the program will be cancelled.

In the C language, we can use two different loops :

while (<boolean expression>) {
    <?instructions...>
}

for (<set variable>; <boolean expression>; <update variable>) {
    <?instructions...>
}

And they are two keywords to manage the loop :

• break : To stop the loop at this point
• continue : To ignore the code below and continue the loop at this point

Loop example for arrays

#include <stdio.h>

int main(void) {
    int values[4] = {4, 2, 9, 1};

    for (int i = 0; i < 4; ++i) {
        printf("values[%i] == %i\n", i, values[i]);
    } 

    return 0;
}

values[0] == 4
values[1] == 2
values[2] == 9
values[3] == 1

We create an iterator named i. The loop lives while i is less than 4. For each round, the variable iterator is incremented by 1.

for (int i = 0; i < 4; ++i) {}

Is the same as

int i = 0;
while (i < 4) {
    ++i;
}

And

int i = 0;
while (1) {
    if (i >= 4) {
        break;
    }
    ++i;
}

---

Exercises

1. Create a function telling if the parameter is odd or even
2. Walk through an array and multiply each element by two
3. Create a guessing program for a magic number. It takes the number from the command line parameters. We will use the int atoi(char[] string) function from <stdlib.h> to convert a string into an integer

---

Solutions

1. Create a function telling if the parameter is odd or even

   void odd_or_even(int x) {
      if (x % 2 == 0) {
          printf("%i is even\n", x);
      } else {
          printf("%i is odd\n", x);
      }
   }
   

2. Walk through an array and multiply each element by two

    int main(void) {
        int values[3] = {6, 8, 10};
        for (int i = 0; i < 3; ++i) {
            values[i] *= 2;
        }
   
        return 0;
    }
   

3. Create a guessing program for a magic number

    #include <stdio.h>
    #include <stdlib.h>
   
    int main(int argc, char** argv) {
        int input = atoi(argv[1]);
        int magic_number = 78;
   
        if (input < magic_number) {
            printf("It's greater\n");
        } else if (input > magic_number) {
            printf("It's less\n");
        } else {
            printf("You win !\n");
        }
   
        return 0;
    }
   

   Or with the defensive programming principle :

    #include <stdio.h>
    #include <stdlib.h>
   
    int main(int argc, char** argv) {
        int input = atoi(argv[1]);
        int magic_number = 78;
   
        if (input < magic_number) {
            printf("It's greater\n");
            return 1;
        }
   
        if (input > magic_number) {
            printf("It's less\n");
            return 1;
        }
   
        printf("You win !\n");
        return 0;
    }
   

   This program is not really good. Later we will see how to ask the user a number in the command shell and we will create a loop to avoid restarting the program for each tentative. I also said we will use the int atoi(char[] string) function from <stdlib.h> to convert a string into an integer, but it's a bad function because if the string doesn't represent a number, the result will be weird. Later, we will see the right functions to use for these cases.