# Eval()vs.substitute()

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The aim of this page is to demonstrate the difference between `eval()`

and `substitute`

.

Note:PGLabs is an efficient way to check code.

$f = Compute(sqrt(3x + 1); $fx = $f->eval(x=>"3"); displays 3.16228

The correct answer is a number.

$f = Compute(sqrt(3x + 1); $fx = $f->substitute(x=>"3"); displays (3.16228)

The correct answer is a formula (hence the parenthesis)

One of the biggest differences between `eval()`

and `substitute`

is when they are used in conjunction with flags such as `reduceConstants`

and `reduceConstantFunctions`

, which is described in more detail at:FormattingCorrectAnswers:_NumbersAndFormulas