Lecture 8: Branching I (if ... then ... else)

Until now, all the instructions that were put within the program were executed. Moreover, they were executed exactly in the order that they were placed. The first line of the program was executed first, then the second, then the third, and so on. This is not always the case. With branching (a branch is a part of a tree) we can control the flow of the program.

Imagine a program that specifies a number and the computer will calculate the square-root of the number. Taking the square-root of a negative number doesn't make sense (unless you are working with complex numbers, off course), so you would like your program to generate an error and stop when the user enters a negative number. You want text like

Negative numbers are not allowed!

to appear on the screen. Obviously, you do not always want this text to appear on the screen; in case the user enters a positive number you just want the square-root to be calculated and appear on the screen:

The square-root of 5 is 2.23607

You would like to have some way to check the number and depending on this result, execute parts of the program.

if ... then ...

The simplest way to have a control over the instructions that will be executed is with the statement if .. then. The full syntax of the statement is

if condition then instruction;

For condition we will substitute our condition and for instruction we will put our instruction(s) that will be executed if and only if the condition was true.
The condition is an expression that results in a value of type boolean (see lecture 4). This can be a variable, for instance: were b declared as boolean, the following is correct:
if b then instruction;
On the other hand, more common are conditions with expressions that compare variables, such as
if (x = y) then instruction;
if (x < y) then instruction;

comparison	meaning
`(a = b)`	`a` equal to `b`
`(a <> b)`	`a` not equal to `b`
`(a < b)`	`a` smaller than `b`
`(a > b)`	`a` larger than `b`
`(a <= b)`	`a` smaller or equal to `b`
`(a >= b)`	`a` larger or equal to `b`

Remember that if we want more than one instruction to be executed, we can group them with a begin ... end combination, so that for the if ... then statement they appear as one.
if (a = b) then
    begin
      instruction1;
      instruction2;
    end;
In this case, both instruction1 and instruction2 will be executed when a is equal to b.

The normal execution of the program will resume after the block of instructions. In the following example, instruction3 and instruction4 will be executed, regardless of the condition (a = b).

if (a = b) then
    begin
      instruction1;
      instruction2;
    end;
instruction3;
instruction4; to be executed:

(a=b) (a<>b)

instruction1
instruction2
instruction3
instruction4 instruction3
instruction4

Note that here the analogy with branching in trees stops. In a tree, the branches never meet again; once we are on a branch, it is never again possible to join the main trunk.

if ... then ... else ...

If we also want to program to do things in case the condition is not true we can do this with if ... then ... else statement. The general form of this instruction is

if condition then
instructionA
else
instructionB;

Note the peculiarity of PASCAL: the instruction immediately before else is not terminated with ;

example:

if (a=b) then
begin
    instruction1;
    instruction2;
end
else
begin
    instruction3;
    instruction4;
end;
instruction5;
instruction6; to be executed:

(a=b) (a<>b)

instruction1
instruction2
instruction5
instruction6 instruction3
instruction4
instruction5
instruction6

Here a complete program that shows the use of branching to calculate the square-root of a number:

PROGRAM SquareRoot;

Var x: real;
root: real;

begin
writeln('Give a number');
readln(x);
if (x<0) then
    writeln('Negative numbers are not allowed!')
else
    begin
      root := Sqrt(x);
      writeln('The square-root of ', x:0:4, ' is ', root:0:4);
    end;
writeln('Have a nice day');
end.

Running the program; two examples:

Give a number
3.68
The square-root of 3.6800 is 1.9183
Have a nice day Give a number
-3.68
Negative numbers are not allowed!
Have a nice day

Lecture 8: ... of roots and branches

Quick test:

To test your knowledge of what you have learned in this lesson, click here for an on-line test. Note that this is NOT the form the final test takes!

Peter Stallinga. Universidade do Algarve, 4 março 2002