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{SECT 0 {SECT 0 {PARA 0 "" 0 "" {TEXT 26 6 "Error:" }{TEXT -1 24 " can
not evaluate boolean" }}{PARA 0 "" 0 "" {TEXT -1 0 "" }}{PARA 0 "" 0 "
" {TEXT -1 298 "Maple can't determine whether something is true or fal
se in an \"if\" statement or \"while\" clause.  This usually arises wh
en a procedure that is meant to work for numeric arguments is called w
ith symbolic arguments instead.  It is often a case of \"premature eva
luation\".  Typical examples of this are" }}{EXCHG {PARA 0 "> " 0 "" 
{MPLTEXT 1 0 48 "f:= proc(x) if x > 1/2 then 3*x else 2*x fi end:" }}}
{EXCHG {PARA 0 "> " 0 "" {MPLTEXT 1 0 23 "plot(f(x), x = 0 .. 1);" }}
{PARA 8 "" 1 "" {TEXT -1 37 "Error, (in f) cannot evaluate boolean" }}
}{EXCHG {PARA 0 "> " 0 "" {MPLTEXT 1 0 29 "evalf(Int(f(x), x = 0 .. 1)
);" }}{PARA 8 "" 1 "" {TEXT -1 37 "Error, (in f) cannot evaluate boole
an" }}}{EXCHG {PARA 0 "> " 0 "" {MPLTEXT 1 0 20 "fsolve(f(x) = 0, x);
" }}{PARA 8 "" 1 "" {TEXT -1 37 "Error, (in f) cannot evaluate boolean
" }}}{PARA 0 "" 0 "" {TEXT -1 0 "" }}{PARA 0 "" 0 "" {TEXT -1 70 "The \+
trouble is that Maple evaluates the inputs to a procedure such as " }
{MPLTEXT 0 21 4 "plot" }{TEXT -1 45 " before it calls the procedure, r
esulting in " }{MPLTEXT 0 21 1 "f" }{TEXT -1 41 " being called with th
e symbolic argument " }{MPLTEXT 0 21 1 "x" }{TEXT -1 39 ".  The proble
m can be fixed as follows:" }}{PARA 0 "" 0 "" {TEXT -1 0 "" }}{EXCHG 
{PARA 0 "> " 0 "" {MPLTEXT 1 0 25 "plot('f(x)', x = 0 .. 1);" }}}
{EXCHG {PARA 0 "> " 0 "" {MPLTEXT 1 0 22 "evalf(Int(f, 0 .. 1));" }}
{PARA 11 "" 1 "" {XPPMATH 20 "6#$\"++++v8!\"*" }}}{EXCHG {PARA 0 "> " 
0 "" {MPLTEXT 1 0 22 "fsolve('f(x)' = 0, x);" }}{PARA 11 "" 1 "" 
{XPPMATH 20 "6#\"\"!" }}}{PARA 0 "" 0 "" {TEXT -1 0 "" }}{PARA 0 "" 0 
"" {TEXT -1 28 "or modify the definition of " }{MPLTEXT 0 21 1 "f" }
{TEXT -1 65 " so that it accepts symbolic inputs (returning unevaluate
d), e.g." }}{EXCHG {PARA 0 "> " 0 "" {MPLTEXT 1 0 132 "f:= proc(x) \n \+
       if not type(x, numeric) then 'f(x)'\n        elif x > 0 then 3*
x \n        else 2*x        \n        fi\n      end:" }}}{EXCHG {PARA 
0 "> " 0 "" {MPLTEXT 1 0 5 "f(x);" }}{PARA 11 "" 1 "" {XPPMATH 20 "6#-
