String Manipulation

String Manipulation Chapter 7

String Processing Even in quantitative sciences, we often encounter letters and/or words that must be processed by code You may want to write code that: • Reads data files that contain both numbers and words • Writes data to files (filenames are strings) • Read a list of files • Combine a string and a number • Label plot titles or axes with nums and words • Prints messages with words and numbers mixed

Common Earth Science Data Sets Advanced National Seismic System (ANSS) • http://www.ncedc.org/anss/ • Near real-time data for all global earthquake events! • Not just numbers USGS Current Water Data • http://waterdata.usgs.gov/nwis/rt • Near real-time data for streams in all 50 states! • Not just numbers

Strings: A Quick Review • Recall that strings are arrays of characters • i.e. a matrix of characters

Strings: A Quick Review • Strings can form rectangular matrices too! • Must follow same matrix rules • Rows and Cols must be consistent

Strings: A Quick Review • Strings can form rectangular matrices too! • Must follow same matrix rules • Rows and Cols must be consistent • Can “pad” each row with blank spaces to make columns consistent • You can read about “strcat”, “srtvcat”, and “char”

Formatted Strings: sprintf What if you want to combine a numeric result with a word? • fprintf: only prints to the command window (can’t store/use the result) • sprintf: works just like fprintf, but makes a string

Custom Plot Labels Using sprintf What if you want to label a plot with numbers from data? • Most MATLAB commands for labeling only accept strings • Use sprintf to make a string with text+number. Then use the string

Other String Manipulation Functions • Attaway also covers • deblank • char • strtrim • upper • lower • These may come in handy someday • You can read about these functions for their usage

Comparing Strings What if you want to see if two strings are identical? • Do not use “==“ • Compares ASCII values • May work, but is bad programming style, and can produce unwanted results • Use “strcmp” or “strncmp” • Compares strings

Comparing Strings What if you want to see if two strings are identical? strcmp • compares two strings • returns logical true if identical strncmp • compares the first n characters of two strings • Returns logical true if identical • You can read about “strfind”, “findstr”, “strrep”, and “strtok”.

“is*” Functions Now that you know about various variable types in MATLAB… • May want to test whether a variable contains a certain type of data • Various “is” functions do this • ischar • isletter • isspace http://www.mathworks.com/help/matlab/ref/is.html

Converting Numbers to Strings int2str • Converts an int to a string • Automatically rounds off if needed num2str • Converts an number (double or int) to a string • Can specify precision • If precision not specified, uses current MATLAB defaults

Converting Strings to Numbers str2num • Converts a string to a number (double)

Practical Example:Reading Data Files What if you have a bunch of files that you want to load? • Don’t load each one, one by one by hand • Waste of time! • Knowledge of string processing can help! • Use “ls” as a function • Store result as character array

Plotting Data From Multiple Files

Plotting Data From Multiple Files Fair Warning: • Data files are read in the order that ls reports. • If not in order, you will have to sort the data.

Final Thoughts • Even though the focus of most scientific tasks is to crunch numbers… • Knowing how to deal with strings is beneficial What’s Next? • What if our data files have a mix of numbers and letters/words? • Need a new class of variable: • cell arrays and structures!

String Manipulation