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CS 170 – Intro to Scientific and engineering Programming

CS 170 – Intro to Scientific and engineering Programming. Variables. Use names to assign result of an expression to a variable Variables do not need to be declared before assignment A single “equal” sign (=) is the assignment operator, LHS = RHS Read this as

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CS 170 – Intro to Scientific and engineering Programming

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  1. CS 170 – Intro to Scientific and engineering Programming

  2. Variables • Use names to assign result of an expression to a variable • Variables do not need to be declared before assignment • A single “equal” sign (=) is the assignment operator, • LHS = RHS • Read this as • evaluate expression on the right-hand side, and then… • assign the result to the variable named on the left-hand-side • Therefore • The right-hand-side needs to be a legal Matlab expression • The left-hand-side needs to be a single variable name (this will get more sophisticated later on) • A semicolon at the end of the RHS expression suppresses the display, but the assignment still takes place.

  3. Examples of Variables and Assignment • Legal • >> A = sqrt(13) • >> B = exp(2); • >> A = 2*B • >> A = A + 1 • >> C = tan(pi/4) • Illegal (all for different reasons) • >> D = sqrt(E) + 1; • >> 3 = E • >> 3*A = 14 • >> F = 2 3

  4. Storing values in variables result = 4+8*sqrt(3) message = “Welcome to CS 170!!” radius = 10 • What does a variable mean? • Why are they useful? • Have you ever used variables before?

  5. Assignment statements • General formatvariable_name = expression • Right hand side can be any expression that evaluates to a value: >> hypotenuse = sqrt(3^2 + 4^2) • Executing an assignment statement: 1. evaluate the expression on the right side 2. assign value of expression to variable named on the left side

  6. Hunting for ‘s >> x = 17 >> x + 1 = 10 >> x = x / x + x >> diff = (x - y) / 2 >> end = x - 18 >> y = x = 2

  7. Assigning variables in a script • Statements in script are executed as if they were typed directly into the Command Window • Variables created in CW can be changed by assignments in scripts and vice versa! • What would happen if the script contained a syntax error?

  8. Variable names • Must start with a letter • May contain any number of letters of digits or underscore characters • Case matters! • Reserved words in Matlab:break case catch continue else elseifend for function global if otherwise persistent return switch try while

  9. Chose meaningful names! • The good, the bad, & the ugly maxArea xxyyzz_3b totalMonthlyHedgeFunds 2pi tf result • Camel-case:myfirsprogrammyFirstProgram

  10. Exercise! • Write a sequence of assignment statements that exchange the values of thing1 & thing2 • Before • After 5 thing1 thing2 3 3 thing1 thing2 5

  11. Floating point numbers • Decimal numbers are represented as a type called double >> number = 8.43 number = 8.4300 • MATLAB normally prints 4 digits after the decimal point and rounds numbers up or down to fit

  12. Really big and really little numbers • MATLAB uses scientific notation to print very large or very small numbers >> number = 1230000000 number = 1.2300e+09 >> number = 0.000789 number = 7.8900e-04 • MATLAB has a special representation for really big numbers >> number = 1.2e+9999 number = Inf • * What do get when you type: 1/0 or 1/infor 0*inf?

  13. Strings • Strings consist of letters, digits, symbols, and white space, surrounded by single quotes: myName= ‘Xenia I am’ eec= ‘ )&it:s;elf,’ thirteen = ‘6 + 7’ • Common ‘s callMe= Ishmael reply = ‘Madam, I’m Adam’

  14. Questions??

  15. Resources • Lecture slides CS112, Ellen Hildreth, http://cs.wellesley.edu/~cs112/

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