Evaluate string representing symbolic expression (2024)

str2sym('sin(pi)')

ans =0

str2sym assumes the = operator represents an equation, not an assignment. Also, str2sym does not add the variables contained in the string to the workspace.

Show this behavior by evaluating 'x^2 = 4'. The str2sym function returns the equation x^2 == 4 but x does not appear in the workspace.

eqn = str2sym('x^2 = 4')

eqn =x^2 == 4

Find the variable in eqn by using symvar. The variable var now refers to x.

var = symvar(eqn)

var =x

Assign values from eqn by solving eqn for var and assigning the result.

varVal = solve(eqn,var)

varVal = -2 2

Assume the file mySym.txt contains this text.

a = 2.431y = a*exp(t)diff(z(t),t) = b*y*z

Evaluate expressions in mySym.txt using str2sym.

filename = 'mySym.txt';filetext = fileread(filename);filetext = splitlines(filetext);str2sym(filetext)

ans = a == 2.431 y == a*exp(t) diff(z(t), t) == b*y*z

The output of str2sym is independent of workspace values, which means the output is reproducible. Show this reproducibility by assigning a value to b and re-evaluating the stored expressions.

b = 5;str2sym(filetext)

ans = a == 2.431 y == a*exp(t) diff(z(t), t) == b*y*z

To use workspace values or a value from input equations, use subs (solve the equation first using solve), as described in Evaluate String as Symbolic Expression and Substitute Workspace Values into String Input.

Show this behavior by reading a differential equation and initial condition from a file. Solve the equation for the condition. Because str2sym does not evaluate y(t) in the equation, the output is reproducible.

filename = 'mySym.txt';filetext = fileread(filename);filetext = splitlines(filetext);eqn = str2sym(filetext(1))

eqn =diff(y(t), t) == -y(t)

cond = str2sym(filetext(2))

cond =y(0) == 2

ySol = dsolve(eqn,cond)

ySol =2*exp(-t)

Show the error between entering a ratio of large integers directly versus the exact string representation.

num = sym(12230984290/38490293482)

num =5724399718238385/18014398509481984

numExact = sym('12230984290/38490293482')

numExact =6115492145/19245146741

error = num - numExact

error =-7827162395/346689742765832461975814144

Show the error between entering a high-precision number directly versus the exact string representation.

num = vpa(8.023098429038490293482)

num =8.0230984290384910195825796108693

numExact = vpa('8.023098429038490293482')

numExact =8.023098429038490293482

error = num - numExact

error =0.00000000000000072610057961086928844451883343504

For details, see Numeric to Symbolic Conversion. For full workflows, see Find Almost Integers with High-Precision Arithmetic and Prime Factorizations.

Create a character vector that represents a hexadecimal value. Convert the value to symbolic decimal number.

H = '0x2A'D = str2sym(H)

D =42

Create a character vector that represents a binary value. Convert the value to symbolic decimal number.

B = '0b101010'D = str2sym(B)

D =42

symstr — String representing symbolic expression
character vector | string | cell array of character vectors

String representing a symbolic expression, specified as a character vector, string, or cell array of character vectors.