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Let us define a matrix funtion: f[x_]:={{x,1},{x^2},1}} then we wants to modify its elements as following

Part[f[x],1,2]=x^3
Part[f[x],2,1]=x

Can we do this? Actually I had written a program like this and the error shows: "f[x] in the part assignment is not a symbol."

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    $\begingroup$ Do you mean you want to change the DownValues of the function f indirectly? Why? what is the point of this? This produces convoluted code which will not be understood 2 days after it is written. But if you just want to modify matrix entry, then all what you have to do is r=f[x]; r[[2,1]]=x that is all. $\endgroup$ – Nasser Dec 24 '17 at 7:40
  • $\begingroup$ Thanks for your reply. I need to do this when I need to redefine some element of a matrix where all other elements remain unchanged.. $\endgroup$ – atanu Dec 28 '17 at 5:55
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You can use ReplacePart on DownValues of f to make changes to the definition of f:

DownValues[f] = ReplacePart[DownValues[f], {{1, 2, 2, 1} -> x, {1, 2, 1, 2} -> x^3}];    
f[t]

{{t, t^3}, {t}, {2, 1}}

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  • $\begingroup$ Thanks a lot for your response.. $\endgroup$ – atanu Dec 28 '17 at 5:50
  • $\begingroup$ @atanu, my pleasure. Welcome to mma.se. $\endgroup$ – kglr Dec 28 '17 at 6:25

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