Maple cannot determine whether something is true or false in an if statement or while clause.

 Example 1

A typical occurrence happens when a procedure that requires numeric arguments is called with symbolic arguments.  Here are three examples with the same underlying problem:

Maple evaluates the inputs to a procedure such as plot before it calls the procedure, resulting in $f$ being called with the symbolic argument $x$. Since $x$ has not yet been assigned a value, Maple cannot determine if $x$ is less than $0$. You can either delay the evaluation of the input using unevaluation quotes, or you can supply the arguments with operator form.

Solution 1

Here we use unevaluation quotes around $f$. For details, see Unevaluated Expressions.

Solution 2

Here, we supply the operator form to the plot function.  (Note it is $f$ not $f\left(x\right)$ in the first argument to plot.)

Solutions for the fsolve and evalf/Int problems

Example 2

In this example, we want to use a loop to find primes less than 15.  The error happens because i needs an initial value.

Solution 1

Before beginning the loop, assign i to a starting value.

Solution 2

Alternatively, use the from clause to set a starting value.

For more information about loops, see The Repetition Statement.