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Python Lab: Control Statements Conditionals, Loops, Iterations

Python Lab: Control Statements Conditionals, Loops, Iterations. Proteomics Informatics, Spring 2014 Week 4 18 th Feb, 2014 Himanshu.Grover@nyumc.org. Recap. Data types ( int , float, str etc.) and values (2, 4.6, “ATCG”) Variables (x = 10, y = “ATCG”)

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Python Lab: Control Statements Conditionals, Loops, Iterations

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  1. Python Lab: Control StatementsConditionals, Loops, Iterations Proteomics Informatics, Spring 2014 Week 4 18th Feb, 2014 Himanshu.Grover@nyumc.org

  2. Recap • Data types (int, float, str etc.) and values (2, 4.6, “ATCG”) • Variables (x = 10, y = “ATCG”) • Data structures (lists, dictionaries) • Operations • Simple python statements and expressions

  3. Today • Control statements: • Control the flow of the program/code • Conditionals and Loops • Allow building complex logic in programs

  4. Eclipse/Pydev setup ??

  5. ‘if/else’ Conditionals • Idea: • Test for condition & take appropriate action • conditional execution vs. branching • Basic structure (Conditional Execution) If <expression>: <statements’ block> (Alternative execution) if <expression1>: <statements1 block> else: <statements2block> • Ex. x = 3 if x% 4 == 0: ## (modulus operator) print “x is divisible by 4” if x < 5: print “low expression” else: print “high expression”

  6. Expressions must evaluate to True or False • Type-specific, built-in vs. user-defined: • “PEPTIDE”.isupper() • Logical or boolean operators (and, or, not) • help build more complex logic • x == y and x != z • x > 5 or x < -5 • x > 5 and x < 10 • not (x > 5 and x < 10) • Relational operators: • x == y (x equal y?) • x > y (x greater than y?) • x < y (x less than y?) • x >= y (x greater than or equal to y?) • x <= y (x less than or equal to y?) • x != y (x not equal to y?) • Membership (x in y) • “PEP” in “PEPTIDE”, “PET” not in “PEPTIDE” • 2 in [1,2,3,4,5]

  7. Other ‘if/else’ Structures • Multiple tests(chaining) if <expression1>: <statements1 block> elif<expression2>: <statements2 block> else: <statements3block> Ex. x = 3 if x < 3: print “low” elifx < 7: print “medium” else: print “high” • Nested conditionals if <expression1>: if <expression2>: <statements2 block> else: <statements3 block> Ex. disease = True x = 3 if disease == True: if x > 0 and x <= 4: print “give drug A” elif x > 7 and x <= 10: print “give drug B” else: print “no drug”

  8. ‘for’ Loops • Idea: repeat action(s) for each item in a sequence or any other iterable object (Ex.str, list, dict, set etc.) • Basic structure for <item> in <sequence>: <statements’ block>

  9. ‘for’ loop: Access Pattern 1 • Sequence scans • Ex.numList = [1,2,3,4] sum = 0 prod = 1 fornum in numList: sum = sum + num prod = prod*num print “sum: %d\tprod: %d”%(sum, prod)

  10. ‘for’ loop: Access Pattern 2 • Counter/index: range function • Greater flexibility for more specialized operations • Ex.numList = [1,2,3,4] sum = 0 prod = 1 forpos in range(len(numList)): sum = sum + numList[pos] prod = product*numList[pos] print “sum: %d\tprod: %d”%(sum, prod)

  11. HW Assignment • Given a peptide sequence and charge state, compute its m/z

  12. Next Class • More examples of loops • Write simple functions

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