%\"fG6#%\"xG" }}}{PARA 0 "" 0 "" {TEXT -1 326 "Another way this error \+
can occur is in the numerical solution of a differential equation when
 the equation contains something symbolic (e.g. a parameter that has n
ot been given a numerical value, or an undefined function).  In some c
ases this arises by a misspelling.  In the example below, the user pro
bably intended to type " }{MPLTEXT 0 21 3 "sin" }{TEXT -1 13 " rather \+
than " }{MPLTEXT 0 21 3 "sim" }{TEXT -1 1 "." }}{EXCHG {PARA 0 "> " 0 
"" {MPLTEXT 1 0 53 "f:= dsolve(\{D(y)(x)=sim(y(x)), y(0)=1\},y(x),nume
ric);" }}{PARA 11 "" 1 "" {XPPMATH 20 "6#>%\"fG:6#%(rkf45_xG6'%\"iG%(r
kf45_sG%)outpointG%#r1G%#r2G6#%aoCopyright~(c)~1993~by~the~University~
of~Waterloo.~All~rights~reserved.G6\"C&>8&-%&evalfG6#9$@$52-%$absG6#,$
F3!\"\"-F<6#,&&%,loc_controlG6#\"\"#\"\"\"F3F?4-%'memberG6$&FD6#\"\"'<
*F?FG!\"#FF$F?\"\"!$FPFR$FGFR$FFFRC%>FD-%%copyG6#=F06#;FG\"#EE\\[l;\"#
=FR\"\"$FR\"#>FR\"\"%$FG!\")\"#?FT\"\"&F^o\"#@FRFNFG\"#AFR\"\"($FG!\"*
\"#BFR\"\")\"&++$\"#CFR\"\"*\"%+5\"#DFR\"#5FRFhnFR\"#6FR\"#7FR\"#8FR\"
#9FR\"#:FR\"#;FRFGFG\"#<FRFFFR>%'loc_y0G-FY6#=F06#;FGFGE\\[l\"FGFT>%'l
oc_y1G-FY6#=F0F[qE\\[l!@$0F;FRC$>&FD6#F[oF3@%1%'DigitsG-%'evalhfG6#F\\
rC$>8%-%*traperrorG6#-F^r6#-%=dsolve/numeric_solnall_rkf45G6,%&loc_FG-
%$varG6#FD-F]s6#Fgp-F]s6#F_q-F]s6#%'loc_F1G-F]s6#%'loc_F2G-F]s6#%'loc_
F3G-F]s6#%'loc_F4G-F]s6#%'loc_F5G-F]s6#%)loc_workG@$/Fbr%*lasterrorGC%
>8'-%+searchtextG6$.F^r-%(convertG6$-%#opG6$FG7#Fbr%%nameG>8(-F\\u6$.%
)hardwareGF_u@%50FjtFR0FhuFR-Fir6,F[sFDFgpF_qFesFhsF[tF^tFatFdt-%&ERRO
RG6#FbrFav7$/%\"xGF7-%$seqG6$/&%$ordG6#,&8$FGFGFG&Fgp6#Faw/FawF\\qF06%
FDFgpF_q" }}}{EXCHG {PARA 0 "> " 0 "" {MPLTEXT 1 0 5 "f(3);" }}{PARA 
8 "" 1 "" {TEXT -1 56 "Error, (in dsolve/numeric/rkf45) cannot evaluat
e boolean" }}}}{SECT 0 {PARA 0 "" 0 "" {TEXT 26 10 "See also: " }
{HYPERLNK 17 "plot" 2 "plot" "" }{TEXT -1 2 ", " }{HYPERLNK 17 "Numeri
cal integration" 2 "evalf/int" "" }{TEXT -1 2 ", " }{HYPERLNK 17 "fsol
ve" 2 "fsolve" "" }{TEXT -1 2 ", " }{HYPERLNK 17 "dsolve/numeric" 2 "d
solve/numeric" "" }}}{SECT 0 {PARA 0 "" 0 "" {TEXT 26 22 "Maple Adviso
r Database" }{TEXT 256 18 "   R. Israel, 1997" }}}}{MARK "0 17 0" 321 
}{VIEWOPTS 1 1 0 1 1 1803 }