Reading and writing data in files

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Harvey Caldwell
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1 Reading and writing data in files It is ften very useful t stre data in a file n disk fr later reference. But hw des ne put it there, and hw des ne read it back? Each prgramming language has its wn peculiar cnventin. Scilab prvides several different functins that can be used t write and read data, each f which is designed fr a particular situatin. In this lessn, I will describe ne methd: nt the mst efficient ne, certainly, but ne which will lk very similar in several ther prgramming languages. If yu learn t use this style, yu can very easily adapt it t ther envirnments. In the examples belw, the data file is a simple ne: it cntains several clumns f ASCII characters, with the same number f clumns n each line, like this: This multi-clumn frmat is very cmmn -- even spreadsheets can read and write it. Reading multiclumn data Let's break the task dwn int several steps. 1. Open the file fr reading 2. Read the next line If there are n mre lines, quit 3. Clse the file Step 1 typically ccurs near the start f a prgram, step 2 inside a lp, and step 3 at the very end. Let's lk at each ne in sme detail. Step 1: Open the file Befre we can read anything frm a file, we need t pen it via the mpen functin. We tell Scilab the name f the file, and it ges ff t find it n the disk. If it can't find the file, it returns with an errr; even if the file des exist, we might nt be allwed t read frm it. S, we need t check the value returned by mpen t make sure that all went well. A typical call lks like this: fid = mpen(filename, "r");

2 if (fid == -1) errr("cannt pen file fr reading"); end There are tw input arguments t mpen: the first is a string with the name f the file t pen, and the secnd is a shrt string which indicates the peratins we wish t undertake. The string "r" means "we are ging t read data which already exists in the file." We assign the result f mpen t the variable fid. This will be an integer, called the "file descriptr," which we can use later n t tell Scilab where t lk fr input. Step 2: Read the next line One usually enters a lp in which ne reads data frm the file repeatedly. We can call the mfscan functin t read a specific number f items frm the next line (4 items, in the example belw): while (dne_yet == 0) [num_read, val(1), val(2), val(3), val(4)] = mfscanf(fid, "%f %f %f %f"); if (num_read <= 0) dne_yet = 1; end We need t pass t the mfscan functin the file descriptr f the file frm which we wish t read, and then a template fr the number and type f items that we wish t read. In the example abve, I am expecting t see fur flatingpint values at the start f the next line in the file. There culd be additinal items after these fur -- they will be ignred. The functin returns tw types f infrmatin. First, hw many f the requested items was it able t read prperly? -1, if there are n mre lines t read, r 0, if the line didn't have any such items, r sme number between 1 and 4 (in this example), fr the number f flating pint values which it did read prperly

3 If there are n mre lines t read, ne usually needs t break ut f a lp, r set a flag t d s. Next, the mfscan functin places the individual items f the line int its secnd (and third, and furth, etc.) utput arguments. Given this line: the abve call t mfscanf wuld result in num_read = 4 val(1) = 1 val(2) = 2 val(3) = 3.5 val(4) = 4.9 But given this line: xyz the abve call t sscanf wuld result in num_read = 3 val(1) = 1 val(2) = 2 val(3) = 3.5 val(4) = is undefined Yu shuld always check t make sure that mfscanf was able t read the number f items expected. Step 3: Clse the file At the very end f the prgram, after all the data has been read, it is gd frm t clse a file: fclse(fid); If, in the middle f a prgram, yu need t start reading data frm the beginning f a file yu've already pened and used, yu can clse the file and then pen it again.

4 Writing multiclumn data Let's break the task dwn int several steps. 1. Open the file fr writing 2. Write the next line 3. Clse the file Step 1 typically ccurs near the start f a prgram, step 2 inside a lp, and step 3 at the very end. Let's lk at each ne in sme detail. Step 1: Open the file Befre we can write anything int a file, we need t pen it via the mpen functin. We tell Scilab the name f the file, and give the secnd argument 'w', which stands fr 'we are abut t write data int this file'. fid = mpen(filename, "w"); if (fid == -1) errr('cannt pen file fr writing'); end When we pen a file fr reading, it's an errr if the file desn't exist. But when we pen a file fr writing, it's nt an errr: the file will be created if it desn't exist. If the file des exist, all its cntents will be destryed, and replaced with the material we place int it via subsequent calls t mfprintf. Be sure that yu really d want t destry an existing file befre yu call mpen! Step 2: Write the next line Placing data int the file is easy: call the mfprint functin, just as yu wuld call mprintf t print values t the screen; but include as the first argument the file descriptr mfprintf(fid, "%f %f %f \n", time, height, velcity); Dn't frget t place a newline character '\n' t mark the end f each line. If yu dn't supply newline characters, all the data will be placed n a single, very lng line. Step 3: Clse the file At the very end f the prgram, after all the data has been written, it is gd frm t clse a file: mclse(fid);

5 Examples Yu can find sme examples f shrt, simple Scilab prgrams which read and write infrmatin frm and t files in the examples sectin f the curse WWW site. Specifically, read_wrds.sci write_file.sci plt_frm_file.sci Practice in class Write a prgram which prints lines t a file called "pwers.txt". The file shuld have 10 lines; in the first clumn is the line number (1, 2, 3,..., 10), in the secnd clumn the square f the line number (1, 4, 9,..., 100), and in the third clumn the cube f the line number (1, 8, 27,..., 1000). S the first tw lines wuld lk like this: When dne, pen yur data file in a brwser windw and call me ver s that I can see it and mark it ff. Write a prgram which reads numbers frm the file "pwers.txt" and calculates, fr each line, the sum f the numbers. S, the first line wuld add up t 3. Yur prgram shuld print ne line per sum, smething like this: = = 14 Surce:

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