how to substitute symbolic variables

hello ,

i want to substitute sin(theta) with theta and cos(theta) with 1 ,

the expression is like

dte1= (Ks*(474.0*te1 - 374.0*te2))/100 - 1.0*L*p*sin(te1) - 1.3*L*p*sin(1.3*te1 - 1.2*te2) - 0.3*L*p*sin(0.3*te1)

I tried "subs" function but it is replacing only the specified variable , i want to replace it like this :

for example : in the above equation i want to replace

sin(1.3*te1 - 1.2*te2) → (1.3*te1 - 1.2*te2)

sin(0.3*te1) → (0.3*te1)

summary : replace sin(theta) with theta , theta can be any symbolic variable

Answer 1

[Ameer Hamza]

First convert to string. Remove sin and convert back to symbolic expression

syms K2 te1 te2 L p Ks

dte1 = (Ks*(474.0*te1 - 374.0*te2))/100 - 1.0*L*p*sin(te1) - 1.3*L*p*sin(1.3*te1 - 1.2*te2) - 0.3*L*p*sin(0.3*te1);

det1_str = string(dte1);
det1_str = strrep(det1_str, 'sin', '');

dte1 = str2sym(det1_str);

[Maaz Nayeem]

thanks for the reply,

but my problem is , i want to substitute sin(theta) with theta ...... theta can be anything not particuarly (1.3*te1 - 1.2*te2) or (0.3*te1) , it can be like (0.2*te2) also

[Ameer Hamza]

Try the updated code.

[Maaz Nayeem]

thank you soo much !!!

similarly can you help me with cos(theta)=1 (entire cos(theta) substitute with 1)

[Ameer Hamza]

That is a bit tricky. Can you give me an example expression?

[Maaz Nayeem]

dte11 = 4.74*Ks - 1.69*L*p*cos(1.3*te1 - 1.2*te2) - 0.09*L*p*cos(0.3*te1) - 1.0*L*p*cos(te1)

[Ameer Hamza]

If the cos() function does not contain any nested brackets then the following code will replace

syms K2 te1 te2 L p Ks
dte1 = (Ks*(474.0*te1 - 374.0*te2))/100 - 1.0*L*p*cos(te1) - 1.3*L*p*cos(1.3*te1 - 1.2*te2) - 0.3*L*p*cos(0.3*te1);
dte1 = vpa(dte1);

dte1_str = string(dte1);
dte1_str = regexprep(dte1_str, 'cos\([a-zA-Z0-9\s\*\-\./]*\)', '1');
eval(['dte1=' char(dte1_str)])

However, this will fail for nested brackets, e.g., cos(2*sin(x+2)^2), there are brackets inside brackets.

[Ameer Hamza]

Check the code for cos() -> 1 function

[Maaz Nayeem]

i got it !

TYSM !!

Answer 2

[Walter Roberson]

mapSymType(mapSymType(dte11, 'sin', @(x) children(x)), 'cos', 1)

There might be a way to do both in one call