Control Flows

Control flow defines the order and the set of instructions that will be executed in the program. In pseudocode, control flow related statements include if and loop, as well as complementary statements such as break and continue.

To illustrate, here is a simple program with control flow statements:

loop I from 0 to 100
    if I % 5 == 0 then
        output I
    end if
end loop

if Statement

The if statement determines whether a piece of code should be executed based on a runtime boolean value. Its syntax is as follows (placeholder values are surronded by <>):

if <conditional_value> then
    <statements>
end if

If <conditional_value> is true, then <statements> will be executed; if <conditional_value> is false, then <statements> will be skipped.

Note

All conditional values in pseudocode must be a boolean. If any other type is encountered, an error will be thrown.

An else component can be added to an if statement. The contained statements will be executed if the conditional value is false:

if <conditional_value> then
    <if_statements>
else
    <else_statements>
end if

Note that if can be nested inside else. This is just a syntax sugar that makes mimicking a switch statement easier. For example, the below code:

if X > 10 then
    output "X is greater than 10"
else if X < 10 then
    output "X is less than 10"
else
    output "X is equal to 10"
end if

is equivalent to:

if X > 10 then
    output "value is greater than 10"
else
    if X < 10 then
        output "value is less than 10"
    else
        output "value is equal to 10"
    end if
end if

The last else statement is optional in the switch-like statement.

while Loop

The while loop repeatedly executes the containing statements according to the given conditional value.

In a loop while statement, the containing statements is repeatedly executed as long as the given conditional value is true:

X = 0
loop while X < 20
    output X, " is less than 20"
    X = X + 1
end loop

In a loop until statement, the containing statements is repeatedly executed until the given conditional value is true (i.e. repeats as long as the conditional value is false):

X = 0
loop until X > 100
    output X, " is not greater than 100"
    X = X + 1
end loop

Note

The loop until statement is just a syntax sugar for the loop while statement, but with an extra not operator that surrounds the whole conditional value.

for Loop

The for loop enumerates a variable (referred to as the “loop variable”) between two values (inclusive). At the end of each iteration, it increments the loop variable by 1. Note that changing the loop variable inside the loop will affect the enumeration of the for loop.

An example of the for loop that enumerates between an interval:

CENTER = 20
loop I from CENTER - 10 to CENTER + 7
    // outputs numbers from 10 to 27 (inclusive)
    output I
end loop

Note

The loop variable will still be defined after the loop has finished.

Note that there is no decremental for loop or for loop with custom increments at the end of each iteration as the IB Pseudocode standard did not mention such feature. This behavior can be trivially implemented with a while loop though.

break and continue

The break and continue statements are used for early-terminating a loop and jump to next iteration respectively.

The break statement exits the current (inner-most) loop immediately, abandoning the rest of the unexecuted statements in the current iteration as well as any subsequent iterations.

For instance, the following code locates the index of the first occurence of 5 in array ARR:

loop I from 0 to ARR.length() - 1
    if ARR[I] == 5 then
        output "5 found at index ", I
        break
    end if
end loop

The continue statement jumps to the end of the current iteration, abandoning all unexecuted code in the current iteration.

For instance, the following code prints out all even numbers from 0 to 100:

loop I from 0 to 100
    if I % 2 == 1 then
        continue
    end if

    output I
end loop

The break and continue works on both for and while loops.