function StrToFloatDef(const Value: String;
DefaultValue: Double=0): Double
The StrToFloatDef function converts the formatted string input parameter into its native value. The decimal separator used in the formatted string is determined by the TFormatSettingsDecimalSeparator property. If the input parameter is not a valid floating-point string, the return value is the specified default value.
Examples
A := StrToFloatDef('1200.548', 0);
X := FloatToStr(A); // X is '1200.548'