add converstion between lognormal form and 90% c.i.

README.md

@@ -345,9 +345,10 @@ It emits one warning about something I already took care of, so by default I've
   - [x] Add prototypes
   - [x] Use named structs
   - [x] Add to header file
-  - [x] Provide example
+  - [x] Provide example algebra
+  - [x] Add conversion between 90% ci and parameters.
+  - [ ] Use that conversion in conjuction with small algebra.
   - [ ] Test results
-  - [ ] Add conversion between 90% ci and parameters.
   - [x] Move to own file? Or signpost in file? => signposted in file.
 - [ ] Disambiguate sample_laplace--successes vs failures || successes vs total trials as two distinct and differently named functions
 - [ ] Write twitter thread.

squiggle.c

@@ -12,7 +12,8 @@
 #define EXIT_ON_ERROR 0
 #define PROCESS_ERROR(error_msg) process_error(error_msg, EXIT_ON_ERROR, __FILE__, __LINE__)
 
-const double PI = 3.14159265358979323846; // M_PI in gcc gnu99
+#define PI 3.14159265358979323846 // M_PI in gcc gnu99
+#define NORMAL95CONFIDENCE 1.6448536269514722
 
 // # Key functionality
 // Define the minimum number of functions needed to do simple estimation
@@ -97,7 +98,6 @@ inline double sample_normal_from_95_confidence_interval(double low, double high,
     // 5. If we want a 90% confidence interval from high to low,
     // we can set mean = (high + low)/2; the midpoint, and L = high-low,
     // Normal([high + low]/2, [high - low]/(2 * 1.6448536269514722))
-    const double NORMAL95CONFIDENCE = 1.6448536269514722;
     double mean = (high + low) / 2.0;
     double std = (high - low) / (2.0 * NORMAL95CONFIDENCE);
     return sample_normal(mean, std, seed);
@@ -504,3 +504,24 @@ lognormal_params algebra_product_lognormals(lognormal_params a, lognormal_params
     };
     return result;
 }
+
+lognormal_params convert_ci_to_lognormal_params(struct c_i x)
+{
+    double loghigh = logf(x.high);
+    double loglow = logf(x.low);
+    double logmean = (loghigh + loglow) / 2.0;
+    double logstd = (loghigh - loglow) / (2.0 * NORMAL95CONFIDENCE);
+    lognormal_params result = { .logmean = logmean, .logstd = logstd}; 
+    return result;
+}
+
+struct c_i convert_lognormal_params_to_ci(lognormal_params y)
+{
+    double h = y.logstd * NORMAL95CONFIDENCE;
+    double loghigh = y.logmean + h;
+    double loglow = y.logmean - h;
+    struct c_i result = { .low=exp(loglow), .high=exp(loghigh)};
+    return result;
+
+}
+

squiggle.h

@@ -72,4 +72,10 @@ typedef struct lognormal_params_t {
 } lognormal_params;
 lognormal_params algebra_product_lognormals(lognormal_params a, lognormal_params b);
 
+
+lognormal_params convert_ci_to_lognormal_params(struct c_i x);
+
+
+struct c_i convert_lognormal_params_to_ci(lognormal_params y);
+
 #endif