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For loops in programming

For loops in programming. Assumes you have seen assignment statements and print statements. Why do we want a loop in a program?. Many programs need to process huge amounts of data Each piece of data needs to go through the same instructions

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For loops in programming

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  1. For loops in programming Assumes you have seen assignment statements and print statements

  2. Why do we want a loop in a program? • Many programs need to process huge amounts of data • Each piece of data needs to go through the same instructions • One option is to write out the instructions over and over again • That’s not very practical or flexible • What if I wrote a program to process invoices for 100 customers and I got some more customers? Rewrite the program?? No! • A loop lets me specify what needs to be repeated and how many times it needs to be repeated

  3. Two kinds of loops • Definite • You know that you want to execute the loop a certain number of times • “there are 10 students’ grades” • “Draw a standard chess board” (always 8 x 8 squares) • “Quiz the user about the capitals of the 50 states” • “do this for every character in the string” • “do this for every item in the list” • “do this for every line in the file” • Indefinite • You don’t know how many times you want to execute the loop but you know there is a condition that is the stopping point • “do this until the user wants to stop” • “let the user enter numbers until they type -1” • “do this until the user wins the game” • “repeat this until this number reaches a certain level”

  4. For loop – a definite loop • The syntax of the for loop statement for variable in some list of values to assign to the variable : (colon!) Follow this line by all statements that should be repeated with an extra indent - This is how the interpreter knows what is in the loop and what’s not • The semantics of the for loop statement • At the start of the loop check to see if the loop should execute • If not, skip the body and continue with next statement after loop • If it should execute, Assign variable with the first item in the list Execute the body • Repeat the test at the top of the loop to see if loop should execute again

  5. Controlling the loop • You don’t want a loop that doesn’t stop! • Python uses several ways to control a loop • The range function (gives integers) • The contents of a list of values (gives the elements whatever type) • The contents of a string (gives characters) • The contents of a file (gives strings)

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