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# Assigning an equation and then plotting it does not work?

Posted 10 years ago
 First I assign an equation to cir1: In[21]:= cir1 = x^2 + y^2 == 1 Out[21]= x^2 + y^2 == 1  then if I plot the equation it works fine ContourPlot[x^2 + y^2 == 1, {x, -1, 1}, {y, -1, 1}]  But if I try the same thing with the variable cir1, I get a blank graph ContourPlot[cir1, {x, -1, 1}, {y, -1, 1}]  Am I missing something simple to make that work? Newbie here... Thanks!
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Posted 10 years ago
 Try with ContourPlot[Evaluate[cir1], {x, -1, 1}, {y, -1, 1}] ContourPlot (and Plot and most plotting functions) protects its argument from outside assignements of variables. Without this protection the following would not work: x=1; Plot[ x^2, {x,0,1}] because it would give Plot[1^2, {x,0,1}]. This (mostly) benign protection can be bypassed with Evaluate, when necessary and with care.
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