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Replace operands

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subsop(object, i1 = new1, i2 = new2, …, <Unsimplified>)


subsop returns a modified copy of the object, but does not change the object itself.

subsop(object, i = new) replaces the operand op(object, i) by new. Operands are specified in the same way as with the function op: i may be an integer or a list of integers. E.g., subsop(object, [j, k] = new) replaces the suboperand op(op(object, j), k). Cf. Example 2. In contrast to op, ranges cannot be used in subsop to specify more than one operand to replace. Several substitution equations have to be specified instead.

If several operands are to be replaced, the specified substitutions are processed in sequence from left to right. Each substitution is carried out and the result is processed further with the next substitution. The intermediate objects are not simplified.

The result of subsop is not evaluated further. It can be evaluated via the function eval. Cf. Example 3.

Operands of expression sequences can be replaced by subsop. Such objects are not flattened.

Note that the order of the operands may change by replacing operands and evaluating the result. Cf. Example 4.

FAIL is returned if an operand cannot be accessed.

Substitution via subsop is faster than via subs or subsex.

The call subsop(object) is allowed; it returns the object without modifications.


Example 1

We demonstrate how to replace one or more operands of an expression:

x := a + b: subsop(x, 2 = c)

subsop(x, 1 = 2, 2 = c)

Also the 0-th operand of an expression (the "operator") can be replaced:

subsop(x, 0 = _mult)

The variable x itself was not affected by the substitutions:


delete x:

Example 2

The following call specifies the suboperand c by a list of integers:

subsop([a, b, f(c)], [3, 1] = x)

Example 3

This example demonstrates the effect of simplification. The following substitution replaces the first operand a by 2. The result simplifies to 3:

subsop(a + 1, 1 = 2)

The option Unsimplified suppresses the simplification:

subsop(a + 1, 1 = 2, Unsimplified)

The next call demonstrates the difference between simplification and evaluation. After substitution of PI for x, the identifier sin is not evaluated, i.e., the body of the system function sin is not executed:

subsop(sin(x),  1 = PI)

Evaluation of sin simplifies the result:


Example 4

The order of operands may change by substitutions. Substituting z for the identifier b changes the internal order of the terms in x:

x := a + b + c: op(x)

x := subsop(x, 2 = z): op(x)

delete x:



Any MuPAD® object

i1, i2, …

Integers or lists of integers

new1, new2, …

Arbitrary MuPAD objects



As the last step of a substitution, the modified object is simplified (however, not evaluated). This option suppresses this final simplification. Cf. Example 3.

Return Values

Input object with replaced operands or FAIL.

Overloaded By



For overloading subsop, it is sufficient to handle the cases subsop(object) and subsop(object, i = new).

The case where the position of the operand to be replaced is given by a list is always handled recursively: First, op is called with the list bar the last element to find the object to substitute in (using the overloading of op if present, storing all the intermediate results), then the substitution is performed on that sub-object (using the overloading of subsop of the form subsop(subobj, i = new)). The result is substituted into the last-but-one result of the recursive op call, again respecting any overloading of subsop, and so on up to the front of the list.

See Also

MuPAD Functions

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