gmtmath − Reverse Polish Notation calculator for data tables |
gmtmath [ −At_f(t).d ] [ −Ccols ] [ −Hnrec ] [ −I ] [ −Nn_col/t_col ] [ −Q ] [ −S[f|l] ] [ −Tt_min/t_max/t_inc|tfile ] [ −V ] [ −bi[s][n] ] [ −bo[s][n] ] operand [ operand ] OPERATOR [ operand ] OPERATOR ... = [ outfile ] |
gmtmath will perform operations like add, subtract, multiply, and divide on one or more table data files or constants using Reverse Polish Notation (RPN) syntax (e.g., Hewlett-Packard calculator-style). Arbitrarily complicated expressions may therefore be evaluated; the final result is written to an output file [or standard output]. When two data tables are on the stack, each element in file A is modified by the corresponding element in file B. However, some operators only require one operand (see below). If no data tables are used in the expression then options −T, −N must be set (and optionally −b). By default, all columns except the "time" column are operated on, but this can be changed (see −C). |
operand |
If operand can be opened as a file it will be read as an ASCII (or binary, see −bi) table data file. If not a file, it is interpreted as a numerical constant or a special symbol (see below). |
outfile is a table data file that will hold the final result. If not given then the output is sent to stdout. |
OPERATORS |
Choose among the following operators: ABS 1 abs (A). |
SYMBOLS |
The following symbols have special meaning: PI 3.1415926... |
−A |
Requires −N and will partially initialize a table with values from the given file containing t and f(t) only. The t is placed in column t_col while f(t) goes into column n_col - 1 (see −N). |
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−C |
Select the columns that will be operated on until next occurrence of −C. List columns separated by commas; ranges like 1,3-5,7 are allowed. −C (no arguments) resets the default action of using all columns except time column (see −N). −Ca selects all columns, including time column, while −Cr reverses (toggles) the current choices. |
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−H |
Input file(s) has Header record(s). Number of header records can be changed by editing your .gmtdefaults4 file. If used, GMT default is 1 header record. Use −Hi if only input data should have header records [Default will write out header records if the input data have them]. |
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−I |
Reverses the output row sequence from ascending time to descending [ascending]. |
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−N |
Select the number of columns and the column number that contains the "time" variable. Columns are numbered starting at 0 [2/0]. |
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−Q |
Quick mode for scalar calculation. Shorthand for −Ca −N1/0 −T0/0/1. |
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−S |
Only report the first or last row of the results [Default is all rows]. This is useful if you have computed a statistic (say the MODE) and only want to report a single number instead of numerous records with identical values. Append l to get the last row and f to get the first row only [Default]. |
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−T |
Required when no input files are given. Sets the t-coordinates of the first and last point and the equidistant sampling interval for the "time" column (see −N). If there is no time column (only data columns), give −T with no arguments; this also implies −Ca. Alternatively, give the name of a file whose first column contains the desired t-coordinates which may be irregular. |
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−V |
Selects verbose mode, which will send progress reports to stderr [Default runs "silently"]. |
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−bi |
Selects binary input. Append s for single precision [Default is double]. Uppercase S (or D) will force byte-swapping. Append n for the number of columns in the binary file(s). |
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−bo |
Selects binary output. Append s for single precision [Default is double]. Uppercase S (or D) will force byte-swapping. Append n for the number of columns in the binary file(s). |
The ASCII output formats of numerical data are controlled by parameters in your .gmtdefaults4 file. Longitude and latitude are formatted according to OUTPUT_DEGREE_FORMAT, whereas other values are formatted according to D_FORMAT. Be aware that the format in effect can lead to loss of precision in the output, which can lead to various problems downstream. If you find the output is not written with enough precision, consider switching to binary output (−bo if available) or specify more decimals using the D_FORMAT setting. |
(1) The operator PLM calculates the associated Legendre polynomial of degree L and order M, and its argument is the cosine of the colatitude which must satisfy -1 <= x <= +1. PLM is not normalized. (2) All derivatives are based on central finite differences, with natural boundary conditions. |
To take log10 of the average of 2 data files, use gmtmath file1.d file2.d ADD 0.5 MUL LOG10 = file3.d Given the file samples.d, which holds seafloor ages in m.y. and seafloor depth in m, use the relation depth(in m) = 2500 + 350 * sqrt (age) to print the depth anomalies: gmtmath samples.d T SQRT 350 MUL 2500 ADD SUB = | lpr To take the average of columns 1 and 4-6 in the three data sets sizes.1, sizes.2, and sizes.3, use gmtmath −C1,4-6 sizes.1 sizes.2 ADD sizes.3 ADD 3 DIV = ave.d To take the 1-column data set ages.d and calculate the modal value and assign it to a variable, try set mode_age = ‘gmtmath −S −T ages.d MODE =‘ To evaluate the dilog(x) function for coordinates given in the file t.d: gmtmath −Tt.d T DILOG = dilog.d To use gmtmath as a RPN Hewlett-Packard calculator on scalars (i.e., no input files) and calculate arbitrary expressions, use the −Q option. As an example, we will calculate the value of Kei (((1 + 1.75)/2.2) + cos (60)) and store the result in the shell variable z: set z = ‘gmtmath −Q 1 1.75 ADD 2.2 DIV 60 COSD ADD KEI =‘ To use gmtmath as a general least squares equation solver, imagine that the current table is the augmented matrix [ A | b ] and you want the least squares solution x to the matrix equation A * x = b. The operator LSQFIT does this; it is your job to populate the matrix correctly first. The −A option will facilitate this. Suppose you have a 2-column file ty.d with t and b(t) and you would like to fit a the model y(t) = a + b*t + c*H(t-t0), where H is the Heaviside step function for a given t0 = 1.55. Then, you need a 4-column augmented table loaded with t in column 0 and your observed y(t) in column 3. The calculation becomes gmtmath −N4/1 −Aty.d -C0 1 ADD −C2 1.55 STEPT ADD −Ca LSQFIT = solution.d Note we use the −C option to select which columns we are working on, then make active all the columns we need (here all of them, with −Ca) before calling LSQFIT. The second and fourth columns are preloaded with t and y(t), respectively, the other columns are zero. If you already have a precalculated table with the augmented matrix [ A | b ] in a file (say lsqsys.d), the least squares solution is simply gmtmath −T lsqsys.d LSQFIT = solution.d |
(1) Files that have the same names as some operators,
e.g., ADD, SIGN, =, etc. should be
identified by prepending the current directory (i.e.,
./LOG). |
Abramowitz, M., and I. A. Stegun, 1964, Handbook of
Mathematical Functions, Applied Mathematics Series, vol.
55, Dover, New York. |
GMT(l), grd2xyz(l), grdedit(l), grdinfo(l), grdmath(l), xyz2grd(l) |