With function replace rule does not work in

Question

If I define an expression , and evaluate it in With function, replace rule does not work.

sumT = TA + TE;
With[
 {TA = 0.1, TE = 0.2},
 {TA + TE, sumT}
 ]

output

{0.3, TA + TE}

Answer 1

try this:

sumT = TA + TE;
With[
 {TA = 0.1, TE = 0.2},
  Evaluate@{TA + TE, sumT}
 ]

Answer 2

Replace With with Block:

sumT = TA + TE;

Block[{TA = 0.1, TE = 0.2}, {TA + TE, sumT}]

{0.3, 0.3}

Block[{TA = 0.1, TE = 0.5}, {TA + TE, sumT}]

{0.6, 0.6}

From its Documentation details:

"Block allows you to set up an environment in which the values of variables can temporarily be changed."

Answer 3

In answer to

Could you give some explanation ?

Just follow the evaluation process

1. With[ {TA = 0.1, TE = 0.2}, {TA + TE, sumT} ]
2. {0.1 + 0.2, sumT}
3. {0.3,sumT}
4. {0.3, TA + TE}

The way With works is that it holds everything unevaluated, performs the substitutions "formally", and then "goes away" and lets the resulting expression continue evaluating. You can think of the TA and TE inside of the With as local variables if you want. Once the With has been handled (step 2), we're now back at the main scope, and sumT will be replaced according to its OwnValues.