Lecture 9: Loops I: for

Loops are for repeating part of the code a number of times. In C there exist three types of loops, There is no difference between the first two types of loops, only a matter of legibility of the program. The do .. while loop differs from the other two by the fact that the condition of exiting the loop is checked at the end, while for the other two (for and while) it is checked in the beginning of the loop.
Todays lecture is about the for loop.

For loop

The most common loop in C is the for loop. This loop, in principle, is used to execute things a predetermined number of times in a countable way. This in contrast to loops that will run while a certain condition is true, as we will learn in the next lecture.

 The general structure of  the for loop is
 
for (startI; testC; stepI)
  instruction;

with startI and stepI general instructions and testC a Boolean condition. This will repeat the instructions instruction and stepI until the test condition results in "false".
 
The instruction is repeated a number of times, determined by the control instructions startI and stepI and the test condition testC. The control instructions startI and stepI can be any instruction or even combination of instructions (separated by commas).
The startI instruction is only executed at the beginning of the loop and only executed once. Immediately after that, the condition testC  is evaluated. If this results is "false" (0), the loop is immediately exited. Hence it is possible with the for loop that the instruction is not even executed a single time (contrary to the do .. while loop as we will see in the next lecture). If the result of the condition is "true" (!=0), the instructions instruction and stepI are executed.
Since the loop is designed for doing things in a countable way, it is advised to use control variables of the integer type as in the next example. Do not forget to declare the variable (see lecture 5).
 

 program code

main()
// for-loop example 
{
  int i;

  for (i=1; i<5; i++) 
    printf("Ola\n");
}

  output 

  Ola
  Ola
  Ola
  Ola

This program is doing the following 
  1) it is attributing 1 to i
  2) it is checking if i is smaller than 5
  3) if not so: EXIT LOOP immediately. Else
  4) execute printf("Ola\n");
  5) add 1 to i
  6) go to step 2)

Multiple instructions


Just like with the if ... else ...  structure, we can also group instructions together with {..} in loops:
 
 program code
for (i = 1; i<5; i++)
 {
    printf("Ola\n");
    printf("It is a nice day\n");
 }
    output
Ola
It is a nice day
Ola
It is a nice day
Ola
It is a nice day
Ola
It is a nice day

Compare this with
 
 program code
for (i = 1; i<5; i++)
  printf("Ola\n");
  printf("It is a nice day\n");
    output
Ola
Ola
Ola
Ola
It is a nice day

Use of the loop variable

Inside the loop the loop variable can be used, but don't mess with it
 
Good code:
 program code

for (i=1; i<5; i++)
  printf("%d Ola\n", i);

     output

  1 Ola 
  2 Ola
  3 Ola
  4 Ola
Bad code:
 program code

for (i=1; i<5; i++)
  {
    printf("%d Ola\n", i);
     i = i + 1;
  }

     output

  1 Ola
  3 Ola
 
The program on the right is an example of bad code. Such style of programming, although at some times it will save space and maybe executing time, makes your program unstructured and very difficult to understand for others! If you want to achieve things like in the program on the right, use other loops, like while or do-while, or , better, use something like in the program below.
 
program code

for (i=1; i<5; i++)
  printf("%d Ola", 2*i-1);		    output
 1 Ola
 3 Ola

Nested loops


The For loops (and any other loop as well) can also be 'nested', which means that they can be put within eachother. We can create double loops, or triple loops (like in the figure below on the left) or any other level. Such structures look like nests of birds and hence the name 'nesting' of loops. Here are some examples

 
           program code
--------------------------------------
main()
// three nested loops
{
 int i, j, k;

 for (i=1; i<=2; i++)
   for (j=1; j<=2; j++)
     for (k=1; k<=2; k++)
       printf("i=%d j=%d k=%d\n", i, j, k);
}
 

    output
-------------
i=1 j=1 k=1
i=1 j=1 k=2
i=1 j=2 k=1
i=1 j=2 k=2
i=2 j=1 k=1
i=2 j=1 k=2
i=2 j=2 k=1
i=2 j=2 k=2
 program code
------------------------------------------
main()
// three nested loops
{
 int i, j, k;

 for (i=1; i<=2; i++)
   for (j=1; j<=2; j++)
     {
       for (k=1; k<=2; k++)
         printf("i=%d j=%d k=%d\n", i, j, k);
       for (k=1; k<=2; k++)
         printf("i=%d j=%d k=%d\n", i, j, k);
     }
}

 

     output
-------------
i=1 j=1 k=1
i=1 j=1 k=2
i=1 j=1 k=1
i=1 j=1 k=2
i=1 j=2 k=1
i=1 j=2 k=2
i=1 j=2 k=1
i=1 j=2 k=2
i=2 j=1 k=1
i=2 j=1 k=2
i=2 j=1 k=1
i=2 j=1 k=2
i=2 j=2 k=1
i=2 j=2 k=2 
i=2 j=2 k=1
i=2 j=2 k=2 

An example. Fibonacci.

Fibonacci numbers are numbers that follow the following rules:
F1 =1
F2 = 1
Fn = Fn-1 + Fn-2

The program below implements this. The user is asked to give the number of Fibonacci numbers to calculate (n) and then, in a for-loop, the next Fibonacci number is calculated, until there are enough numbers.

main()
/* program to calculate the first n Fibonacci numbers
   using the algorithm
   F(1) = 1, F(2) = 1 and F(i) = F(i-1) + F(i-2) */
{
  int i, n, fi, fi1, fi2;

  // first ask the user for the number of Fibonacci numbers to calculate
  printf("How many Fibonacci numbers do you want to see?\n");
  scanf("%d", &n);
  fi1 = 1;               // variables to store F(i-1) and F(i-2)
  fi2 = 1;               // initialize them to their starting value
  printf("1 1 ");        // print the first 2 Fibonacci numbers
  // we are going to do something in a countable way, so we will use
  // the for-loop structure:
  for (i=3; i<=n; i++)   // print the rest
    {
      fi = fi1 + fi2;    // calculate the next number
      fi2 = fi1;         // calculate the new F(i-1) and F(i-2)
      fi1 = fi;
      printf("%d ", fi); // print the result
    }
}
Later we will learn a much more elegant way to calculate Fibonacci numbers using recursive programming.

Quick test:

To test your knowledge of what you have learned in this lesson, click here for an on-line test.
Peter Stallinga. Universidade do Algarve, 30 October 2002